U.S. patent application number 12/961893 was filed with the patent office on 2011-09-01 for enhanced resolution mass spectrometer and mass spectrometry method.
Invention is credited to John Fjeldsted, William Frazer, August Hidalgo.
Application Number | 20110210240 12/961893 |
Document ID | / |
Family ID | 41795246 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110210240 |
Kind Code |
A1 |
Hidalgo; August ; et
al. |
September 1, 2011 |
Enhanced Resolution Mass Spectrometer and Mass Spectrometry
Method
Abstract
A mass spectrum is generated by a process in which, from a mass
scan signal comprising original samples defining a peak, a subset
of the original samples defining the peak is selected. One or more
synthesized samples are synthesized from the subset of the original
samples. The one or more synthesized samples provide a temporal
resolution greater than the temporal resolution of the original
samples. The one or more synthesized samples are summed with
respective temporally-aligned accumulated samples to produce the
mass spectrum. The accumulated samples are obtained from mass scan
signals generated during respective previously-performed mass scan
operations.
Inventors: |
Hidalgo; August; (Loveland,
CO) ; Fjeldsted; John; (Loveland, CO) ;
Frazer; William; (Loveland, CO) |
Family ID: |
41795246 |
Appl. No.: |
12/961893 |
Filed: |
December 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12242110 |
Sep 30, 2008 |
7863556 |
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12961893 |
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12188932 |
Aug 8, 2008 |
7908093 |
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12242110 |
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11412887 |
Apr 27, 2006 |
7412334 |
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12188932 |
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Current U.S.
Class: |
250/282 ;
250/288 |
Current CPC
Class: |
H01J 49/0036
20130101 |
Class at
Publication: |
250/282 ;
250/288 |
International
Class: |
H01J 49/26 20060101
H01J049/26 |
Claims
1. A method for generating a mass spectrum, the method comprising:
from a mass scan signal comprising original samples defining a
peak, selecting a subset of the original samples defining the peak,
the original samples having a temporal resolution; synthesizing
from the subset of the original samples one or more synthesized
samples providing a temporal resolution greater than the temporal
resolution of the original samples; and summing the one or more
synthesized samples with respective temporally-aligned accumulated
samples to produce the mass spectrum, the accumulated samples
obtained from mass scan signals generated during respective
previously-performed mass scan operations.
2. The method of claim 1, in which the synthesizing comprises
subjecting the subset of the original samples to interpolation to
generate the synthesized samples.
3. The method of claim 2, in which: the original samples in the
subset and the synthesized samples collectively constitute an
augmented subset; the synthesizing additionally comprises
suppressing at least one temporally-extreme one of the original
samples in the augmented subset to generate a truncated subset; and
the summing additionally comprises summing the original samples in
the truncated subset with respective temporally-aligned ones of the
accumulated samples.
4. The method of claim 1, in which: the synthesizing generates a
single synthesized sample comprising a time component and an
amplitude component, the time component having a greater temporal
resolution than the original samples; and the synthesizing
comprises: subjecting the original samples in the subset to a
centroid calculation to obtain the time component of the
synthesized sample, and generating the amplitude component of the
synthesized sample from at least one of the original samples in the
subset.
5. The method of claim 4, in which the synthesizing additionally
comprises, prior to the subjecting, associating each of the
original samples in the subset with a respective time value or mass
value.
6. The method of claim 5, in which the generating the amplitude
component of the synthesized sample comprises subjecting two or
more of the original samples in the subset and the respective time
values thereof to interpolation to generate a two-dimensional
sample having an amplitude component calculated by the
interpolation and a time component equal to the time component of
the synthesized sample, the amplitude component of the
two-dimensional sample providing the amplitude component of the
synthesized sample.
7. The method of claim 4, in which the generating the amplitude
component of the synthesized sample comprises selecting one of the
original samples in the subset as the amplitude component of the
synthesized sample.
8. The method of claim 4, in which the summing comprises summing
the amplitude component of the synthesized sample with the
amplitude component of the one of the accumulated samples having a
time component equal to the time component of the synthesized
sample to generate the amplitude component of a new accumulated
sample having a time component equal to the time component of the
synthesized sample.
9. The method of claim 8, additionally comprising mapping the time
components of the accumulated samples to respective memory
locations.
10. The method of claim 8, additionally comprising generating a
respective one of the accumulated samples by a process comprising
accumulating the amplitude components of synthesized samples
obtained from the sequences of original samples generated during
the previously-performed mass scan operations and having equal time
components.
11. The method of claim 1, additionally comprising generating the
accumulated samples by a process comprising subjecting the mass
scan signals generated during the respective previously-performed
mass scan operations to respective selecting, synthesizing and
summing.
12. The method of claim 1, in which the summing comprises: summing
each of the one or more synthesized samples with a respective
temporally-aligned one of the accumulated samples read from a
memory location to generate a new accumulated sample; and storing
the new accumulated sample at the memory location from which the
one of the accumulated samples was read.
13. A mass spectrometer, comprising: a sample selector operable to
select, from a mass scan signal comprising original samples
defining a peak, a subset of the original samples defining the
peak, the original samples having a temporal resolution; a sample
synthesizer operable to synthesize from the subset of the original
samples one or more synthesized samples providing a temporal
resolution greater than the temporal resolution of the original
samples; and a sample combiner operable to sum the one or more
synthesized samples with respective temporally-aligned accumulated
samples to produce a mass spectrum, the accumulated samples
generated by the sample selector, the sample synthesizer and the
sample summer from mass scan signals obtained during respective
previously-performed mass scan operations.
14. The mass spectrometer of claim 13, in which the sample
synthesizer comprises an interpolator operable to subject the
subset of the original samples to interpolation to generate the
synthesized samples.
15. The mass spectrometer of claim 14, in which: the original
samples in the subset and the synthesized samples collectively
constitute an augmented subset; the sample synthesizer additionally
comprises a sample suppressor operable to suppress at least one
temporally-extreme one of the original samples in the augmented
subset to generate a truncated subset; and the sample combiner
comprises a memory and a summer, the memory operable to store the
accumulated samples, the summer operable to sum the synthesized
samples in the truncated subset with the temporally-aligned
accumulated samples stored in the memory.
16. The mass spectrometer of claim 15, in which the summer is
additionally operable to sum the original samples in the truncated
subset with respective temporally-aligned accumulated samples.
17. The mass spectrometer of claim 15, in which: the memory
comprises a memory location in which a respective one of the
accumulated samples is stored; the summer is operable to perform
operations comprising summing the one of the accumulated samples
read from the memory location with a respective one of the
synthesized samples in the truncated subset to generate a new
accumulated sample; and the memory is operable to store the new
accumulated sample at the memory location.
18. The mass spectrometer of claim 13, in which: the sample
synthesizer generates a single synthesized sample from the original
samples in the subset, the synthesized sample comprising a time
component and an amplitude component; and the sample synthesizer
comprises: a centroid calculator operable to subject the original
samples in the subset to a centroid calculation to obtain the
temporal component of the synthesized sample, and an amplitude
component generator operable to generate the amplitude component of
the synthesized sample from at least one of the original samples in
the subset.
19. The mass spectrometer of claim 18, in which the sample
synthesizer additionally comprises a time value generator operable
to generate a time value for each of the original samples in the
subset.
20. The mass spectrometer of claim 19, in which the amplitude
component generator is operable to subject two or more of the
original samples in the subset and the respective time values
thereof to interpolation to generate a two-dimensional sample
having an amplitude component calculated by the interpolation and a
time component equal to the time component of the synthesized
sample, the amplitude component of the two-dimensional sample
providing the amplitude component of the synthesized sample.
21. The mass spectrometer of claim 18, in which the amplitude
component generator is operable to select one of the original
samples in the subset as the amplitude component of the synthesized
sample.
22. The mass spectrometer of claim 18, in which the sample combiner
is operable to combine the amplitude component of the synthesized
sample with the amplitude component of the one of the accumulated
samples having a time component equal to the time component of the
synthesized sample to generate the amplitude component of a new
accumulated sample having a time component equal to the time
component of the synthesized sample.
23. The mass spectrometer of claim 18, in which the time components
of the accumulated samples are mapped to respective memory
locations.
24. The mass spectrometer of claim 18, in which the sample combiner
is operable to generate the accumulated samples by accumulating the
amplitude components of synthesized samples generated during the
previously-performed mass scan operations and having equal time
components.
25. A computer-readable medium in which is fixed a program operable
to cause a computational device to perform a method that generates
a mass spectrum, the method comprising: selecting, from a mass scan
signal comprising original samples defining a peak, a subset of the
original samples defining the peak, the original samples having a
temporal resolution; synthesizing from the subset of the original
samples one or more synthesized samples providing a temporal
resolution greater than the temporal resolution of the original
samples; and summing the one or more synthesized samples with
respective temporally-aligned accumulated samples to produce the
mass spectrum, the accumulated samples obtained from mass scan
signals generated during respective previously-performed mass scan
operations.
26. The computer-readable medium of claim 25, in which: the
synthesizing comprises: subjecting the subset of the original
samples to interpolation to generate the synthesized samples, the
original samples in the subset and the synthesized samples
collectively constituting an augmented subset, and suppressing at
least one temporally-extreme one of the original samples in the
augmented subset to generate a truncated subset; and the summing
additionally comprises summing the original samples in the
truncated subset with respective temporally-aligned accumulated
samples.
27. The computer-readable medium of claim 25, in which: the
synthesizing generates a single synthesized sample comprising a
time component and an amplitude component, the time component
having a greater temporal resolution than the original samples; and
the synthesizing comprises: associating the original samples in the
subset with respective time values or mass values to generate an
augmented subset of respective two-dimensional samples, subjecting
the two-dimensional samples in the augmented subset to a centroid
calculation to obtain the time component of the synthesized sample,
and generating the amplitude component of the synthesized sample
from at least one of the original samples in the subset.
Description
RELATED APPLICATION
[0001] This application is a Continuation-in-Part of co-pending
U.S. patent application Ser. No. 12/188,932 of Fjeldsted et al.
filed on 8 Aug. 2008 and entitled Mass Spectrometer and Method for
Enhancing Resolution of Mass Spectra, which is a Continuation of
U.S. patent application Ser. No. 11/412,887 of Fjeldsted et al.
filed on 27 Apr. 2006 and entitled Mass Spectrometer and Method for
Enhancing Resolution of Mass Spectra, now U.S. Pat. No. 7,412,334,
the entire disclosures of which are incorporated into this
application by reference. U.S. patent application Ser. No.
12/188,932 will be referred to in this disclosure as the parent
application.
BACKGROUND
[0002] In time-of-flight mass spectrometers (TOFMS), a mass sample
to be analyzed is ionized, the resulting ions are accelerated in a
vacuum by an electrical pulse having a known potential, and the
flight times of the ions of different masses at an ion detector are
measured. The more massive the ion, the longer is the flight time.
The relationship between the flight time and the mass, m, of ions
of a given mass can be written in the form:
time=k {square root over (m)}+c
where k is a constant related to flight path and ion energy, and c
is a small delay time that may be introduced by the signal cable
and/or detection electronics. When the term mass is used in this
disclosure in the context of mass spectrometry, it is to be
understood to mean mass-to-charge ratio. The process of
accelerating the ions of the mass sample and detecting the arrival
times of the ions of different masses at the ion detector will be
referred to herein as a mass scan operation.
[0003] The ion detector generates electrons in response to ions
incident thereon. The electrons constitute an electrical signal
whose amplitude is proportional to the number of electrons. There
is only a statistical correlation between the number of electrons
generated in response to a single ion incident on the ion detector.
In addition, more than one ion at a time may be incident on the ion
detector due to ion abundance.
[0004] In the mass spectrometer, an ion accelerator generates a
short pulse of ions by applying an electrical pulse having a known
voltage to ions received from the ion source. Immediately after
leaving the ion accelerator, the ions are bunched together but,
within the ion pulse, ions of different masses travel at different
speeds. The flight time required for the ions of a given mass to
reach the ion detector depends on the speed of the ions, which in
turn, depends on the mass of the ions. Consequently, as the ion
pulse approaches the ion detector, the ion pulse is separated in
space and in time into discrete packets, each packet containing
ions of a single mass. The packets reach the ion detector at
different arrival times that depend on the mass of the ions
therein.
[0005] The mass spectrometer generates what will be referred to a
mass scan signal in response to a single pulse of ions accelerated
by a single electrical pulse. The mass scan signal is a digital
signal that represents the output of the ion detector as a function
of time. The time represents the time-of-flight of the ions from
the ion accelerator to the ion detector. The number of electrons
generated by the ion detector in a given time interval constitutes
an analog ion detection signal that is converted to the mass scan
signal by an analog-to-digital converter (A/D converter). The mass
scan signal represents the output of the ion detector as a function
of the flight time taken by the ions to reach the ion detector. The
mass scan signal is a temporal sequence of digital samples output
by the A/D converter after the ions have been accelerated. The
conversion time of the A/D converter effectively divides the time
axis into discrete bins and the A/D converter outputs a single
digital sample for each bin on the time axis.
