U.S. patent application number 15/213512 was filed with the patent office on 2017-01-26 for portable laser induced breakdown spectroscopy systems.
The applicant listed for this patent is THERMO SCIENTIFIC PORTABLE ANALYTICAL INSTRUMENTS INC.. Invention is credited to Michael E. DUGAS, Brendan FALVEY, Haowen LI, Yu SHEN, Rong SUN, Peidong WANG.
Application Number | 20170023484 15/213512 |
Document ID | / |
Family ID | 57837031 |
Filed Date | 2017-01-26 |
United States Patent
Application |
20170023484 |
Kind Code |
A1 |
WANG; Peidong ; et
al. |
January 26, 2017 |
PORTABLE LASER INDUCED BREAKDOWN SPECTROSCOPY SYSTEMS
Abstract
An embodiment of a laser induced breakdown system is described
that comprises a portable device that includes: a laser configured
to produce a beam comprising a plurality of repeating pulses; a
processor configured to open a data acquisition window after a
delay period, wherein the delay period begins upon production of
one of the pulses; one or more optical elements configured to
direct the beam at a sample and collect emitted light from a plasma
continuum; and an optical detector configured to produce a
plurality of signal values from the emitted light from the plasma
continuum collected during the data acquisition window, wherein the
processor is configured to identify an element from the signal
values.
Inventors: |
WANG; Peidong; (Carlisle,
MA) ; SUN; Rong; (Winchester, MA) ; FALVEY;
Brendan; (Wilmington, MA) ; LI; Haowen;
(Lexington, MA) ; SHEN; Yu; (Waltham, MA) ;
DUGAS; Michael E.; (Londonderry, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THERMO SCIENTIFIC PORTABLE ANALYTICAL INSTRUMENTS INC. |
Tewksbury |
MA |
US |
|
|
Family ID: |
57837031 |
Appl. No.: |
15/213512 |
Filed: |
July 19, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62194493 |
Jul 20, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01J 3/0213 20130101;
G01J 3/10 20130101; G01J 3/027 20130101; G01J 3/0286 20130101; G01J
3/443 20130101; G01N 2201/0221 20130101; G01J 3/0264 20130101; G01N
21/718 20130101; G01J 3/0237 20130101; G01J 3/0272 20130101; G01N
2201/127 20130101 |
International
Class: |
G01N 21/71 20060101
G01N021/71; G01J 3/443 20060101 G01J003/443 |
Claims
1. A laser induced breakdown system, comprising: a portable device
that comprises: a laser configured to produce a beam comprising a
plurality of repeating pulses; a processor configured to open a
data acquisition window after a delay period, wherein the delay
period begins upon production of one of the pulses; one or more
optical elements configured to direct the beam at a sample and
collect emitted light from a plasma continuum; and an optical
detector configured to produce a plurality of signal values from
the emitted light from the plasma continuum collected during the
data acquisition window, wherein the processor is configured to
identify an element from the signal values.
2. The laser induced breakdown system of claim 1, further
comprising: a second optical detector that detects a time point of
an actual pulse from the laser.
3. The laser induced breakdown system of claim 2, wherein: the
processor initiates the timing of the delay period using the time
point of the actual pulse.
4. The laser induced breakdown system of claim 1, further
comprising: a temperature detector that acquires a temperature
measurement.
5. The laser induced breakdown system of claim 4, wherein: the
temperature detector is positioned inside a housing of the portable
device.
6. The laser induced breakdown system of claim 4, wherein: the
temperature measurement is acquired at substantially the same time
as when the laser produces the beam.
7. The laser induced breakdown system of claim 6, wherein: the
processor uses the temperature measurement to compensate for a
temperature related difference in a calibration curve.
8. The laser induced breakdown system of claim 7, wherein: the
temperature related difference in a calibration curve comprises a
difference in a slope of the calibration curve.
9. The laser induced breakdown system of claim 7, wherein: the
processor compensates for a temperature related difference using a
change in the delay period.
10. The laser induced breakdown system of claim 1, wherein: the
processor is configured to open a second data acquisition window
after a second delay period.
11. The laser induced breakdown system of claim 10, wherein: the
first data acquisition window and the second data acquisition
window comprise different durations of time.
12. The laser induced breakdown system of claim 10, wherein: the
first data acquisition window and the second data acquisition
window comprise overlapping durations of time.