[0006] Because the relationship between the amplitude of the ion
detection signal output by the ion detector and the number of ions
incident on the ion detector is a statistical one, a single mass
scan signal will not accurately represent the mass spectrum of the
sample. In addition, the ion detection process is subject to noise
from a number of different noise sources. Such noise causes the ion
detector to generate an output signal even in the absence of ions
incident on the ion detector. To overcome these problems, the mass
spectrometer generates multiple mass scan signals and sums the
most-recently generated mass scan signal with an accumulation of
all previously-generated mass scan signals to generate a mass
spectrum having a defined statistical accuracy and signal-to-noise
ratio.
[0007] The resulting mass spectrum is subject to mass resolution
limitations originating from the ion accelerator and the ion
detector and its associated circuitry. The mass spectrometer and
mass spectrometry method disclosed in the parent application
decreased the mass resolution limitations originating from the ion
detector and its associated circuitry leaving the ion accelerator
as the primary limiter of mass resolution. This has prompted
improvements in the precision of the mass accelerator so that, once
more, the ion detector and its associated circuitry have become
contributors to mass resolution limitations.
[0008] Accordingly, what is needed is to reduce the mass resolution
limitations imposed by the ion detector and its associated
circuitry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The disclosure can be better understood with reference to
the following drawings. The elements of the drawings are not
necessarily to scale relative to each other. Instead, emphasis is
placed upon clear illustration. Furthermore, like reference
numerals designate corresponding parts throughout the several
views.
[0010] FIG. 1 is a block diagram showing an example of a
conventional mass spectrometer.
[0011] FIG. 2A is a graph illustrating an exemplary analog pulse
exhibited by an ion detection signal output by an ion detector,
such as is depicted in FIGS. 1 and 5, during a first mass scan
operation.
[0012] FIG. 2B is a graph illustrating an exemplary analog pulse
exhibited by an ion detection signal output by an ion detector,
such as is depicted in FIGS. 1 and 5, during a second mass scan
operation and corresponding to the analog pulse shown in FIG.
2A.
[0013] FIG. 2C is a graph illustrating an exemplary analog pulse
exhibited by an ion detection signal output by an ion detector,
such as is depicted in FIGS. 1 and 5, during a third mass scan
operation and corresponding to the analog pulses shown in FIGS. 2A
and 2B.
[0014] FIG. 2D is a graph illustrating an exemplary analog pulse
exhibited by an ion detection signal output by an ion detector,
such as is depicted in FIGS. 1 and 5, during a fourth mass scan
operation and corresponding to the analog pulses shown in FIGS.
2A-2C.
[0015] FIGS. 3A-3D are graphs illustrating exemplary samples
obtained by digitizing the analog pulses shown in FIGS. 2A-2D,
respectively.
[0016] FIG. 4 is a graph illustrating an exemplary peak exhibited
by a mass spectrum generated by the mass spectrometer shown in FIG.
1 summing the samples shown in FIGS. 3A-3D.
[0017] FIG. 5 is a block diagram showing an example of a mass
spectrometer in accordance with an embodiment of the parent
application.
[0018] FIG. 6 is a block diagram illustrating an exemplary sampling
system, such as that depicted in FIG. 5.
[0019] FIG. 7 is a flowchart illustrating an exemplary architecture
and functionality of the sample adjuster depicted in FIG. 6.
[0020] FIGS. 8A-8D are graphs illustrating examples of the active
samples output by the sample adjuster shown in FIG. 6 upon
processing, as input, the samples shown in FIGS. 3A-3D,
respectively.
[0021] FIG. 9 is a graph illustrating an exemplary peak exhibited
by a mass spectrum generated by the mass spectrometer shown in FIG.
5 summing the active samples shown in FIGS. 8A-8D.
[0022] FIG. 10 is a block diagram showing an example of a mass
spectrometer in accordance with an embodiment of the invention.
[0023] FIG. 11 is a block diagram showing an example of a first
embodiment of the sample processor of the mass spectrometer shown
in FIG. 10.
[0024] FIGS. 12A-12F are graphs illustrating the operation of the
sample processor shown in FIG. 11.
[0025] FIG. 13 is a block diagram showing a first example of a
second embodiment of the sample processor of the mass spectrometer
shown in FIG. 10.
[0026] FIGS. 14A-14D are graphs illustrating the operation of the
sample processor shown in FIG. 13.
[0027] FIG. 15 is a block diagram showing a second example of the
second embodiment of the sample processor of the mass spectrometer
shown in FIG. 10.
[0028] FIG. 16 is a flow chart illustrating the operation of the
processor of the sample processor shown in FIG. 15.
[0029] FIG. 17 is a flow chart showing an example of a method in
accordance with an embodiment of the invention for generating a
mass spectrum.
[0030] FIG. 18 is a flow chart showing an example of the
synthesizing and the summing shown in FIG. 17.
[0031] FIG. 19 is a flow chart showing another example of the
synthesizing and the summing shown in FIG. 17.
DETAILED DESCRIPTION
[0032] One embodiment of the invention provides a method for
generating a mass spectrum in which, from a mass scan signal
comprising original samples defining a peak, a subset of the
original samples defining the peak is selected. The original
samples have a temporal resolution. One or more synthesized samples
are synthesized from the subset of the original samples. The one or
more synthesized samples provide a temporal resolution greater than
the temporal resolution of the original samples. The one or more
synthesized samples are summed with respective temporally-aligned
accumulated samples to produce the mass spectrum. The accumulated
samples are obtained from mass scan signals generated during
respective previously-performed mass scan operations.
[0033] Another embodiment of the invention provides a mass
spectrometer comprising a sample selector, a sample synthesizer and
a sample combiner. The sample selector is operable to select, from
a mass scan signal comprising original samples defining a peak, a
subset of the original samples defining the peak. The original
samples have a temporal resolution. The sample synthesizer is
operable to synthesize from the subset of the original samples one
or more synthesized samples that provide a temporal resolution
greater than the temporal resolution of the original samples. The
sample combiner is operable to sum the one or more synthesized
samples with respective temporally-aligned accumulated samples to
produce a mass spectrum. The accumulated samples are generated by
the sample selector, the sample synthesizer and the sample combiner
from mass scan signals generated during respective
previously-performed mass scan operations.
[0034] Another embodiment of the invention provides a
computer-readable medium in which is fixed a program operable to
cause a computational device to perform a method that generates a
mass spectrum. In the method performed in response to the program,
from a mass scan signal comprising original samples defining a
peak, a subset of the original samples defining the peak is
selected. The original samples have a temporal resolution. One or
more synthesized samples are synthesized from the subset of the
original samples. The one or more synthesized samples provide a
temporal resolution greater than the temporal resolution of the
original samples. The one or more synthesized samples are summed
with respective temporally-aligned accumulated samples to produce
the mass spectrum. The accumulated samples are obtained from mass
scan signals generated during respective previously-performed mass
scan operations.
[0035] In one example, the one or more synthesized samples are
generated by subjecting the original samples in the subset to
interpolation. Typically, at least one temporally-extreme one of
the original samples in the subset comprising the synthesized
samples is suppressed to generate a truncated subset, and the
original samples in the truncated subset are summed with respective
temporally-aligned ones of the accumulated samples to generate
respective new accumulated samples.
[0036] In another example, a single synthesized sample having a
time component and an amplitude component is generated from the
original samples in the subset. The original samples in the subset
are subject to a centroid calculation to obtain the time component
of the synthesized sample, and the amplitude component of the
synthesized sample is generated from at least one of the original
samples in the subset. In embodiments, the amplitude component of
the synthesized sample is generated from the original samples in
the subset by selection or by interpolation.
[0037] FIG. 1 is a block diagram showing an example of a
conventional time-of-flight mass spectrometer 10. A mass sample to
be analyzed is introduced into an ion source 11 that ionizes the
sample. The ions so produced are accelerated by applying a
potential between the ion source 11 and an electrode 12. The mass
spectrum of the mass sample to be analyzed is generated by
accumulating the mass scan signals generated by respective mass
scan operations. At the beginning of each mass scan operation, a
controller 15 causes a pulse source 17 to apply a short electrical
pulse between the electrode 12 and ion source 11. The controller 15
also resets the contents of a write address register 21. Subsequent
periods of a conversion clock signal provided by a clock 24
increment the address register 21, and an analog ion detection
signal generated by an ion detector 25 is digitized by an
analog-to-digital converter (A/D converter) 27 to generate a
digital mass scan signal composed of a temporal sequence of digital
samples. Unless explicitly stated that a sample is an analog
sample, the word sample, as used herein, refers to a digital
sample, i.e., a digital value that represents the amplitude of a
respective analog sample of the analog signal. An adder 33 sums an
accumulated sample (if any) stored in memory 29 at the address
specified by the address register 21 with the sample provided by
A/D converter 27 to generate a new accumulated sample. The new
accumulated sample is then stored back in memory 29 at the address
specified by the address register 21. Similar operations are
performed for each remaining value of the write address in the
range of write addresses generated by address register 21 to
generate the remainder of a mass spectrum stored in memory 29. The
range of write addresses extends from zero to a value approximately
equal to (.DELTA.mf).sup.2, where .DELTA.m is the range of masses
resolved by mass spectrometer 10 and f is the frequency of the
conversion clock signal generated by clock 24 and applied to A/D
converter 27.
[0038] As noted above, the flight time required by an ion to
traverse the distance between the electrode 12 and the ion detector
25 provides a measure of the mass of the ion. The value in address
register 21 when the ion is incident on the ion detector 25 is
proportional to the flight time. Hence, after mass spectrometer 10
has performed a number of mass scan operations, memory 29
accumulates data that indicates the abundance of ions with a given
mass as a function of the mass of the ions. In other words, the
data stored in memory 29 represents a mass spectrum of the sample
being analyzed.
[0039] Various devices, such as a Faraday cup, multichannel plate
(MCP), electron multiplier (continuous structure as well as dynode
structure), conversion dynode, Daly ion detector, and combinations
thereof, may be used to implement the ion detector 25. The ion
detection signal generated by the ion detector 25 depends on the
number of ions incident on the ion detector 25 in a time
corresponding to the sampling time of A/D converter 27. Moreover,
in a time-of-flight mass spectrometer, heavier ions arrive at the
ion detector 25 after lighter ions. The ion detection signal output
by the ion detector 25 as a function of flight time exhibits pulses
that can be identified as originating from ions of specific masses.
A pulse in the ion detection signal is due to ions of a particular
mass being incident on the ion detector 25 during a small interval
of time. Ions of the same mass are generally bunched together as
they travel toward and are incident on the ion detector 25 and will
be referred to hereafter as an ion packet. Thus, ions within the
same ion packet have the same mass. Further, the pulses exhibited
by the ion detection signal generated by the ion detector 25 will
be referred to below as analog pulses.
[0040] In general, the number ions in each ion packet is relatively
small, and hence the statistical accuracy of the mass scan signal
obtained in a single mass scan operation is usually insufficient.
In addition, there can be a significant amount of noise in the
system. The noise is generated both in the ion detector 25, the
analog signal path, and in the A/D converter 27.
[0041] To improve statistical accuracy, the mass scan signals
generated by a large number of respective mass scan operations are
accumulated to produce the mass spectrum of the mass sample. At the
beginning of the mass spectrum measurement process, the controller
15 stores zeros in all of the memory locations in memory 29 and
initiates the first mass scan operation. The first mass scan
operation causes a first mass scan signal to be stored in memory 29
as a mass spectrum. When the first mass scan operation is
completed, the controller 15 resets the address register 21 and
initiates another mass scan operation by causing the pulse source
17 to pulse the electrode 12. The second mass scan signal generated
by the second mass scan operation is added to the mass spectrum
stored in memory 29 to generate a new mass spectrum having a better
statistical accuracy than the previous mass spectrum. The process
just described is repeated until the new mass spectrum has the
desired statistical accuracy.
[0042] Small variations in the mass scan signals degrade the mass
resolution of the mass spectrum defined by the accumulated samples
stored in memory 29. For example, clock jitter may cause small
timing variations in the mass scan signals, and the effect of these
small timing variations on the mass spectrum can become significant
as many different mass scan signals are accumulated. Further,
variations in the pulse source 17 may cause the electrodes 12 to
ionize the mass sample of the ion source 11 such that ions of the
same mass have slightly different initial energies. As a result,
ions of the same mass may be incident on the ion detector 25 at
slightly different times. In addition, the ion detector 25 has
finite rise and fall times. Thus, even if ions of the same mass
were incident on the ion detector 25 at exactly the same time, the
pulse exhibited by the ion detection signal output by the ion
detector 25 would have a pulse width spanning a finite range of
time. The analog signal path, including the analog portion of A/D
converter 27, may further increase the width of the pulses
exhibited by the ion detection signal output by the ion detector
25. These and other variations can significantly degrade the mass
resolution of the mass spectrum.
[0043] To better illustrate the foregoing, refer to FIGS. 2A-2D,
which respectively depict exemplary pulses 41-44 exhibited by the
ion detection signal output by the ion detector 25 during
corresponding temporal portions of four mass scan operations
performed by mass spectrometer 10. As shown in FIGS. 2A-2D, each
pulse 41-44 has a finite pulse width, which is related to the rise
and fall times of the ion detector 25. Further, ions of the same
mass may be incident on the ion detector 25 at different times due
to the variations described above, thereby increasing the finite
pulse widths of the pulses 41-44.
[0044] The pulses 41-44 depicted in FIGS. 2A-2D, respectively, are
corresponding pulses in the analog ion detection signal output by
the ion detector 25 during respective mass scan operations
performed by mass spectrometer 10. As used in this disclosure,
pulses are corresponding if they are caused by ions of the same
mass incident on ion detector 25. Thus, the pulses 41-44 shown in
FIGS. 2A-2D are caused by ions of the same mass and, ideally, each
would occur at the same time (x) after the start of its respective
mass scan operation. The ion detection signals that exhibit pulses
41-44 are each digitized to produce respective mass scan signals
and the samples constituting the mass scan signals are accumulated
to define a single peak in the mass spectrum. However, as can be
seen by comparing FIGS. 2A-2D, variations in the pulse source 17
and/or the ion detector 25 cause small timing offsets among the
pulses 41-44. The maximum of the pulse 41 shown in FIG. 2A occurs
at time x after the start of the first mass scan operation, but the
maximum of the pulse 42 shown in FIG. 2B occurs at a time greater
than x after the start of the second mass scan operation, the
maximum of the pulse 43 shown in FIG. 2C occurs at a time less than
x after the start of the third mass scan operation, and the maximum
of the pulse 44 shown in FIG. 2D occurs at a time less than x after
the start of the fourth mass scan operation.
[0045] The analog ion detection signals that exhibit the pulses
41-44, respectively, are digitized by the A/D converter 27 (FIG. 1)
to generate respective mass scan signals that are output by the A/D
converter. FIGS. 3A-3D respectively depict mass scan signals that
exhibit peaks 45-48, respectively. Each of the points constituting
the mass scan signals shown in FIGS. 3A-3D represents a sample of
one of the ion detection signals exhibiting pulses 41-44 shown in
FIGS. 2A-2D, respectively. In particular, FIG. 3A depicts a mass
scan signal exhibiting a peak 45 obtained by digitally sampling the
ion detection signal exhibiting pulse 41 shown in FIG. 2A, FIG. 3B
depicts a mass scan signal exhibiting a peak 46 obtained by
digitally sampling the ion detection signal exhibiting the pulse 42
shown in FIG. 2B, FIG. 3C depicts a mass scan signal exhibiting a
peak 47 obtained by digitally sampling the ion detection signal
exhibiting the pulse 43 shown in FIG. 2C, and FIG. 3D depicts a
mass scan signal exhibiting a peak 48 obtained by digitally
sampling the ion detection signal exhibiting the pulse 44 shown in
FIG. 2D.
[0046] FIG. 4 depicts a mass spectrum exhibiting a peak 49
resulting from accumulating the mass scan signals exhibiting peaks
45-48 shown in FIGS. 3A-3D as would be performed by the
conventional mass spectrometer 10 (FIG. 1). The peak 49 has a
relatively large width (z-y) in the time domain. This is due not
only to the non-zero pulse widths of the pulses 41-44 but also to
the jitter collectively exhibited by pulses 41-44. The
above-described temporal offsets of the pulses 41-44 increase the
overall width of the peak 49.
[0047] FIG. 5 is a block diagram showing an example of a
time-of-flight mass spectrometer 50 in accordance with an
embodiment disclosed in the parent application. To simplify the
description of FIG. 5 and subsequent drawings, those elements that
serve functions analogous to elements described above with
reference to FIG. 1 are indicated by the same reference
numerals.
[0048] In the example shown in FIG. 5, the mass spectrometer 50 is
composed of an ion source 11, a controller 15, a pulse source 17, a
write address register 21, a clock 24, an ion detector 25, memory
29, an adder 33, and a sampling system 51. The elements 17, 21, 24,
25, 27, 29, and 33 perform essentially the respective functions as
the elements with the same reference numerals in FIG. 1.
[0049] FIG. 6 is a block diagram showing an example of sampling
system 51. In the example shown, sampling system 51 is composed of
an A/D converter 27, a buffer 77 and a sample adjuster 78.
[0050] In a manner similar to that described above with reference
to FIG. 1, a mass sample to be analyzed is introduced into the ion
source 11 that ionizes the mass sample. A pulse from the pulse
source 17 applied to the electrode 12 accelerates the ions in the
ion source 11 toward the ion detector 25, which detects the
accelerated ions. The ion detector 25 outputs an analog ion
detection signal whose amplitude is indicative of the number of
ions incident on the ion detector.
[0051] In a manner similar to that described above with reference
to FIG. 1, the analog ion detection signal output by the ion
detector 25 shown in FIG. 5 is sampled by the A/D converter 27
shown in FIG. 6. Referring to FIG. 6, a number of the samples
output by the A/D converter 27 are temporarily stored in a buffer
77 and such samples are processed by a sample adjuster 78, which
will be described in more detail below. The samples output by the
sample adjuster 78 constitute an adjusted mass scan signal that is
summed by a summer 33 (FIG. 5) with the mass spectrum obtained by
accumulating the adjusted mass scan signals generated by
previously-performed mass scan operations to generate a new mass
spectrum, and the new mass spectrum is stored in memory 29.
[0052] Thus, the mass spectrometer 50 shown in FIG. 5 generates a
mass spectrum by accumulating the mass scan signals respectively
generated by a large number of mass scan operations. At each
address location in memory 29 is stored an accumulated sample that
provides one data point of the mass spectrum represented by the
accumulated samples stored in respective memory locations in memory
29.
[0053] The controller 15 and the sample adjuster 78 can be
implemented in hardware, software, or a combination thereof. As an
example, the controller 15 and/or the sample adjuster 78 may be
implemented in software and executed by a programmable logic array,
a digital signal processor (DSP), a central processing unit (CPU),
or other type of apparatus for executing the instructions of the
controller 15 and/or the sample adjuster 78. In other embodiments,
the controller 15 and/or the sample adjuster 78 can be implemented
in firmware or hardware, such as logic gates, for example.
[0054] The sample adjuster 78 is configured to identify peaks in
each mass scan signal received from the A/D converter 27. Further,
for each identified peak, the sample adjuster 78 is configured to
designate at least one of the samples as an active sample. As used
in this disclosure, an active sample is a sample that is not to be
suppressed by the sample adjuster 78.
[0055] For each peak identified in each mass scan signal received
from A/D converter 27, sample adjuster is configured to identify a
predefined number of the samples having the highest values as the
active samples for the peak. Thus, the active samples for a given
peak are the highest-value ones of the samples defining the peak.
In one embodiment, as will be described in more detail below, for
each peak, the sample adjuster 78 identifies only the one sample
having the highest value (i.e., the highest-value one of the
samples defining the peak) as the active sample. In this example,
each peak has only one active sample. In other embodiments, for
each peak, the sample adjuster 78 identifies two or more of the
samples having the highest values as the active samples for the
peak.
[0056] The sample adjuster 78 allows all active samples to pass to
memory 29 unsuppressed but suppresses all of the other samples
constituting the mass scan signal (i.e., each sample not identified
as an active sample by the sample adjuster 78). As used in this
disclosure, a sample is suppressed when it is assigned a value
lower than the actual value assigned to it by the A/D converter 27,
or it is prevented from affecting the respective accumulated sample
constituting one data point of the mass spectrum accumulated in
memory 29. In an example, the sample adjuster 78 suppresses a
sample by assigning such sample a value of zero (0). Thus, each
suppressed sample does not affect the mass spectrum accumulated in
memory 29.
[0057] Various techniques exist that may be employed by the sample
adjuster 78 to identify peaks in the mass scan signal constituted
by the samples generated by A/D converter 27. In one embodiment,
the sample adjuster 78 identifies a peak in a region of the mass
scan signal in which at least a minimum number, p, of consecutive
samples having increasing values is immediately followed by at
least a minimum number, q, of consecutive samples having decreasing
values. Note that the numbers p and q may be specified by a user or
predefined within the sample adjuster 78. Further, numbers p and q
may be equal.
[0058] When sample adjuster 78 identifies a peak in the mass scan
signal, it additionally identifies as a maximum sample the sample
within the above-described two strings having the highest value.
Such a sample is typically identified by the sample adjuster 78 as
an active sample for the identified peak. Moreover, the sample
adjuster 78 allows each sample identified as an active sample to
pass unchanged through the sample adjuster 78 as part of the
adjusted mass scan signal, and suppresses each of the other
samples.
[0059] To better illustrate the foregoing, assume that the ion
detector 25 of mass spectrometer 50 outputs the ion detection
signals exhibiting corresponding pulses 41-44 shown in FIGS. 2A-2D
in consecutive mass scan operations, as described above with
reference to the conventional spectrometer 10. In such an example,
the A/D converter 27 receives the ion detection signals exhibiting
the pulses 41-44 shown in FIGS. 2A-2D, and, in response thereto,
outputs respective mass scan signals exhibiting the peaks 45-48
shown in FIGS. 3A-3D, respectively. Referring to FIGS. 3A-3D,
assume that samples 85-88 are the maximum samples of peaks 45-48,
respectively, and that the sample adjuster 78 is configured to
identify, for each peak, only the peak's maximum sample as a
respective active sample. In such an example, the sample adjuster
78, upon identifying the peak 45 as a peak and identifying the
sample 85 as the maximum sample of peak 45, suppresses all of the
samples defining peak 45 except the maximum sample 85.
[0060] Various techniques exist and may be used to identify the
maximum sample of the peak 45 and to suppress all of the samples of
the peak 45 except the maximum sample 85. FIG. 7 illustrates an
exemplary process that may be used to achieve the foregoing. The
sequence of samples generated by A/D converter 27 and constituting
a mass scan signal are written to and read out of the buffer 77
(FIG. 6) on a first-in, first-out (FIFO) basis. During the first
mass scan operation, samples defining the peak 45 are among those
written into the buffer 77 by the A/D converter 27 as the A/D
converter 27 samples the ion detection signal exhibiting pulse 41.
In block 112, the sample adjuster 78 analyzes the samples stored in
the buffer 77 to determine whether the samples define a peak. For
examples, the sample adjuster 78 compares the samples in the buffer
77 and determines that these samples define a peak when such
samples include at least a number p of consecutive samples of
increasing values followed by at least a number q of consecutive
samples of decreasing values.
[0061] Other techniques for identifying a peak, such as peak 45, in
a mass scan signal are known and may be used in other embodiments.
As an example, the sample adjuster 78 may identify any sample as
defining a peak if it is immediately preceded by a sample of lower
value and is followed by a sample of lower value within the next
two samples.
[0062] If the samples in buffer 77 do not define a peak, then the
sample adjuster 78 reads and suppresses the next sample in the
buffer 77. In particular, the sample adjuster 78 reads the next
sample in the buffer 77 and outputs a value of zero, as shown by
blocks 120 and 122, effectively replacing the sample's actual value
with the value of zero (0). The suppressed sample output by the
sample adjuster 78 is then summed by summer 33 with the accumulated
sample read from the memory 29 at the address specified by the
address register 21. Note that, as each sample is read out of the
buffer 77 by the sample adjuster 78, a new sample is written to the
buffer 77 by the A/D converter 27. If the current mass scan
operation being performed by the mass spectrometer 50 is not yet
complete, then the sample adjuster 78 makes a "no" determination in
block 124 and returns to block 112, where it once more analyzes the
samples currently stored in the buffer 77. These samples include
sample newly-written to the buffer 77 by A/D converter 27.
[0063] Once the sample adjuster 78 has determined in block 115 that
the samples temporarily stored in buffer 77 define a peak, such as
peak 45, then, in block 133, the sample adjuster 78 identifies the
one or more active samples of the peak. In the instant example,
assume that the sample adjuster 78 only identifies the maximum
sample for each peak as the active sample for the peak. Thus, when
the sample adjuster 78 makes a "yes" determination in block 115, in
block 133, the sample adjuster 78 identifies the highest-value one
of the samples defining the peak and stored in the buffer 77 as the
active sample for the peak. Thus, the sample adjuster 78 can
compare the samples stored in the buffer 77 with one another to
find the sample with the highest value and identify this sample as
the active sample for the peak. Other techniques for identifying
the active sample or samples of a peak may be employed in other
embodiments.
[0064] In block 136, the sample adjuster 78 reads the next sample
from the buffer 77 on a FIFO basis and, in block 138, determines
whether this sample was identified in block 133 as an active
sample. If not, the sample adjuster 78 suppresses this sample. In
particular, upon reading the next sample in block 136, the sample
adjuster 78 outputs a value of zero, as shown by block 141,
effectively replacing the actual value of the sample with the value
of zero (0).