13. The laser induced breakdown system of claim 10, wherein: the
processor obtains the plurality of signal values using the first
delay period and a plurality of signal values using the second
delay period; and the processor calculates a differential value
between one or more of the signal values from the first delay
period and the corresponding signal values from the second delay
period.
14. The laser induced breakdown system of claim 13, wherein: the
processor selects one of the first or second delay periods based on
the differential value.
15. The laser induced breakdown system of claim 1, wherein: the
data acquisition window does not comprise a duration substantially
greater than the delay period.
16. The laser induced breakdown system of claim 1, further
comprising: a variable filter element that modifies a power level
of the beam.
17. The laser induced breakdown system of claim 16, further
comprising: a temperature detector that detects ambient
temperature, wherein the processor modifies the relative position
of the variable filter with the beam using the ambient
temperature.
18. A method, comprising: identifying an element using a portable
device that performs a method comprising: producing a beam from a
laser comprising a plurality of repeating pulses; directing the
beam at a sample; collecting emitted light in response to the beam;
opening a data acquisition window after a delay period, wherein the
delay period begins upon production of one of the pulses; producing
a plurality of signal values from the emitted light collected
during the data acquisition window; and identifying the element
from the signal values.
19. The method of claim 18, wherein the method performed by the
portable device further comprises: detecting a time point of an
actual pulse from the laser.
20. The method of claim 19, wherein the method performed by the
portable device further comprises: initiating the timing of the
delay period using the time point of the actual pulse.
21. The method of claim 18, wherein the method performed by the
portable device further comprises: acquiring a temperature
measurement.
22. The method of claim 21, wherein: the temperature measurement is
acquired from inside a housing of the portable device.
23. The method of claim 21, wherein: the temperature measurement is
acquired at substantially the same time as when the laser produces
the beam.
24. The method of claim 21, wherein the method performed by the
portable device further comprises: compensating for a temperature
related difference in a calibration curve using the temperature
measurement.
25. The method of claim 24, wherein: the temperature related
difference in a calibration curve comprises a difference in a slope
of the calibration curve.
26. The method of claim 24, wherein: the processor compensates for
a temperature related difference using a change in the delay
period.
27. The method of claim 18, wherein the method performed by the
portable device further comprises: opening a second data
acquisition window after a second delay period.
28. The method of claim 27, wherein: the first data acquisition
window and the second data acquisition window comprise different
durations.
29. The method of claim 27, wherein: the first data acquisition
window and the second data acquisition window comprise overlapping
durations.
30. The laser induced breakdown system of claim 27, wherein the
method performed by the portable device further comprises:
obtaining the plurality of signal values using the first delay
period and a plurality of signal values using the second delay
period; and calculating a differential value between one or more of
the signal values from the first delay period and the corresponding
signal values from the second delay period.
31. The method of claim 30, wherein the method performed by the
portable device further comprises: selecting one of the first or
second delay periods based on the differential value. The method of
claim 18, wherein: the data acquisition window does not comprise a
duration substantially greater than the delay period.
33. The method of claim 18, wherein the method performed by the
portable device further comprises: modifying a power level of the
beam using a variable filter element.
34. The method of claim 33, wherein the method performed by the
portable device further comprises: acquiring a temperature
measurement; and modifying a relative position of the variable
filter element with the beam using the temperature measurement.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority benefit of U.S.
provisional patent application Ser. No. 62/194,493 for "Portable
Laser Induced Breakdown Spectroscopy Systems" by Peidong Wang, et
al., filed Jul. 20, 2015, the entire disclosure of which is
incorporated herein by reference
FIELD OF THE INVENTION
[0002] The invention relates to hand held Laser Induced Breakdown
(LIBS) devices and methods for their use.
BACKGROUND OF THE INVENTION
[0003] It is generally appreciated that elemental analysis
techniques have important applications to determine the elemental
composition of a material in various forms. Elemental analysis
techniques range from destructive (e.g.--material is destroyed in
testing) to semi-destructive (e.g.--material is sampled or surface
damaged) to fully non-destructive (e.g.--material is left fully
intact). One view of elemental analysis is via the periodic chart
to define which elements can be detected via a particular technique
or device design. There are often challenges with certain elements
due to interfering signals, weak signals, or the inability to cause
atomic excitation. This category of techniques can include what are
referred to as Inductively Coupled Plasma-Atomic Emission
Spectroscopy (e.g. ICP-AES), ICP-Mass Spectrometry (e.g. ICP-MS),
Electrothermal Atomization Atomic Absorption Spectroscopy (e.g.