[0065] However, if the value read from the buffer 77 in block 136
was identified in block 133 as an active sample, then in block 144,
the sample adjuster 78 outputs the sample without changing its
value. The sample currently output by the sample adjuster 78 in
either block 141 or block 144 is output to summer 33, which sums
the sample with the accumulated sample read from memory 29 at the
address specified by the address register 21 to generate a new
accumulated sample that is written in memory 29 at the same
address. The new accumulated sample is one data point of the new
mass spectrum being generated in memory 29 by the current mass scan
operation. Further, in block 145, the sample adjuster 78 determines
whether any additional active samples were identified in block 133
for the peak identified in block 115. In the instant example, only
one active sample is identified in block 133 for each peak. Thus,
in this example, a "no" result should be obtained in block 145, and
the sample adjuster 78 goes to block 124. However, in other
examples in which more than one active sample is identified for
each peak, a "yes" result may be obtained in block 145. In such a
case, the sample adjuster 78 returns to block 136.
[0066] In mass spectrometer 50, for each peak in the mass scan
signal, rather than A/D converter 27 outputting all of the samples
defining the peak to the summer 33 as is done in the conventional
mass spectrometer 10, the sample adjuster 78 outputs only one or
more active samples, and suppresses the remaining samples. For
example, instead of summer 33 receiving all of the samples defining
peak 45 shown in FIG. 3A, as in mass spectrometer 10, in mass
spectrometer 50, summer 33 receives only the single active sample
86 shown in FIG. 8A. As shown in FIG. 8A, all of the samples
defining the peak 45 except for a single active sample, i.e., the
maximum sample 85, are suppressed by the sample adjuster 78. Thus,
only the maximum sample 85 of the identified peak actually changes
any of the accumulated samples stored in the memory 29 and,
therefore, affects the mass spectrum defined by the accumulated
samples stored in memory 29.
[0067] During subsequent mass scan operations, the above-described
process is repeated for the respective mass scan signals that
exhibit peaks 46-48 output by the ADC 27. In particular, in the
next mass scan operation, the A/D converter 27 outputs the mass
scan signal exhibiting the peak 46 shown in FIG. 3B. The sample
adjuster 78, however, suppresses all of the samples defining peak
46 except for the maximum sample 86. Thus, the sample adjuster 78
converts the mass scan signal exhibiting peak 46 shown in FIG. 3B
into the adjusted mass scan signal exhibiting maximum sample 86
shown in FIG. 8B. In the next mass scan, the A/D converter 27
outputs the mass scan signal exhibiting peak 47 shown in FIG. 3C
and suppresses all of the samples defining peak 47 except for the
maximum sample 87. Thus, the sample adjuster 78 converts the mass
scan signal exhibiting peak 47 shown in FIG. 3C into the adjusted
mass scan signal exhibiting maximum value 87 shown in FIG. 8C.
Further, in the next mass scan, the A/D converter 27 outputs the
mass scan signal exhibiting peak 48 shown in FIG. 3D and suppresses
all of the samples defining peak 48 except for the maximum sample
88. Thus, the sample adjuster 78 converts the mass scan signal
exhibiting peak 48 shown in FIG. 3D into the adjusted mass scan
signal including maximum sample 88 shown in FIG. 8D.
[0068] FIG. 9 depicts an exemplary peak 149 that constitutes part
of a mass spectrum obtained by accumulating the adjusted mass scan
signals shown in FIGS. 8A-8D. As a result of the processing
performed by the sample adjuster 78, as described above, the peak
149 has a width (b-a) that is narrower than that of the peak 49 of
the mass spectrum generated by the conventional mass spectrometer
10 and shown in FIG. 4. Accordingly, the processing performed by
the sample adjuster 78 enhances the resolution of the mass spectrum
defined by the accumulated samples stored in the memory 29.
[0069] It is possible for multiple samples defining the same peak
to have the same value. For example, a sample on the leading edge
of a peak may have the same value as a sample on the trailing edge
of the same peak. If two or more samples defining the same peak are
equal and are the highest-value ones of the samples defining the
peak, then the sample adjuster 78 may be configured to select in
block 133 of FIG. 7 any of the equal-value samples as the active
sample for the peak.
[0070] For example, when the two highest-value samples defining a
given peak are equal in value, the sample adjuster 78 may always
select the earlier of the two equal samples or, in another
embodiment, may always select the later of the two equal samples.
In another embodiment, the sample adjuster 78 may select the
earlier and the later of the two equal samples per peak
alternately. For example, for the first peak for which the two
highest-value samples are equal, the sample adjuster 78 selects the
earlier of the two equal samples as the first peak's maximum
sample. For the second peak for which the two highest-value samples
are equal, the sample adjuster 78 selects the later of the two
equal samples as the second peak's maximum sample. For the next
peak for which the two highest-value samples are equal, the sample
adjuster 78 select the earlier of the two equal samples as the
peak's maximum sample, and so on for the remaining peaks.
[0071] In addition, as described above, it is unnecessary for the
sample adjuster 78 to allow only one sample to pass unsuppressed.
For example, the sample adjuster 78 may allow the three
highest-value samples per peak to pass unsuppressed. Other numbers
of samples may be allowed to pass unsuppressed through the sample
adjuster 78 per peak in other examples.
[0072] Generally, below a certain threshold number of samples per
peak identified as active samples, increasing the number of samples
per peak identified as active samples and therefore allowed to pass
unsuppressed decreases the mass resolution of the peaks of the mass
spectrum defined by the accumulated samples stored in memory 29 but
increases the accuracy with which the centers of the peaks are
represented in the mass spectrum. Thus, a trade-off between mass
resolution and center-of-peak accuracy has to be made when
selecting the number of samples per peak that the sample adjuster
78 identifies as active samples. The threshold number of samples is
apparatus-dependent. In an exemplary embodiment of mass
spectrometer 50 described above with reference to FIG. 5, the
threshold number of samples per peak is three samples per peak.
[0073] Specifically, to enhance the mass resolution of the mass
spectrum at the expense of reduced center-of-peak accuracy, sample
adjuster 78 is configured to identify fewer of the samples defining
each peak as active samples. For example, to maximize the mass
resolution, sample adjuster 78 should be configured to identify
only one of the samples defining the peak as an active sample, as
described above. However, to enhance center-of-peak accuracy in the
mass spectrum at the expense of reduced mass resolution, sample
adjuster 78 should be configured to identify more than one of the
samples defining the peak as active samples. For example, to
maximize center-of-peak accuracy at the expense of reduced mass
resolution, sample adjuster 78 should be configured to identify as
active samples a number of the samples defining the peak equal to
the above-described threshold number. Moreover, sample adjuster 78
may be configured to identify as active samples a number of samples
defining each peak selected to provide a compromise between mass
resolution and center-of-peak accuracy.
[0074] The number of samples per peak identified as active samples
and, therefore, allowed by the sample adjuster 78 to pass
unsuppressed is predefined in at least some embodiments. For
example, a user may specify such number prior to the mass spectrum
measurement process being performed. Alternatively, the sample
adjuster 78 may have a default number of samples that it selects as
active samples unless the user specifies a different number. In
another embodiment, the sample adjuster 78 is hard coded to allow a
certain number of samples to pass unsuppressed for each peak. Other
techniques for controlling which samples are suppressed and
unsuppressed are possible in other embodiments.
[0075] Regardless of the number of samples that sample adjuster 78
is configured to identify as active samples for a given peak, it is
generally desirable for the samples having the highest values to be
so identified. For example, if only one sample is to be identified
as the active sample for a peak and, therefore, to remain
unsuppressed, then it is desirable for the identified sample for
the peak to be the sample with the highest value (i.e., the maximum
sample for the peak). If three samples are to be identified as
active samples for a peak, then it is again desirable for the
identified samples for the peak to be the samples with the highest
values. Ensuring that the highest-value samples are identified as
the active samples generally increases the accuracy of the mass
spectrum defined by the accumulated samples stored in memory
29.
[0076] In a practical example of the choice of the number of
samples identified as active samples, a good compromise between
mass resolution and center-of-peak accuracy was obtained by
identifying the three highest-value ones of the samples
representing the peak as the active samples. However, if the two
highest-value samples were equal in value, then four samples were
identified, with the two highest-value samples constituting the
middle two samples.
[0077] Until recently, the performance of the ion accelerator
composed of ion source 11, electrode 12 and pulse source 17 has
limited the mass resolution of practical examples of embodiments of
mass spectrometer 50 in which the active samples were identified as
just described. However, recent improvements in the precision of
the ion accelerator require commensurate improvements in the mass
resolution of the ion detection system without the reduction in the
center-of-peak accuracy that reducing the number of active samples
would entail. The samples identified by sample adjuster 78 as
active samples constitute a subset of the samples defining a peak
in the mass scan signal.
[0078] FIG. 10 is a block diagram showing an example of a
time-of-flight mass spectrometer 100 in accordance with an
embodiment of the invention. To simplify the description of FIG. 10
and subsequent drawings, elements functionally analogous to
elements described above with reference to FIGS. 1 and 5 have the
same reference numerals and will not be described again in
detail.
[0079] In the example shown in FIG. 10, mass spectrometer 100 is
composed of ion source 11, controller 15, pulse source 17, clock
24, ion detector 25, A/D converter 27, and a sample processor 110.
Sample processor 110 is composed of a sample selector 120, a sample
synthesizer 130 and a sample combiner 140. In mass spectrometer
100, A/D converter 27 is connected to receive the ion detection
signal from ion detector 25 and is operable in a manner similar to
that described above to convert the analog ion detection signal
received from ion detector 25 during a mass scan operation to a
temporal sequence of original samples that will be referred to
herein as a mass scan signal. An original sample is a digital
sample. A/D converter 27 has a sampling rate that defines the
temporal resolution of the original samples. The mass scan signal
comprises sets of original samples defining respective peaks. In a
minimalist example, the mass scan signal comprises a set of
original samples defining a peak. Sample selector 120 is operable
to select, from the mass scan signal comprising the original
samples defining the peak, a subset of the original samples
defining the peak. Sample selector 120 outputs the selected samples
to sample synthesizer 130. Sample synthesizer 130 is operable to
generate one or more synthesized samples from the subset of the
original samples selected by sample selector 120. A synthesized
sample is a digital sample. The one or more synthesized samples
provide a temporal resolution greater than the temporal resolution
of the original samples. Sample combiner 140 is operable to sum the
one or more synthesized samples with respective temporally-aligned
accumulated samples to produce a mass spectrum. An accumulated
sample is a digital sample. The accumulated samples are generated
by sample selector 120, sample synthesizer 130 and sample combiner
140 from mass scan signals generated during respective
previously-performed mass scan operations.
[0080] FIG. 11 is a block diagram showing an example of one
embodiment 210 of sample processor 110 in which the sample
synthesizer subjects the original samples in the subset to
interpolation to generate the synthesized samples that provide a
greater temporal resolution than the original samples. In the
example shown, sample processor 210 is composed of a sample
selector 220, a sample synthesizer 230 and a sample combiner
240.
[0081] Sample selector 220 identifies each peak defined by the
original samples constituting the mass scan signal output by A/D
converter 27, and selects from the mass scan signal for output to
sample synthesizer 230 a respective subset of the original samples
defining the peak. The subset of original samples is composed of a
predetermined number of the original samples and will be referred
to herein as an original subset. In an example, for each peak
identified in the mass scan signal, sample selector 220 selects an
original subset composed of three original samples unless two of
the original samples in the subset are equal in value. When two of
the original samples are equal in value, sample selector 220
selects an original subset composed of four active samples, as
described above. Alternatively, the number of samples in the
original subset is determined adaptively in response to the
amplitude of the maximum-amplitude sample in the original
subset.
[0082] In the example shown, sample selector 220 is composed of
buffer 77 and sample adjuster 78 described above with reference to
FIGS. 5 and 6. The active samples output by sample adjuster 78 for
each peak identified in the mass scan signal constitute the
original subset of the original samples defining the peak selected
by sample selector 220. In other examples, sample selector 220 is
composed of elements different from buffer 77 and sample adjuster
78, and is operable to identify each peak defined by the original
samples constituting the mass scan signal output by A/D converter
27, and to select for output to sample synthesizer 230 a respective
original subset of the original samples defining the peak.
[0083] The example of sample synthesizer 230 shown in FIG. 11 is
composed of an interpolator 232 and a sample suppressor 234. For
each peak identified by sample selector 220 in the mass scan
signal, interpolator 232 receives from sample selector 220 the
original subset of the original samples defining the peak. Sample
synthesizer 220 subjects the original samples within the original
subset to interpolation to generate the synthesized samples and
adds the synthesized samples to the original subset to generate an
augmented subset. In the augmented subset, at least one of the
synthesized samples is interposed between two adjacent ones of the
original samples. In one example, a single synthesized sample is
interposed between two adjacent ones of the original samples. In
another example, two or more synthesized samples are interposed
between two adjacent ones of the original samples.
[0084] Sample synthesizer 230 passes the augmented subset of
samples composed of the original samples received from sample
selector 220 and the synthesized samples generated by sample
synthesizer 230 to sample suppressor 234. Sample suppressor 234
suppresses at least one of the original samples in the augmented
subset to generate a truncated subset having a smaller temporal
span than the original subset of original samples output by sample
selector 210. The temporal span of a subset is the time difference
between the earliest sample and the latest sample in the subset.
The temporal order of the original samples is the order in which
the original samples were generated by A/D converter 27. The at
least one original sample that is suppressed is a
temporarily-extreme one of the original samples in the augmented
subset. In other words, the at least one original sample that is
suppressed is either or both of the earliest original sample and
the latest original sample in the augmented subset. In some
embodiments, at least one of the synthesized samples in the
augmented subset is additionally suppressed. Sample suppressor 234
outputs the truncated subset to sample combiner 240.
[0085] The example of sample combiner 240 shown is composed of a
memory address generator 221, a memory 229 and a summer 233. A
first input of summer 233 is connected to receive the samples in
each truncated subset from the output of sample synthesizer 230.
Memory address generator 221, memory 229 and summer 233 are
interconnected in an arrangement similar to that of memory address
generator 21, memory 29 and summer 33 described above with
reference to FIGS. 1 and 5, i.e., the data output DO of memory 229
is connected to a second input of summer 233, and the output of
summer 233 is connected to the data input DI of memory 229. Memory
229 additionally has an address input ADR connected to receive the
memory address generated by memory address generator 221. In one
embodiment, sample suppressor 234 outputs the samples constituting
the truncated subset serially, memory 229 and summer 233 have a
data width equal to that of the original samples, and memory
address generator 221, memory 229 and summer 233 operate a rate of
n times that of the conversion clock signal generated by clock 24,
where n is the denominator of the temporal offset between an
original sample and an adjacent synthesized sample in the augmented
subset expressed as a fraction of the period of the conversion
clock. In this case, the range of memory addresses generated by
memory address generator 221 for a given mass range is n times that
generated by memory address generator 21. In another embodiment,
sample suppressor 234 outputs the samples constituting the
truncated subset in parallel, memory 229 and summer 233 have a data
width equal to n times that of the original samples, memory address
generator 221, memory 229 and summer 233 operate a rate equal to
that of the conversion clock signal generated by clock 24. In this
case, the range of memory addresses generated by memory address
generator 221 for a given mass range is equal to that generated by
memory address generator 21, but each memory location is n times as
wide. In another embodiment, a combination of a wider data width
(e.g., n times that of the original samples) and a higher
operational rate (e.g., n times that of the conversion clock) is
used. In all such embodiments, for a given mass range, memory 229
is n times larger than memory 29.
[0086] In some embodiments, the size of memory 229 is limited by
the amount of memory available in an application-specific
integrated circuit (ASIC) used to implement the circuitry
downstream of A/D converter 27 so that the size of memory 229 may
be no larger than that of memory 29 of mass spectrometer 50
described above with reference to FIG. 5. Memory 29 has only one,
single-width memory location per conversion clock period. In
embodiments of mass spectrometer 100 in which the size of memory
229 is the same as that of memory 29, the mass range that can be
detected is 1/ n of that of mass spectrometer 50 because n memory
locations are needed per conversion clock period or each memory
location is n times as wide.
[0087] Prior to the beginning of each mass scan operation performed
by mass spectrometer 100, controller 15 provides a reset signal to
a reset input R of address generator 221. The reset signal sets the
memory address output by address generator 221 to zero or another
predetermined value. Then, during the following mass scan, address
generator 221 counts the conversion clock signal generated by clock
24 to generate a respective memory address. In an embodiment in
which sample suppressor 234 outputs the samples in the truncated
subset serially, address generator 221 generates n memory addresses
for each of the samples output by A/D converter 27 during the mass
scan operation. The n memory addresses are typically consecutive.
In an embodiment in which sample suppressor 234 outputs the samples
in the truncated subset in parallel, address generator 221
generates a respective single memory address for each of the
samples output by A/D converter 27 during the mass scan
operation.
[0088] Summer 233 sums each of the samples in the truncated subset
received from sample suppressor 234 with a respective
temporally-aligned accumulated sample read from memory 229 to
generate a new accumulated sample that is stored in memory 229.
Specifically, in an embodiment in which sample suppressor 234
outputs the samples in each truncated subset serially, the
accumulated sample is read from a memory location in memory 229
specified by the current memory address generated by memory address
generator 221. Summer 233 sums the current sample received from
sample suppressor 234 with the accumulated sample read from memory
220 to generate a new accumulated sample. The current sample is a
synthesized sample or an original sample. The new accumulated
sample is then written back in memory 229 at the memory location
specified by the current memory address received from memory
address generator 221. The process just described is repeated for
each of the samples (i.e., each of the original samples and each of
the synthesized samples) in the truncated subset output by sample
suppressor 234. Reading the accumulated sample from a memory
location in memory 229 specified by the value of the memory address
generated by memory address generator 221 when summer 223 receives
the current sample from sample suppressor 234 and writing the new
accumulated sample at the same memory location in memory 229
provides the temporal alignment between the current sample and the
accumulated sample with which the current sample is summed. The
memory address generated by memory address generator 221 increments
after each new accumulated sample has been written back in memory
229.
[0089] In an embodiment in which sample suppressor 234 outputs n of
the samples constituting each truncated subset in parallel, a block
of n accumulated samples is read from a memory location in memory
229 specified by the current memory address generated by memory
address generator 221. In implementations in which the number of
samples in the truncated subset is less than or equal to n, sample
suppressor 234 outputs all of the samples in the truncated subset
in a single period of the conversion clock. In implementations in
which the number of samples in the truncated subset is more than n,
sample suppressor 234 requires two or more periods of the
conversion clock to output all of the samples in the truncated
subset. Some of the n samples output in parallel by sample
suppressor may be suppressed samples having a value of zero. Summer
233 sums the n samples received from sample suppressor 234 with the
block of n accumulated samples read from memory 229 to generate a
block of n new accumulated samples. The block of new accumulated
samples is then written back in memory 229 at the memory location
specified by the current memory address received from memory
address generator 221. In embodiments in which the number of
samples in the truncated subset is greater than n, the process just
described is repeated in the next period of the conversion clock to
subject the remaining samples in the truncated subset to
accumulation. The process just described generates a respective new
accumulated sample from each of the samples (i.e., each of the
original samples and each of the synthesized samples) in the
truncated subset of samples output by sample suppressor 234.
Reading the block of accumulated samples from a memory location in
memory 229 specified by the value of the memory address generated
by memory address generator 221 when summer 223 receives the n
samples from sample suppressor 234 and writing the block of new
accumulated samples at the same memory location in memory 229
provides the temporal alignment between each sample received from
sample suppressor 234 and the respective accumulated sample with
which the sample is summed. The memory address generated by memory
address generator 221 increments after each block of new
accumulated samples has been written back in memory 229.
[0090] In both of the serial and parallel embodiments described
above, the read function of memory 229 is inhibited during the
first mass scan operation in each mass spectrum measurement
process. Inhibiting the read function causes memory 229 to output a
value of zero at its data output DO. Consequently, each sample
received from sample suppressor 234 effectively overwrites any
residual accumulated sample stored in memory 229 during the first
mass scan operation. Alternatively, instead of inhibiting the read
function of memory 229 during the first mass scan operation, a gate
is interposed between the data output DO of memory 229 and the
second input of summer 233 to supply a value of zero to the second
input of summer 233 only during the first mass scan operation. In a
further alternative, a value of zero is stored in each memory
location in memory 229 at the start of each mass spectrum
measurement process, which makes it unnecessary to inhibit the read
function of memory 229 during the first mass scan operation.
[0091] Successive mass scan operations accumulate in memory 229 a
raw mass spectrum of progressively increasing accuracy. When the
raw mass spectrum accumulated in memory 229 achieves a specified
accuracy, a processor (not shown) reads the raw mass spectrum from
memory 229 and subjects each peak exhibited by the raw mass
spectrum to a centroid calculation to determine the time value
represented by the peak. The processor then converts the time value
represented by each peak to a corresponding mass using the
time-to-mass conversion equation described below.
[0092] Operation of an example of sample processor 210 will now be
described with reference to FIGS. 12A-12F. FIG. 12A shows part of
the mass scan signal output by A/D converter 27 during a mass scan
operation. The mass scan signal is composed of a temporal sequence
of original samples. In the part of the mass scan signal shown, the
original samples define a peak 250. An exemplary original sample is
shown at 261. FIG. 12A is similar to FIG. 3A except that the
original samples constituting the mass scan signal are represented
by vertical bars rather than by points.
[0093] FIG. 12B shows an original subset 252 of the original
samples that define peak 250 output to sample synthesizer 232 by
sample selector 220. The time axes of FIGS. 12B-12F are expanded
relative to the time axis of FIG. 12A to enable the synthesized
samples and the original samples from which they are derived to be
shown more clearly. In the example shown, original subset 252 is
composed of three original samples 261, 262, 263. In other
examples, original subset 252 is composed of more or fewer original
samples. In the example shown in FIG. 12B, on the time axis, each
original sample 262, 263 in original subset 252 is separated from
the previous original sample 261, 262, respectively, by one period
t of the conversion clock.
[0094] Interpolator 232 receives original samples 261-263
constituting original subset 252 and performs an interpolation
operation that generates synthesized samples 264 and 265.
Interpolator 232 adds synthesized samples 264 and 265 to original
subset 252 to form an augmented subset 254 composed of five
samples. In the example shown, interpolator 232 has subject
original samples 261-263 to linear interpolation to generate
synthesized samples 264 and 265 shown in FIG. 12C. In other
examples, interpolator 232 subjects samples 261-263 to polynomial
interpolation, to spline interpolation or to a curve fitting
operation to generate the synthesized samples.
[0095] In the example shown in FIG. 12C, on the time axis, each
original sample 262, 263 in augmented subset 254 remains separated
from the previous original sample 261, 262, respectively, by one
period t of the conversion clock signal. Additionally, in augmented
subset 254, synthesized sample 264 is separated from original
sample 261 by one half t/2 of one period of the conversion clock
signal, and synthesized sample 265 is separated from original
sample 262 by one half t/2 of one period of the conversion clock
signal. With the addition of synthesized samples 264, 265 shown in
FIG. 12C, the samples constituting augmented subset 254 have a
temporal resolution twice that the original samples constituting
subset 252 shown in FIG. 12B, and the denominator n of the temporal
offset t/2 between the original samples and respective adjacent
synthesized samples in augmented subset 254 is equal to 2.
[0096] Interpolator 232 outputs augmented subset 254 composed of
original samples 261-263 and synthesized samples 264, 265 to sample
suppressor 234. Sample suppressor 234 suppresses at least one of
the temporally-extreme ones of the samples 261-265 constituting
augmented subset 254 to generate a truncated subset 256 having a
smaller temporal span than original subset 252. In the example
shown, sample suppressor 234 suppresses the earliest original
sample 261 and the latest original sample 263 in augmented subset
254. Original samples 261 and 263 are earliest and latest in the
order in which original samples 261-263 were generated by A/D
converter 27. Suppressing original samples 261 and 263 generates
truncated subset 256 of samples composed of, in temporal order,
synthesized sample 264, original sample 262 and synthesized sample
265, as shown in FIG. 12D. It can be seen by comparing FIG. 12D
with 12B that, whereas in original subset 252 shown in FIG. 12B,
the latest sample 263 was two conversion clock signal periods later
than the earliest sample 261, in truncated subset 254, the latest
sample 265 is only one conversion clock signal period later than
the earliest sample 264. Consequently, samples 264, 262 and 265
constituting truncated subset 254, when combined by sample combiner
240 with the respective temporally-aligned accumulated samples
stored in memory 229 add to the raw mass spectrum stored in memory
229 information regarding the shape of peak 250 having a temporal
resolution twice that of original samples 261-263 that defined peak
250.
[0097] Another example of the operation of sample processor 210 is
shown in FIGS. 12E and 12F. In the example shown in FIG. 12E,
interpolator 232 has interposed three synthesized samples 271, 272,
273 between original samples 261, 262, and has interposed three
synthesized samples 274, 275, 276 between original samples 262,
263. Interpolator 232 has added synthesized samples 271-276 to
original subset 252 to form an augmented subset 255. Within
augmented subset 255, synthesized samples 271, 272, 273 are
respectively separated from original sample 261 by one quarter
(t/4), one half, and three quarters of one period of the conversion
clock signal generated by clock 24, and synthesized samples 274,
275, 276 are respectively separated from original sample 262 by one
quarter, one half and three quarters of one period of the
conversion clock signal. Thus, with the addition of synthesized
samples 271-276, the samples constituting augmented subset 255 have
a temporal resolution four times that of the original samples
261-263 constituting original subset 252, and the denominator n of
the temporal offset t/4 between the original samples and respective
adjacent synthesized samples in augmented subset 255 is equal to
4.
[0098] In the example shown in FIG. 12F, sample suppressor 234
suppresses the earliest original sample 261, the earliest
synthesized samples 271, 273, the latest synthesized samples 275,
276, and the latest original sample 263 in augmented subset 255 to
form a truncated subset 257. Original samples 261 and 263 are the
earliest and the latest original samples in the order in which
original samples 261-263 were generated by A/D converter 27.