ETA-AAS), X-Ray Fluorescence Spectroscopy (e.g. XRF), X-Ray
Diffraction (e.g. XRD), and Laser Induced Breakdown Spectroscopy
(e.g. LIBS). Limits of detection are a key performance
specification of any technique or instrument. Elemental analysis
may be either qualitative (e.g. easier) or quantitative (e.g. more
difficult) and often requires calibration to known standards.
[0004] As described above, the periodic table is often used to
define the elements which a system can detect and quantify. A key
element of interest in the analysis of metals is Carbon which is
known as a "light" element according to the periodic table. For
example, the carbon content of many steel compositions defines the
material properties and compatibility of a particular composition
with other metals. It is generally appreciated that XRF devices and
in particular portable instruments are not able to reliably detect
and quantify light elements such as the carbon content of a
material. The defining characteristic of low carbon steel (used
extensively to transport chemicals in piping) is the presence of
approximately 300 ppm of carbon which would require a limit of
detection of less than 100 ppm to reliably quantify (limit of
quantification, LOQ, or .about.3 times LOD). Often these materials
need to be tested at the point of use to confirm suitability for
the purpose.
[0005] Laser Induced Breakdown Spectroscopy is an atomic emission
spectroscopy technique which uses laser pulses to induce
excitation. The interaction between the focused laser pulses and
the sample creates plasma composed of ionized matter. Plasma light
emissions can provide spectral data regarding the chemical
composition of many different kinds of materials. LIBS can provide
an easy to use, rapid, and in situ chemical analysis with adequate
precision, detection limits, and cost. Importantly, LIBS can very
accurately detect and quantify the light elements that other
technologies cannot.
[0006] Laser interactions with matter are governed by quantum
mechanics which describe how photons are absorbed or emitted by
atoms. If an atom absorbs a photon one or more electrons move from
the ground state to a higher energy quantum state. Electrons tend
to occupy the lowest possible energy levels, and in the
cooling/decay process the atom emits a photon. The different energy
levels of different atoms produce different photon energies for
each kind of atom, with narrowband emissions due to their
quantization. These emissions correspond to the spectral emission
lines found in LIBS spectra and their features and their associated
energy levels are well known for each atom.
[0007] There are three basic stages in the plasma life time. The
first stage is the ignition process which includes the initial bond
breaking and plasma formation during the laser pulse. This is
affected by the laser type, laser power, and pulse duration. The
second stage in plasma life is the most critical for optimization
of LIBS spectral acquisition and measurement because the plasma
causes atomic emission during the cooling process. After ignition,
the plasma will continue expanding and cooling. At the same time,
the electron temperature and density will change. This process
depends on ablated mass, spot size, energy coupled to the sample,
and environmental conditions (state of the sample, pressure, etc.).
The last stage of the plasma life is not interesting for LIBS
measurements. A quantity of ablated mass is not excited as vapor or
plasma, hence this amount of material is ablated as particles and
these particles create condensed vapor, liquid sample ejection, and
solid sample exfoliation, which do not emit radiation. Moreover,
ablated atoms become cold and create nanoparticles in the
recombination process of plasma.
[0008] In general there is a desire to move analytical techniques
from the laboratory to the field at the point of use by using
devices that are easily portable and supportable with a minimum of
additional requirements. There are often significant costs and
technical challenges associated with laboratory testing, in
particular when there are significant time delays between the
sampling and the test result, or if sampling itself presents an
issue. Example markets include in-process pipe testing, scrap metal
sorting, incoming material inspection, and positive material
identification. Portable XRF devices have been very successful in
these markets but have technical limitations in certain
applications such as with the detection of light elements as
described above. Further, while it is appreciated that Optical
Emission Spectroscopy (e.g. OES) devices can detect light elements
it is also known to be very challenging to execute in a portable
form.
[0009] There are many challenges associated with bringing
technologies from the laboratory to the field. In a laboratory
setting, one can usually control many of the analysis variables and
perform various sample preparation steps to get an accurate and
repeatable result in often "ideal" conditions. Bringing the
technology to the field, in particular to outdoor and often remote
locations, introduces a host of variables that cannot be completely
controlled. Most importantly the operating environment can vary
widely including temperatures ranging from -5 to 50.degree. C. and
beyond. Additionally, sample preparation may be limited by the
other tools available in the field and the technical skill of the
operator. The portable instruments need to be rugged, easy to use,
and minimize the amount of user intervention to get repeatable
results.
[0010] Without the ability to control all of the analysis variables
like in a laboratory setting, it becomes important that the
portable instrument is able to operate effectively over a broad
range of variables that may occur in a real field setting. The
calibration of the instrument may typically occur at the factory in
controlled conditions, and various factors can be intentionally
altered (temperature, pressure, sample types, power settings). But,
it is unlikely that every possible condition could be envisioned as
well as impractical to calibrate for all potential operating
conditions that may occur in the field during manufacture. Further,
calibration processes typically cannot compensate for certain
changes in operation due to environmental conditions, performance
changes over a lifetime of use, etc. For example, the power output
of a laser may vary based on a number of factors that include
temperature, decrease caused by usage over time, or other
factors.
[0011] Therefore, it is appreciated that there is a strong need for
a portable LIBs system and methods that enable adjustment to
operating conditions in order to compensate for many of the
uncontrolled variables and give a result comparable to those found
in a laboratory environment.
SUMMARY
[0012] An embodiment of a laser induced breakdown system is
described that comprises a portable device that includes: a laser
configured to produce a beam comprising a plurality of repeating
pulses; a processor configured to open a data acquisition window
after a delay period, wherein the delay period begins upon
production of one of the pulses; one or more optical elements
configured to direct the beam at a sample and collect emitted light
from a plasma continuum; and an optical detector configured to
produce a plurality of signal values from the emitted light from
the plasma continuum collected during the data acquisition window,
wherein the processor is configured to identify an element from the
signal values.
[0013] In some embodiments the portable laser induced breakdown
system further comprises a second optical detector that detects an
actual pulse that signals the beginning of the delay period and/or
a temperature detector that detects ambient temperature, wherein
the controller modifies duration of the delay period using the
ambient temperature.
[0014] Also in some embodiments, the portable laser induced
breakdown system further comprises an attenuator element that
modifies a power level of the beam. Further some embodiments also
include a temperature detector that detects ambient temperature and
a processor that modifies the relative position of the variable
filter with the beam using the ambient temperature.
[0015] In addition, an embodiment of a method is described that
comprises identifying an element using a portable device that
performs a method comprising: producing a beam from a laser
comprising a plurality of repeating pulses; directing the beam at a
sample; collecting emitted light in response to the beam; opening a
data acquisition window after a delay period, wherein the delay
period begins upon production of one of the pulses; producing a
plurality of signal values from the emitted light collected during
the data acquisition window; and identifying the element from the
signal values.
[0016] The above embodiments and implementations are not
necessarily inclusive or exclusive of each other and may be
combined in any manner that is non-conflicting and otherwise
possible, whether they are presented in association with a same, or
a different, embodiment or implementation. The description of one
embodiment or implementation is not intended to be limiting with
respect to other embodiments and/or implementations. Also, any one
or more function, step, operation, or technique described elsewhere
in this specification may, in alternative implementations, be
combined with any one or more function, step, operation, or
technique described in the summary. Thus, the above embodiment and
implementations are illustrative rather than limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The above and further features will be more clearly
appreciated from the following detailed description when taken in
conjunction with the accompanying drawings. In the drawings, like
reference numerals indicate like structures, elements, or method
steps and the leftmost digit of a reference numeral indicates the
number of the figure in which the references element first appears
(for example, element 120 appears first in FIG. 1). All of these
conventions, however, are intended to be typical or illustrative,
rather than limiting.