Synthesized samples 271, 272 and 275, 276 are the earliest two and
the latest two synthesized samples in the order in which the
synthesized samples 264-267 were generated by interpolator 232.
Suppressing original samples 261 and 263 and synthesized samples
271, 272, 275 and 276 generates a truncated subset 257 of samples
composed of, in temporal order, synthesized sample 273, original
sample 262 and synthesized sample 274, as shown in FIG. 12F. It can
be seen by comparing FIG. 12F with FIG. 12B that, whereas in
original subset 252 shown in FIG. 12B, the latest sample 263 was
two conversion clock signal periods later than the earliest sample
261, in truncated subset 257, the latest sample 274 is only one
half of one conversion clock signal period later than the earliest
sample 273. Consequently, samples 273, 262 and 274 constituting
truncated subset 257, when combined by sample combiner 240 with the
respective temporally-aligned accumulated samples stored in memory
229, add to the raw mass spectrum stored in memory 229 information
regarding the shape of peak 250 having a higher temporal resolution
than that of original samples 261-263 that defined peak 250.
[0099] In an alternative embodiment, in FIG. 12E, interpolator 232
generates only the synthesized samples 273 and 274 that constitute
part of truncated subset 257 output by sample suppressor 234. In
such embodiment, the operation of sample suppressor 234 is
simplified because sample the suppressor suppresses no synthesized
samples, and suppresses only temporally-extreme original samples
261 and 263. In an example similar to that shown in FIG. 12E,
interpolator 232 generates only synthesized samples 273 and 274
with respective timing offsets of plus and minus t/4 relative to
the timing of original sample 262. Consequently, the augmented
subset output by interpolator 232 is composed of original samples
261, 262 and 263, and synthesized samples 273 and 274 in an
irregular temporal order 261, 273, 262, 274, 263. Sample suppressor
234 then suppresses only original samples 261 and 263 to generate
truncated subset 257 composed of original sample 262 and
synthesized samples 273 and 274 in a regular temporal order 273,
262, 274.
[0100] In the example described above with reference to FIGS. 12C
and 12D, the samples in each truncated subset have a temporal
resolution twice that and, hence, a mass resolution 2 times that,
of the original samples output by A/D converter 27. In the example
described above with reference to FIGS. 12E and 12F, the samples in
each truncated subset have a temporal resolution four times that
and, hence, a mass resolution twice that, of the original samples.
In each mass scan operation except the first, the samples in the
truncated subsets output by sample synthesizer 230 are added to
respective temporally-aligned accumulated samples stored in memory
229 to generate a raw mass spectrum whose accuracy progressively
increases as the number of mass scan operations performed
increases. The temporal resolution, and, hence, mass resolution, of
the raw mass spectrum generated by mass spectrometer 100 is greater
than the temporal resolution and mass resolution of the raw mass
spectrum generated from the same number of mass scan operations by
an embodiment of mass spectrometer 50 having the same conversion
clock frequency by respective resolution ratios somewhat less than
the above-described resolution ratios between the samples in the
truncated subset and the original samples. Once sufficient mass
scan operations have been performed to obtain a raw mass spectrum
of a specified accuracy, the accumulated samples defining each peak
in the raw mass spectrum are subject to a centroid calculation as
described above to determine the time value represented by the
peak, and the time value is converted to a mass value.
[0101] FIG. 13 is a block diagram showing a first example of
another embodiment 310 of sample processor 110 shown in FIG. 10 in
which the sample synthesizer subjects the original samples in the
subset to a centroid calculation to generate the time component of
a single synthesized sample having a time component and an
amplitude component. The time component of the single synthesized
sample provides a greater temporal resolution than the original
samples. In the example shown, sample processor 310 is composed of
a sample selector 320, a sample synthesizer 330 and a first example
342 of a sample combiner 340. Sample combiner 340 includes a memory
329. In a manner similar to that of sample selector 220 described
above with reference to FIG. 11, sample selector 320 identifies
each peak defined by the original samples constituting the mass
scan signal output by A/D converter 27 and selects from the mass
scan signal a subset of the original samples defining the peak for
output to sample synthesizer 330. The subset is composed of a
predetermined number of the original samples and will be referred
to herein as an original subset. The number of samples constituting
the original subset output by sample selector 320 is typically
larger than the number of original samples constituting the
original subset output by sample selector 220 described above with
reference to FIG. 11. Alternatively, the number of samples in the
original subset is determined adaptively in response to the
amplitude of the maximum-amplitude sample in the subset. In the
example shown, sample selector 320 is composed of buffer 77 and
sample adjuster 78 described above with reference to FIGS. 5, 6 and
11. The active samples output by sample adjuster 78 for each peak
identified in the mass scan signal constitute the original subset
of the original samples defining the peak. In other examples, the
sample selector is composed of circuit elements different from
buffer 77 and sample adjuster 78, and that collectively perform
functions that are the same as or equivalent to those described
above.
[0102] The example of sample synthesizer 330 shown is composed of a
centroid calculator 332, an amplitude component generator 334 and a
time value generator 336. Each of centroid calculator 332 and
amplitude component generator 334 receives each original subset of
original samples output by sample selector 320 and additionally
receives corresponding time values from time value generator 336.
Amplitude component generator 334 receives each original subset of
original samples output by sample selector 320 either directly, or
indirectly via centroid calculator 332, as shown. Amplitude
component generator 334 outputs the amplitude component of each
synthesized sample to sample combiner 340.
[0103] Referring additionally to FIG. 10, prior to the beginning of
each mass scan performed by mass spectrometer 100, controller 15
provides a reset signal to a reset input R of time value generator
336. The reset signal sets the time value output by time value
generator 336 to zero or another predetermined value. Then, during
the following mass scan operation, time value generator 336 counts
the conversion clock signal generated by clock 24 to generate a
time value for each of the original samples output by A/D converter
27 during the mass scan. Time value generator 336 outputs each time
value it generates to centroid calculator 332.
[0104] In another embodiment, a mass value converter (not shown) is
interposed between time value generator 336 and centroid calculator
332. The mass value converter converts each time value t generated
by time value generator 336 to a respective mass value m by
subjecting the time value to processing in accordance with the
following mass conversion equation:
m = ( t - c k ) 2 , ##EQU00001##
where c and k are constants. Alternatively, the mass value
converter can perform the mass conversion using a look-up table.
The mass value converter outputs the respective mass value to
centroid calculator 332 instead of the time value output by time
value generator 336. In embodiments in which time value generator
336 is followed by a mass converter, the term mass value should be
substituted for the term time value in the description set forth
below.
[0105] In an embodiment in which no mass value converter is
interposed between time value generator 336 and centroid calculator
332, centroid calculator 332 associates each original sample in the
original subset received from sample selector 320 with its
respective time value to produce a respective two-dimensional
sample. The two-dimensional sample has an amplitude component
contributed by the amplitude represented by the original sample and
a time component contributed by the time value received from time
value generator 336. The two dimensional samples generated from the
original samples in the original subset constitute an augmented
subset. Centroid calculator 332 discards time values corresponding
to the original samples suppressed by sample selector 320.
[0106] Centroid calculator 332 subjects the two-dimensional samples
in the augmented subset to a centroid calculation to determine the
time coordinate of the centroid of the peak represented by the
original samples in the original subset. In this disclosure,
references to the centroid of a peak are to be regarded as
referring to only to the time coordinate (or mass coordinate) of
the centroid. Thus, the centroid calculation generates only a
single result indicating a time (or a mass if each two-dimensional
sample has a mass component instead of a time component).
Algorithms for performing a centroid calculation are known in the
mass spectrometry art and will therefore not be described here in
depth. In one example, the two-dimensional samples in an augmented
subset composed of N two-dimensional samples are regarded as
defining a polygon having N+2 vertices. The area A of such polygon
is given by:
A = 1 2 i = 0 N + 1 ( t i a i + 1 - t i + 1 a i ) ,
##EQU00002##
and the coordinate C.sub.t of the centroid of the polygon on the
time (or mass) axis is given by:
C t = 1 6 A i = 0 N + 1 ( t i + t i + 1 ) ( t i a i + 1 + t i + 1 a
i ) , ##EQU00003##
where t.sub.i and a.sub.i are the coordinates on the time axis and
the amplitude axis, respectively, of the i-th vertex of the
polygon. The amplitudes represented by the amplitude components of
the two-dimensional samples in the subset provide the coordinates
of N of the vertices on the amplitude axis, and the time components
of the two-dimensional samples provide the coordinates of the N
vertices on the time axis. The coordinates of the remaining two
vertices on the amplitude axis are zero and the coordinates of the
remaining two vertices on the time axis are respectively equal to
the time components of the earliest and latest of the
two-dimensional samples in the augmented subset. Centroid
calculator 332 outputs the coordinate C.sub.t of the centroid on
the time axis to sample combiner 342 as the time component of the
single synthesized sample generated by sample synthesizer 330. The
synthesized sample represents the peak originally represented by
the original samples in the original subset. Centroid calculator
332 is configured to calculate time axis coordinate C.sub.t with a
temporal resolution greater than the temporal resolution of the
original samples generated by A/D converter 27. For example,
centroid calculator 332 is configured to calculate time axis
coordinate C.sub.t as a binary number having at least one bit more
than the binary numbers used to represent the time components of
the two-dimensional samples. In an example, centroid calculator
calculates the time axis coordinate with a temporal resolution of
eight (three bits more than the time values) or sixteen times (four
bits more than the time values) the time resolution of the original
samples. Other temporal resolutions are possible.
[0107] Additionally, in sample synthesizer 330, amplitude component
generator 334 receives the original samples in each original subset
output by sample selector 320, and from at least one of the
original samples generates the amplitude component of the
respective synthesized sample. In the example shown, amplitude
component generator 334 receives each augmented subset of
two-dimensional samples from centroid calculator 332. The original
samples in the original subset constitute the amplitude components
of the two-dimensional samples in the augmented subset.
Consequently, amplitude component generator 334 can be regarded as
generating the amplitude component of the synthesized sample from
at least one of the original samples in the original subset even
when the amplitude component generator receives the original
samples as the amplitude components of the two-dimensional samples
in an augmented subset. In other examples, amplitude component
generator 334 receives each original subset of original samples
directly from sample selector 320.
[0108] Processes that amplitude component generator 334 may perform
to generate the amplitude component of the synthesized sample
representing each original subset include processes based on
selection and processes based on interpolation. In processes based
on selection, one of the original samples in the original subset is
selected as the amplitude component of the synthesized sample. In
processes based on interpolation, two or more of the original
samples in the original subset and their respective time values are
subject to interpolation to generate the amplitude component of the
synthesized sample.
[0109] In one example of a process based on selection, amplitude
component generator 334 directly or indirectly receives the
original subset of original samples, and selects a predetermined
one of the original samples in the original subset for output to
sample combiner 342 as the amplitude component of the synthesized
sample. For example, the amplitude component generator selects the
original sample at the temporal mid-point of the original subset
for output to the sample combiner as the amplitude component of the
synthesized sample.
[0110] In another example of a process based on selection,
amplitude component generator 334 directly or indirectly receives
the original subset of original samples, and selects the one of the
original samples in the original subset having the greatest
amplitude as a maximum-amplitude sample for output to sample
combiner 342 as the amplitude component of the synthesized sample.
Alternatively, the processing performed by sample adjuster 78
identifies the maximum-amplitude sample in each original subset. In
this case, amplitude component generator 334 selects the original
sample identified by sample adjuster 78 as the maximum-amplitude
sample for output to sample combiner 342 as the amplitude component
of the synthesized sample.
[0111] In another example of a process based on selection,
amplitude component generator 334 receives the augmented subset of
two-dimensional samples generated by centroid calculator 332, and
additionally receives the time component calculated by centroid
calculator 332. Amplitude component generator 334 selects the
amplitude component of the two-dimensional sample whose time
component is closest in value to the time component of the
synthesized sample for output to sample combiner 342 as the
amplitude component of the synthesized sample. In an example, the
time component of the synthesized sample is 51/4, two of the
two-dimensional samples have respective time components of 5 and 6,
and amplitude component generator 334 selects the amplitude
component of the two-dimensional sample having the time component
of 5 for output to the sample combiner as the amplitude component
of the synthesized sample. The time component of the selected
two-dimensional sample is closest in value to the time component of
the synthesized sample. Amplitude component generator 334 is
additionally configured to determine which two-dimensional sample
to select in the event that the time component of the synthesized
sample equally close to the time components of two of the
two-dimensional samples. Circuitry and algorithms for selecting and
outputting one of a subset of original samples in accordance with a
selection criterion are known in the art and will therefore not be
described in detail here.
[0112] In an example of a process based on interpolation, amplitude
component generator 334 subjects the two-dimensional samples in the
augmented subset to interpolation to generate the amplitude
component of the synthesized sample. Specifically, amplitude
component generator receives the augmented subset of
two-dimensional samples generated by centroid calculator 332 and
additionally receives the time component calculated by centroid
calculator 332. Amplitude component generator 334 subjects two or
more of the two-dimensional samples in the augmented subset to
interpolation to generate a new two-dimensional sample whose
amplitude component is calculated by the interpolation process and
whose time component is equal to the time component calculated by
centroid calculator 332. Amplitude component generator 334 outputs
the amplitude component of the new two-dimensional sample to sample
combiner 342 as the amplitude component of the synthesized sample.
Alternatively, amplitude component generator 334 outputs the entire
new two-dimensional sample to sample combiner 342 as the
synthesized sample. In this case, the time component of the
synthesized sample is output to sample combiner 342 by amplitude
component generator 334 instead of by centroid calculator 332 as
shown in FIG. 13.
[0113] Amplitude component generator 334 may use such interpolation
processes as linear interpolation, spline interpolation, polynomial
interpolation and curve fitting. Circuitry and algorithms for
subjecting two or more two-dimensional samples in an augmented
subset to interpolation to generate a new two-dimensional sample
whose amplitude component is calculated by the interpolation
process are known in the art and will therefore not be described in
detail here.
[0114] The synthesized sample generated by sample synthesizer 330
has an amplitude component and a time (or mass) component as just
described, and represents the peak in the mass scan signal defined
by the subset of original samples selected by sample selector 320.
Sample combiner 342 receives the synthesized sample from sample
synthesizer 330 in lieu of the original samples selected by sample
selector 320. Sample combiner 342 receives none of the original
samples generated by A/D converter 27.
[0115] The example 342 of sample combiner 340 shown in FIG. 13 is
composed of a memory 329 and a summer 333 connected to one another
in an arrangement similar to that of memory 29 and summer 33
described above with reference to FIGS. 1 and 5, i.e., the data
output DO of memory 329 is connected to the second input of summer
333, and the output of summer 333 is connected to the data input DI
of memory 329. Memory 329 additionally has an address input ADR
connected to receive the time component of the synthesized sample
generated by sample synthesizer 330. Specifically, the address
input ADR of memory 329 is connected to the output of centroid
calculator 332 in sample synthesizer 330. The first input of summer
333 is connected to receive the amplitude component of the
synthesized sample generated by sample synthesizer 330.
Specifically, the first input of summer 333 is connected to the
output of amplitude component generator 334 in sample synthesizer
330. At the beginning of each mass spectrum measurement operation,
a value of zero is stored in each memory location in memory 329 as
an initial accumulated sample. Alternatively, the read function of
memory 329 is inhibited the first time during the mass spectrum
measurement process that a read attempt is made at a given memory
location.
[0116] Sample combiner 342 combines the synthesized samples
received from sample synthesizer 330 with respective
temporally-aligned accumulated samples to produce respective new
accumulated samples that collectively constitute a raw mass
spectrum. The accumulated samples are generated by sample selector
320, sample synthesizer 330 and sample combiner 340 from mass scan
signals generated during respective previously-performed mass scan
operations. Specifically, for each synthesized sample received from
sample synthesizer 330, the time component of the synthesized
sample specifies an address in memory 329 where an accumulated
sample is stored. Memory 329 performs a read operation in which the
accumulated sample stored at the address specified by the time
component of the synthesized sample is output to summer 333. Summer
333 sums the accumulated sample read from memory 329 with the
amplitude value of the synthesized sample received from sample
synthesizer 330 to generate a new accumulated sample that is output
to memory 329. Memory 329 then performs a write operation in which
the new accumulated sample received from summer 333 is stored at
the address specified by the time component of the synthesized
sample. Reading the accumulated sample from a location in memory
329 specified by the time component of the current synthesized
sample generated by sample synthesizer 330 and writing the new
accumulated sample at the same location in memory 329 provides the
temporal alignment between the synthesized sample and the
accumulated sample with which the synthesized sample is summed.
[0117] The synthesized samples generated in successive mass scan
operations accumulate in memory 329 to produce a raw mass spectrum
having a progressively increasing accuracy. When the raw mass
spectrum accumulated in memory 329 achieves a specified accuracy, a
processor (not shown) reads the raw mass spectrum from memory 329
and subjects each peak exhibited by the raw mass spectrum to a
centroid calculation to determine the time value represented by the
peak. The processor then converts the time value represented by
each peak to a corresponding mass using the time-to-mass conversion
equation described above. This last calculation is unnecessary in
embodiments in which a mass value converter is interposed between
time value generator 336 and centroid calculator 332, as described
above.
[0118] Operation of an example of sample processor 310 will now be
described with reference to FIGS. 14A-14D. FIG. 14A shows part of
the mass scan signal output by A/D converter 27 during a mass scan
operation. The mass scan signal is composed of a temporal sequence
of original samples. In the part of the mass scan signal shown, the
original samples define a peak 350. An exemplary original sample is
shown at 361.
[0119] FIG. 14B shows an original subset 352 of the original
samples defining peak 350 output to sample synthesizer 330 by
sample selector 320. In the example shown, original subset 352 is
composed of eleven original samples. In other examples, original
subset 352 is composed of more or fewer original samples. In the
example shown in FIG. 14B, on the time axis, each original sample
in original subset 352 is separated from the previous original
sample by one period t of the conversion clock signal.
[0120] In sample synthesizer 330, centroid calculator 332 receives
the original samples constituting original subset 352 from sample
selector 320 and associates each original sample in the original
subset with its respective time value to generate a respective
two-dimensional sample having an amplitude component contributed by
the amplitude represented by the original sample and a time
component contributed by the respective time value received from
time value generator 336. For example, original sample 361 is
associated with its respective time value t.sub.5 to generate a
two-dimensional sample 371 having an amplitude component a.sub.5
equal to the amplitude represented by original sample 361 and a
time component t.sub.5 equal to the time value received from time
value generator 336 for original sample 361, as shown in FIG. 14C.
The two-dimensional samples having amplitude components contributed
by a respective one of the original samples constituting original
subset 352 collectively constitute an augmented subset 354.
[0121] Centroid calculator 332 additionally subjects the
two-dimensional samples constituting augmented subset 354 to a
centroid calculation to determine the time axis coordinate C.sub.t
of the centroid of the peak represented by the two-dimensional
samples in the augmented subset. In the example shown, amplitude
components and the time components of the two-dimensional samples
in augmented subset 354 define the coordinates on the amplitude
axis and the time axis, respectively, of the vertices of a polygon
374. Time axis coordinate C.sub.t calculated by centroid calculator
332 has a temporal resolution greater than that of the
two-dimensional samples constituting augmented subset 354. This is
illustrated in FIG. 14C by the temporal offset between time axis
coordinate C.sub.t and the time components t.sub.6 and t.sub.7 of
the closest temporally-adjacent two-dimensional samples 372 and
376, respectively. Centroid calculator 332 outputs time axis
coordinate C.sub.t to sample combiner 342 as the time component of
the synthesized sample generated by sample synthesizer 330.
[0122] Also in sample synthesizer 330, amplitude component
generator 334 generates the amplitude component of the synthesized
sample representing the original subset from the original samples
in the subset. Specifically, amplitude component generator 334
receives the original samples in the original subset directly or
indirectly from sample selector 320 and generates the amplitude
component of the synthesized sample by selecting the one of the
original samples constituting original subset 352 or by subjecting
two or more of the original samples in the original subset to
interpolation. In an example, amplitude component generator 334
identifies original sample 362 having the greatest amplitude in
original subset 352 shown in FIG. 14B as a maximum-amplitude sample
and outputs the maximum-amplitude sample to sample combiner 342 as
the amplitude component of the synthesized sample.
[0123] FIG. 14D schematically represents part of memory 329 in
which the accumulated samples generated by accumulating the
amplitude components of the synthesized samples representing peak
350 are stored. In the example shown, centroid calculator 332
calculates the time components of the synthesized samples with a
temporal resolution four times that of the original samples
generated by A/D converter 27. The portion of memory 329 shown has
memory locations with memory addresses 6 and 7 respectively
corresponding to time component values t.sub.6 and t.sub.7 shown in
FIG. 14C. In addition, since the time components of the synthesized
samples have a temporal resolution four of times that of the
original samples, memory 329 additionally has memory locations with
memory addresses 61/4, 61/2, 63/4 interposed between memory
addresses 6 and 7 and corresponding to time component values
t.sub.6-1/4, t.sub.6-1/2 and t.sub.6-3/4, respectively, interpose
at t/4 intervals between time component values t.sub.6 and t.sub.7.
In the example shown, the time component of the synthesized sample
provided by the time axis coordinate C.sub.t calculated as
described above with reference to FIG. 14C is t.sub.6-3/4, which
corresponds to memory address 63/4. The scale of the amplitude axis
shown in FIG. 14D differs from that shown in FIGS. 14A-14C.
[0124] At 381-385, FIG. 14D shows accumulated samples that have
been generated by sample selector 320, sample synthesizer 330 and
sample combiner 342 from mass scan signals generated during
respective previously-performed mass scan operations. Accumulated
samples 381-385 are stored in memory 329 at memory addresses 6,
61/4, 61/2, 63/4 and 7, respectively. Sample synthesizer 330 next
generates a synthesized sample having, in this example, the
amplitude of maximum-amplitude original sample 362 as its amplitude
component and time component of t.sub.6-3/4, as described above.
Consequently, when sample combiner 342 receives the synthesized
sample, the time component t.sub.6-3/4 of the synthesized sample
causes accumulated sample 384 to be read from memory address 63/4
in memory 329 and to be input to summer 330. Summer 330 sums
accumulated sample 384 with the amplitude component of the
synthesized sample to generate a new accumulated sample 392. Again
in response to the time component t.sub.6-3/4 of the synthesized
sample, the new accumulated sample is written back in memory 329 at
memory address 63/4.
[0125] Each synthesized sample generated by mass spectrometer 100
incorporating the example of sample processor 310 whose operation
was just described has a temporal resolution four times that and,
hence, a mass resolution twice that, of the original samples output
by A/D converter 27. In each mass scan operation, each synthesized
sample generated by sample synthesizer 330 is added to the
accumulated sample stored in the location in memory 329 having the
memory address corresponding to the time component of the
synthesized sample to generate a new accumulated sample that
constitutes part of a raw mass spectrum. The accuracy of the raw
mass spectrum progressively increases as the number of mass scan
operations increases. The temporal resolution, and, hence, mass
resolution, of the raw mass spectrum is greater than the temporal
resolution and mass resolution of the raw mass spectrum generated
from the same number of mass scan operations by an embodiment of
mass spectrometer 50 having the same conversion clock frequency by
respective resolution ratios somewhat less than the resolution
ratios between the synthesized samples and the original samples.
Once sufficient mass scan operations have been performed to obtain
a raw mass spectrum of a specified accuracy, the accumulated
samples defining each peak in the raw mass spectrum are subject to
a centroid calculation as described above to determine the time
value represented by the peak, and the time value is converted to a
mass value.
[0126] For a given range of mass detection and a given conversion
clock frequency, the size of memory 329 in sample combiner 342 is p
times that of memory 29 of mass spectrometer 50 described above
with reference to FIG. 5, where p is the ratio of the temporal
resolution of the synthesized samples generated by sample
synthesizer 330 and that of the original samples generated by A/D
converter 27. However, in embodiments of mass spectrometer 50 and
mass spectrometer 100 in which the circuitry downstream of A/D
converter 27 is implemented using the same type of integrated
circuit, the fixed amount of memory available within the integrated
circuit prevents memory 329 from being made any larger than memory
29. In this case, the greater mass resolution of mass spectrometer
100 is obtained at the cost of a reduction in the mass range that
can be detected to 1/ p that of mass spectrometer 50 described
above with reference to FIG. 5.
[0127] However, mass spectra are typically sparse, and each peak in
the mass scan signal generated in each mass scan operation is
represented by a single synthesized sample. Consequently, when the
final mass scan has been performed and the raw mass spectrum has
been generated, a value of zero remains in a majority of the memory
locations in memory 329 in the above-described sample combiner 342.
By configuring the sample combiner differently from sample combiner
342 described above with reference to FIG. 13, memory is used more
efficiently and the mass resolution can be increased without a
corresponding reduction in mass range.
[0128] FIG. 15 is a block diagram showing another example of sample
processor 310 described above with reference to FIG. 10
incorporating a second example 344 of sample combiner 340. In the
example shown, sample processor 310 is composed of sample selector
320, sample synthesizer 330 and sample combiner 344. Sample
selector 320 and sample synthesizer 330 are described above with
reference to FIG. 13 and will not be described again here.
[0129] Sample combiner 344 is composed of a synthesized sample
counter 341, a buffer memory 343, a processor 345, a main memory
347 and summer 333. Sample counter 341 has a reset input R
connected to receive a reset signal from controller 15 (FIG. 10), a
data input DI connected to receive the time component of the
synthesized samples output by sample synthesizer 330, and a count
output CO. Buffer memory 343 has a data input DI connected to
receive the both the time component and the amplitude component of
each synthesized samples output by sample synthesizer 330. Buffer
memory 343 additionally has a write address input WADR connected to
the count output CO of sample counter 341, a read address input
RADR and a data output DO. Processor 345 has a first address output
ADR1 connected to the read address input RADR of buffer memory 343,
a data input DI connected to the data output DO of buffer memory
343, a second address output ADR2 and a data output DO. Main memory
347 and summer 333 are connected to one another in an arrangement
similar to that of memory 329 and summer 333 described above with
reference to FIG. 13. Main memory 347 has an address input ADR
connected to the second address output ADR2 of processor 345. The
first input of summer 333 is connected to the data output DO of
processor 345.