[0018] FIG. 1 is a simplified graphical representation of one
embodiment of a portable LIBS instrument;
[0019] FIG. 2 is a simplified graphical representation of one
embodiment of a cutaway view of the portable LIBS instrument of
FIG. 1;
[0020] FIG. 3 is a simplified graphical representation of one
embodiment of a continuum of LIBs plasma generated by laser
pulse;
[0021] FIGS. 4A and 4B are simplified graphical representations of
one embodiment of data depicting calibration curve or LOD changes
vs. different time-delay;
[0022] FIGS. 5A and 5B are simplified graphical representations of
one embodiment of data depicting variation of calibration curve or
LOD changes vs. different temperatures;
[0023] FIG. 6 is a simplified graphical representation of one
embodiment of data acquisition using multiple windows;
[0024] FIG. 7 is a simplified graphical representation of one
embodiment of a differential spectrum from two delay periods;
[0025] FIG. 8 is a simplified graphical representation of one
embodiment of a attenuator system;
[0026] FIG. 9 is a simplified graphical representation of one
embodiment of an attenuator system;
[0027] FIG. 10 is a simplified graphical representation of one
embodiment of an attenuator system;
[0028] FIG. 11 is a simplified graphical representation of one
embodiment of an attenuator system;
[0029] FIG. 12 is a simplified graphical representation of one
embodiment of transmission data through the attenuator system of
FIG. 11;
[0030] FIG. 13 is a simplified graphical representation of one
embodiment of transmission data through the attenuator system of
FIG. 11;
[0031] FIG. 14 is a simplified graphical representation of one
embodiment of an attenuator system;
[0032] FIG. 15 is a simplified graphical representation of one
embodiment of transmission data through the attenuator system of
FIG. 14; and
[0033] FIG. 16 is a simplified graphical representation of one
embodiment of transmission data through the attenuator system of
FIG. 14.
[0034] Like reference numerals refer to corresponding parts
throughout the several views of the drawings.
DETAILED DESCRIPTION OF EMBODIMENTS
[0035] As will be described in greater detail below, embodiments of
the described invention include LIBS systems and methods for
addressing laser power variations that occur in applications with
portable devices. More specifically, embodiments include LIBS
platforms enabled to attenuate the power of a beam emitted from a
laser source in order to compensate for changes in laser power
output. Also, the described embodiments include LIBS platforms
enabled to modulate the initiation of a "delay period" used to
begin a window of data acquisition when the noise floor has been
reduced.
[0036] Some or all of the embodiments described herein may include
one or more elements for operational control of a portable LIBS
device. For example, embodiments may include one or more processor
or controller elements that execute control logic, data
acquisition, and/or data processing operations for the portable
LIBS device. Embodiments may also include readable and writeable
memory devices that store data that may include reference material
data, calibration data, sample material data, performance metrics,
etc. Also in the described embodiments, LIBS devices may include
one or more optical elements for directing a beam to a sample and
collecting light from the sample as well as one or more detection
elements (e.g. CCD, photodiode, etc.) that receive light collected
from the sample and in some embodiments process the collected light
into signals that can be interpreted by the processor or controller
elements.
[0037] An example of portable LIBS 100 is illustrated in FIG. 1
that includes trigger 105 that a user may employ to initiate a
laser to fire and to begin data acquisition processes, and nose 107
constructed and arranged to interface with a sample and includes
optical elements to direct laser pulses at the sample and acquire
spectral information in response to the laser pulses. FIG. 1 also
illustrates gas chamber 110 that is a storage element used to
provide a gas through nose 107 which creates a microenvironment at
an interrogation region of the sample, battery 120 that provides a
power resource for LIBS 100, and display 130 that provides the user
with useful information that includes process and/or result
information. Also, embodiments of LIBS 100 include a housing
constructed of a lightweight, durable, and rigid material that
defines an internal space within LIBS 100 where components may be
arranged.
[0038] An example of a cutaway view of portable LIBS 100 is also
provided in FIG. 2. For example, FIG. 2 includes processor 220,
optical detector 215, temperature detector 225, and laser 205 that
provides beam 250 through optical elements and to a sample via nose
107. Light emission 260 is also collected by optical elements and
directed to optical detector 230. In the present example, the
optical elements may include typical elements known in the art such
as lenses, mirrors, fibers, etc. Optionally, some embodiments of
LIBS 100 may include variable filter 210 as will be described in
greater detail below. It will be appreciated that the embodiments
illustrated in FIG. 2 are exemplary and should not be considered as
limiting, and that components such as optical elements (e.g.
lenses, mirrors, fibers, etc.), memory, and other components not
illustrated are considered to be within the scope of the described
embodiments.
[0039] FIG. 3 provides an illustrative example of relative optical
signal intensity of a typical continuum of LIBS plasma generated by
laser pulse 300 at time 0. Laser pulse 300 initially produces
highly ionized plasma (also referred to as a super continuum
plasma) that generally emits a high level of background intensity
305 which makes it difficult to effectively identify individual
element signatures. In typical embodiments background intensity 305
decays more quickly than ionic intensity 310 or atomic intensity
320 through electron-ion recombination (e.g. atomic transitions) so
that after a period of nanoseconds (ns) to microseconds (.mu.s)
subsequent to pulse 300 (e.g. length of period depending on
specific characteristics of LIBS 100 and/or conditions). The amount
of time from laser pulse 300 until the start of data acquisition is
sometimes referred to as a "delay period" (.tau..sub.d) represented
as a point in time noted as "Delay" in FIG. 3. In the example of
FIG. 3, a data acquisition window opens after the delay period to
obtain the elemental information contained in the spectra and
maximize the ratio of signal of interest/unwanted signal. In the
present example, the LIBS 100 instrument acquires signal intensity
data for a specified amount of time that may be referred to as a
"window" illustrated in FIG. 3 as the time between the "Delay" line
and the "End" line. It is typically desirable that the data
acquisition window closes when the signals from ionic intensity 310
or atomic intensity 320 have decayed below a threshold level. It
will also be appreciated that different delay periods (.tau..sub.d)
may be used and that the data acquisition window may vary, but in
general may not be an amount of time that is substantially greater
than the delay period (e.g. >>10 .tau..sub.d).
[0040] Elements present in the sample can be qualitatively
identified by their spectral lines detected from the sample. For
quantitative analysis the detected intensities for the elements are
compared to intensities on a calibration curve that is typically a
substantially linear relationship between the element response to
the laser pulse and the concentration range of the element. The
slope of the substantially linear calibration curve represents the
change of signal for a given incremental change in concentration.
Those of ordinary skill in the art appreciate that the signal
intensity is influenced by the laser power and other factors. Thus
changes in the laser power influence the slope and thus accuracy of
quantitative result.
[0041] One of the key variables in performing an accurate LIBS
analysis is the amount of laser power deposited on the sample
surface. As those of skill in the art appreciate, the level of
laser power has a direct effect on the plasma characteristics and
in turn the optical signal that is generated. An approach typically
employed in laboratory settings is to use what is referred to as an
active Q-switched laser so that the power can be modulated for
consistency. As those of ordinary skill in the art appreciate the
technique of "Q-switching" (also referred to as "giant pulse
formation") generally refers to the production of a pulsed beam by
a laser where the pulses typically exhibit significantly higher
peak power than can be produced by a laser operating in a
continuous output mode. There are two general categories of
Q-switched laser, the first is referred to as an "active" version
that includes some sort of mechanical control (e.g. a shutter,
wheel, mirror, etc.) positioned within the laser cavity that
enables external control of the pulse repetition rate. The second
is referred to as a "passive" version that employs an absorber
material in the laser cavity (e.g. an ion doped crystal) that does
not generally allow for direct control of the pulse repetition rate
and typically results in increased "jitter" (e.g. a variation in
pulse periodicity and power level).
[0042] In the case of active Q-switch lasers, the pulse repetition
rate of the laser can be precisely determined beforehand and
therefore proper signal acquisition can also be pre-determined.
However, active Q-Switch lasers are significantly more expensive,
larger than passive versions, complex to construct, sensitive to
the surrounding environmental conditions, and need a high voltage
power input to activate the Q switch. For applications with LIBS
100 it is typically desirable to use a passive Q-switch laser as
opposed to an active Q-switch laser due to the smaller size, lower
cost, ease of operation, increased durability, and lower power
consumption requirements. For example, a pulsed Nd:YAG laser that
operates at about 1064 nm may be employed in embodiments of LIBS
100.
[0043] Particularly important for applications with LIBS 100,
changes in the environment can cause calibration curve variations
(e.g. to the slope of the calibration curve) which can produce a
substantial effect on the data acquired. One significant factor is
a change in the ambient temperature in which portable LIBS 100
instrument operates. Temperature differences cause a corresponding
laser power change of a certain magnitude relative to the degree of
change in temperature from the temperature used to produce a
calibration curve. For example, even a relatively small change in
ambient temperature for the laser or LIBS 100 instrument from a
temperature to which a LIBS 100 instrument is calibrated can result
in a significant change in laser power output. In many embodiments
this occurs without instructional or other input from the control
elements in LIBS 100 since the laser power in a passively
Q-switched laser system cannot be easily changed. Some embodiments
of LIBS 100 may include one or more temperature sensors to measure
the temperature inside and/or outside of the instrument. For
instance, in some embodiments temperature detector 225 is
positioned within the housing of LIBS 100 that measures the
temperature of the internal environment where laser 205 is located.