[0130] At the start of each mass spectrum measurement process
performed by mass spectrometer 100, controller 15 supplies a reset
signal to sample counter 341 to reset the count output by the
sample counter to zero or another predetermined value. Such reset
operation is unnecessary in embodiments in which sample counter 341
is operated as a stack. During the first mass scan operation
performed by mass spectrometer 100, for each peak defined by the
original samples constituting the mass scan signal, sample
synthesizer 330 generates a respective synthesized sample that
represents the peak. Sample synthesizer 330 outputs the synthesized
sample to sample combiner 344. Specifically, sample synthesizer 330
outputs the time component of the synthesized sample to sample
counter 341 and outputs both the amplitude component and the time
component of the synthesized sample to buffer memory 343. Sample
counter 341 detects the time component received at its data input
DI and, in response to each change in the time component
corresponding to sample synthesizer 330 outputting another
synthesized sample, increments the count output at count output CO
by one.
[0131] Buffer memory 343 stores each synthesized sample received
from sample synthesizer 330 at a respective memory location whose
address depends on the count received from sample counter 341 at
write address input WADR.
[0132] FIG. 16 is a flow chart showing an example of the processing
performed by processor 345 to generate a raw mass spectrum from the
synthesized samples generated by sample synthesizer 330 and
temporarily stored in buffer memory 343. Processor 345 may
alternatively perform processing different from that illustrated in
FIG. 16 to generate a raw mass spectrum from the synthesized
samples generated by sample synthesizer 330 and temporarily stored
in buffer memory 343. In block 410, after at least one synthesized
sample has been stored in buffer memory 343, processor 345 reads a
synthesized sample out from the buffer memory. In an example, the
processor outputs successive buffer memory addresses to the read
address input RADR of buffer memory 343. In response to the memory
addresses, buffer memory 343 outputs to processor 345 the
synthesized samples stored in the memory locations defined by the
memory addresses.
[0133] In block 412, processor 345 compares the time component of
the synthesized sample read from buffer memory 343 in block 410
with a time component map generated by the processor to determine
whether the time component of the synthesized sample is already
mapped to a respective memory location in main memory 347. The time
component map will be described in greater detail below. Since no
time component map exists when the first mass scan operation is
performed, none of synthesized samples read from buffer memory 343
during the first mass scan operation has a time component already
mapped to a respective memory location.
[0134] In block 414, processor 345 performs a test to determine
whether the comparison performed in block 412 indicated that the
time component of the synthesized sample is already mapped to a
respective memory location in main memory 347. A YES result in
block 414 causes execution to advance to block 430, which will be
described below. A NO result in block 414 causes processor 345 to
perform blocks 420-424 in which it maps the time component of the
synthesized sample to a respective memory location in main memory
347 and writes the amplitude component of the synthesized sample at
that memory location.
[0135] Specifically, in block 420, processor 345 performs a test to
determine whether a memory address is available in main memory 347
to which the time component of the synthesized sample read in block
410 can be mapped. A YES result in block 420 causes execution to
advance to block 422, which will be described below. A NO result in
block 420 causes processor 345 to stop execution. This is done to
allow mass spectrometer 100 to be adjusted in a manner that will
prevent main memory 347 from overflowing when the mass spectrum
measurement process is repeated. Typically, main memory 347 will
overflow when sample adjuster 78 detects false peaks caused by
noise in the analog ion detection signal output by ion detector 25.
Increasing the threshold of ion detector 25 reduces the noise level
in the ion detection signal, which reduces the number of peaks
detected by sample adjuster 78 to one within the capacity of main
memory 347.
[0136] In block 422, processor 345 maps the time component of the
synthesized sample read in block 410 to a respective memory address
within main memory 347. The memory mapping process just described
generates the time component map used in block 412 to determine the
memory location in main memory 347 where the amplitude components
of synthesized samples having the same time component are
accumulated.
[0137] In block 424, processor 345 writes the amplitude component
of the synthesized sample at the memory location in main memory 347
to which the amplitude component of the synthesized sample was
mapped in block 422. Execution then advances to block 440, which
will be described below.
[0138] A synthesized sample whose time component is already mapped
to a respective memory location in main memory 347 returns a YES
result in block 414. This causes processor 345 to execute blocks
430-436 in which the amplitude component of the synthesized sample
is accumulated at the memory location in main memory 347 mapped to
the amplitude component of the synthesized sample. In block 430,
processor 345 uses the memory map generated in block 422 to map the
time component of the synthesized sample read from buffer memory
343 in block 410 to the respective memory address in main memory
347. Processor 345 outputs the memory address to the address input
ADR of main memory 347. In block 432, processor 345 causes main
memory 347 to perform a read operation in which the main memory
outputs to the second input of summer 333 the accumulated sample
stored at the memory address received in block 420.
[0139] In block 434, processor 345 outputs the amplitude component
of the synthesized sample to the first input of summer 333. Summer
333 then sums the accumulated sample read from memory 347 with the
amplitude component of the synthesized sample received from
processor 345 to generate a new accumulated sample that is output
to memory 347. Alternatively, processor 345 sums the amplitude
component of the synthesized sample and the accumulated sample to
generate the new accumulated sample. In this case, summer 333 is
omitted. In block 436, processor 345 causes main memory 347 to
perform a write operation in which the new accumulated sample
output by summer 333 is written at the memory address received in
block 430. Execution then advances to block 440, described
below.
[0140] The sample accumulation process performed in blocks 430-436,
in which a memory location in main memory 347 is mapped to the time
component of the current synthesized sample, the accumulated sample
is read from that memory location in main memory 347 and summed
with the amplitude component of the synthesized sample to generate
a new accumulated sample, and the new accumulated sample is written
at the same location in main memory 347, provides the temporal
alignment between the synthesized sample and the accumulated sample
with which the synthesized sample is summed. Moreover, through the
memory mapping process, synthesized samples generated in different
mass scan operations and having equal time components are
accumulated at the memory location in main memory 347 mapped to the
time component.
[0141] In block 440, processor 345 performs a test to determine
whether synthesized samples that have not been read by processor
345 remain in buffer memory 343. A NO result in block 440 causes
execution to stop. A YES result in block 440 causes execution to
return via block 442 to block 410, where processor 345 reads the
next synthesized sample from buffer memory 343 as described
above.
[0142] Mapping memory locations in main memory 347 to respective
time components greatly increases the efficiency with which the
main memory is used since substantially fewer of the memory
locations store a value of zero when the final mass scan operation
has been performed. Accordingly, an embodiment of main memory 347
of a given size is capable of storing a raw mass spectrum having a
greater temporal (and, hence, mass) resolution and a greater mass
range than a same-size embodiment of memory 329 described above
with reference to FIG. 13.
[0143] The synthesized samples generated by sample synthesizer 330
from the mass scan signals generated in successive mass scans
accumulating in memory 347 produce a raw mass spectrum having a
progressively increasing accuracy. When the raw mass spectrum
accumulated in main memory 347 achieves a specified accuracy,
processor 345 reads the accumulated samples from main memory 347 in
ascending or descending time component order and subjects the raw
mass spectrum to a peak detection operation that identifies each
peak exhibited by the raw mass spectrum. Processor 345 then
subjects the accumulated samples defining each peak to a centroid
calculation to determine the time value represented by the
respective peak. The time values needed for reading out the
accumulated samples in ascending or descending time component order
and for the centroid calculation are determined using the memory
map generated in block 420. The memory map is used to reverse map
the memory locations in main memory 347 from which the accumulated
samples are read to the respective time components mapped to those
memory locations. The processor then converts the time value
represented by each peak to a corresponding mass using the
time-to-mass conversion equation described above. This last
calculation is unnecessary in embodiments in which a mass value
converter is interposed between time value generator 336 and
centroid calculator 332, as described above.
[0144] In the above-described embodiments of mass spectrometer 100,
sample processors 110, 210 and 310 can be implemented in hardware
such as an integrated circuit having bipolar, N-MOS, P-MOS or CMOS
devices. Design libraries comprising designs for such circuit
elements suitable for implementing the above-described functions of
sample processors 110, 210 and 310 are commercially available can
be used to design such hardware implementation of sample processors
110, 210 and 310.
[0145] Sample processors 110, 210 and 310 can alternatively be
implemented in pre-fabricated hardware devices such as an
application-specific integrated circuit (ASIC) or a
field-programmable gate array (FPGA). Design libraries comprising
designs for implementing the above-described functions of sample
processors 110, 210 and 310 in such pre-fabricated hardware devices
are commercially available can be used to configure such
pre-fabricated hardware devices to implement sample processors 110,
210 and 310.
[0146] Sample processors 110, 210 and 310 can alternatively be
implemented in software running on a suitable computational device
(not shown) such as a microprocessor or a digital signal processor
(DSP). Sample processors 110, 210 and 310 may additionally
constitute part of a digital signal processor. Programming modules
capable of programming a computational device to provide the
above-described functions of sample processors 110, 210 and 310 are
commercially available and may be used to program a computational
device to provide a software implementation of sample processors
110, 210 and 310. In such software implementations of sample
processors 110, 210 and 310, the various functions described in
this disclosure are typically ephemeral, and exist only temporarily
as the program executes.
[0147] The program in response to which the computational device
operates can be fixed in a suitable computer-readable medium (not
shown) such as a floppy disk, a hard disk, a CD-ROM, a DVD-ROM, a
flash memory, a read-only memory or a programmable read-only
memory. The program is then transferred to a non-volatile memory
that forms part of the computational device, or is external to the
computational device. Alternatively, the program can be transmitted
to the non-volatile memory of the computational device by a
suitable data link.
[0148] FIG. 17 is a flow chart showing an example of a method 500
in accordance with an embodiment of the invention for generating a
mass spectrum. In block 520, from a mass scan signal comprising
original samples defining a peak, a subset of the original samples
defining the peak is selected. In block 530, one or more
synthesized samples are synthesized from the subset of the original
samples. The one or more synthesized samples provide a temporal
resolution greater than the temporal resolution of the original
samples. In block 550, the one or more synthesized samples are
summed with respective, temporally-aligned accumulated samples to
produce the mass spectrum. The accumulated samples are obtained
from mass scan signals generated during respective
previously-performed mass scan operations.
[0149] In an embodiment, the one or more synthesized samples are
summed with respective temporally-aligned accumulated samples by
summing each of the synthesized samples with a respective
temporally-aligned accumulated sample read from a respective memory
location to generate a new accumulated sample. The new accumulated
sample is then stored at the memory location from which the
accumulated sample was read.
[0150] FIG. 18 is a flow chart showing an example of the
synthesizing performed in block 530 and the summing performed in
block 550. In block 532, the one or more synthesized samples are
synthesized by subjecting the original samples in the subset to
interpolation to generate the synthesized samples. In block 534,
the original samples in the subset and the synthesized samples
constitute an augmented subset, and at least one temporally-extreme
one of the original samples in the augmented subset is suppressed
to generate a truncated subset. In block 552, the original samples
in the truncated subset are additionally summed with respective
temporally-aligned accumulated samples.
[0151] FIG. 19 is a flow chart showing another example of the
synthesizing performed in block 530 and the summing performed in
block 550. In this example, the synthesizing performed in block 530
generates a single synthesized sample comprising a time component
and an amplitude component. In block 540, each of the original
samples in the subset is associated with a respective time value to
generate an augmented subset of respective two-dimensional samples.
Alternatively, a mass value is used instead of the time value. In
block 542, the two-dimensional samples in the augmented subset are
subject to a centroid calculation to obtain the time component of
the synthesized sample. In block 544, the amplitude component of
the synthesized sample is generated from at least one of the
original samples in the subset. In block 560, the amplitude
component of the synthesized sample is summed with the amplitude
component of the one of the accumulated samples having a time
component equal to the time component of the synthesized sample to
generate the amplitude component of a new accumulated sample having
a time component equal to the time component of the synthesized
sample. Additionally, in block 562, the time components of the
accumulated samples are mapped to respective memory locations. This
may be done by storing the accumulated sample at a memory location
defined by the time component of the synthesized sample, as
described above with reference to FIG. 13. Alternatively, a memory
mapping scheme similar to that described above with reference to
FIG. 16 can be used.
[0152] In an embodiment, a respective accumulated sample is
generated by a process in which the amplitude components of
synthesized samples generated during the previously-performed mass
scan operations and having equal time components are accumulated.
In another embodiment, the accumulated samples are generated by
subjecting original samples obtained in the previously-performed
mass scan operations to respective selecting, synthesizing and
summing, as described above with reference to FIG. 17.
[0153] This disclosure describes the invention in detail using
illustrative embodiments. However, the invention defined by the
appended claims is not limited to the precise embodiments
described.
* * * * *