The timing of the temperature measurement may occur at
substantially the same time as a laser pulse fires from the laser,
before the laser pulse fires from the laser, or after the laser
pulse fires from the laser. Typically, there is a high degree of
correlation between the degree of temperature change and the degree
of laser power output change that enables accurate prediction.
[0044] Another change in the environment that can result in an
effect on the data acquired from a sample includes gas pressure
level and gas flow from LIBS 100 that creates a microenvironment in
an interrogation region. For example, some embodiments of LIBS 100
benefit from use of an inert gas (e.g. Argon) to create a stable
environment for plasma generation and detection. Variations in
pressure and flow rate of the gas can have a destabilizing effect
on the environment that affects the data acquired from the plasma.
In the present example, LIBS 100 includes gas chamber 110 that acts
as a reservoir of the inert gas that is delivered to that
interrogation region associated with the sample via plumbing
through nose 107. Also, nose 107 may include features that improve
the retention of the inert gas in the interrogation region to
reduce the fluctuation of gas concentration during the desired data
acquisition period. Such features may include a "skirt" structure
constructed from a flexible polymer or other flexible material that
conforms to the surface contours of a sample and creates a seal
separating the internal microenvironment from the external
environment.
[0045] FIGS. 4A and 4B as well as 5A and 5B provide illustrative
examples of data illustrating the effects of differences in delay
period and temperature changes on a calibration curve. More
specifically, FIG. 4A provides an example of calibration curve
changes vs 5 different time-delay periods. The data is plotted as a
ratio of the relative intensity detected to the intensity of a
known reference peak on the Y axis (e.g. the ratio value of the
intensity of the known reference peak is set at 1) to the percent
concentration of a material on the X axis. In the example in FIG.
4A, the bottom calibration curve correlates to a 500 ns delay
period with the remaining 4 time delay periods incrementing by 250
ns (e.g. 750 ns, 1000 ns, 1250 ns, and 1500 ns) with a progressive
increase in the slope of each calibration curve with the
corresponding increase in the delay period. Therefore, for the
difference is greater at higher concentrations. FIG. 4B illustrates
a progressive increase in the limit of detection (e.g. LOD) as the
delay period progressively increases.
[0046] Similarly, FIG. 5A provides an example of variation of
calibration curve changes with the same delay period at different
temperatures (e.g. 25.degree. C. bottom curve, 40.degree. C. middle
curve, and 55.degree. C. top curve). Similar to FIG. 4A, the graph
is plotted as a ratio of the relative intensity detected to the
intensity of a known reference peak on the Y axis (e.g. the ratio
value of the intensity of the known reference peak is set at 1) to
the percent concentration of a material on the X axis. In the case
of FIG. 5A, the progressive increase in the slope of each
calibration curve occurs with the corresponding increase in the
temperature. FIG. 4B illustrates a substantial degree of
fluctuation in the limit of detection (e.g. LOD) as the temperature
progressively increases with the lowest limit of detection
occurring at 40.degree. C.
[0047] In some embodiments, the predictable relationship between
the time delay and the calibration curve can be employed to
advantageously select a desired calibration curve that could
include compensation for other factors affecting the calibration
curve such as temperature. For example, the relationship between
the calibration curve and the temperature is also very predictable
and thus the accuracy of quantification of a material concentration
can be improved by compensating for the effects of temperature on
the calibration curve via an adjustment of the delay period and
corresponding affect on the calibration curve. Thus, in the present
example, a temperature measurement may be taken using temperature
detector 225 or other temperature sensor at the substantially the
same time that laser 205 fires. Processor 220 then calculates the
difference in slope of the calibration curve at the detected
temperature from the calibration temperature using the relationship
of temperature to the calibration curves. Processor 220 further
calculates a delay period to compensate for the temperature slope
difference using the relationship of delay period to the
calibration curves. The compensatory delay period may then be used
to acquire the intensity data from the sample.
[0048] In the same or alternative embodiments, another timing data
acquisition mode of the invention includes implementing a plurality
of data acquisition "windows". In the described embodiment, each
window can be optimized for each application and/or elemental
signal in order to maximize the ratio of signal of
interest/unwanted signal. Also, in some or all of the described
embodiments the delay period triggering the start of an acquisition
window and duration of time for each window may be dependent upon
the characteristics of the analytical line. For example, an atomic
line for Antimony (Sb) 259.804 nm (5.82 eV) and an ionic line for
Iron (Fe) 259.872 nm (9.25 eV) cannot be optically separated using
a standard acquisition window approach. In the present example,
selection of an acquisition period for a window of 80-160 .mu.s
after the laser pulse results in a reduction of what may be
referred to as the Background Equivalent Concentration (BEC) of Sb
which allows for better detection limit for Fe.
[0049] FIG. 6 provides an illustrative example of one embodiment of
multiple windows, the timing and duration of each may be optimized
to maximize the ratio of signal of interest/unwanted signal for an
application or element of interest. In the example of FIG. 6,
Window 1 maximizes the data quality obtained from ionic intensity
310 and is timed to begin after background intensity 305 has
substantially decayed and atomic intensity 320 is below its
maximum. Similarly, Window 2 maximizes the data quality obtained
from atomic intensity 320 and is timed to begin after ionic
intensity 310 has substantially decayed and before atomic intensity
320 substantially decays. In the example of FIG. 6 Window 1 and
Window 2 are separated from each other by some magnitude of time,
however it will be appreciated that in some embodiments Window 2
may begin immediately after Window 1 ends. It will further be
appreciated that in some embodiments Window 1 and Window 2 may
overlap by some degree with Window 2 beginning at some time within
the duration of Window 1.
[0050] In some embodiments, multiple delay periods may be used with
each delay period shifted by some degree of time. Each delay period
may be associated with a laser pulse or separate laser firing
events. A shift may include moving the delay period to begin
earlier or later by some degree relative to a previous delay
period, typically on the order of 10-50 nanoseconds, and acquiring
intensity information at each acquisition window associated with
the shifted delay period. The resulting data may be used to
optimize the signal used for subsequent calculations and which may
be useful for "peak finding" applications. In the same or
alternative embodiments, the delay period or series of shifted
delay periods may be employed to identify a material or family of
materials such as a matrix material that comprises one or more
other materials of interest. For example, a particular delay period
may be used to identify a matrix material in about 1 sec, and a
second particular delay period may be used with a subsequent laser
firing event to identify a material of interest in the matrix
material.
[0051] Further, in some embodiments the data from two different
delay periods may be used to find peaks that have differences or
similarities in their lifetimes. As described above, the
differential information is useful to select delay periods provide
the desired emphasis on one or more selected peaks. Also, in some
embodiments the differential spectrum could be used to identify
minute contamination of a certain material of interest by another
material or element. For example, FIG. 7 provides an illustrative
example of a first normalized spectrum obtained using a 500 ns
delay and a second normalized spectrum obtained using a 1000 ns
delay. The bottom line illustrates a differential spectrum obtained
by subtracting the values of the 1000 ns spectrum from the values
of the 500 ns spectrum.
[0052] Also, embodiments of the presently described invention
include approaches to address the effect of timing jitter with the
laser in a LIBS 100 instrument. Typical implementations of LIBS
instrumentation, such as those in laboratory environments, utilize
active Q-switch lasers with very predictable laser pulse timing and
thus accurately time the start of the delay period to coincide with
the expected laser pulse timing. As described above, embodiments of
LIBS 100 instruments may utilize a passive Q-switch laser that
exhibits an undesirable timing jitter effect and thus one
embodiment of the invention includes timing the start of the delay
period (e.g. via an electronic shutter associated with a detector)
to coincide with the timing of the actual laser pulse as opposed
coinciding with the timing of the expected laser pulse. In the
described embodiments one or more detector elements may be
incorporated into LIBS 100 that detect jitter effects associated
with the passive Q-switch laser. The detector elements may include
optical detectors that detect the timing of laser pulses and/or
power output, or temperature detectors to detect that ambient
temperature that can be correlated to an expected change in laser
power output.
[0053] For example, a detector element such as detector 215 may be
positioned to receive a signal from the optical path of beam 250
from laser 205. In the described example, detector 215 may
communicate with processor 220 and include a fast photo diode
employed to detect the time of firing of the actual laser pulse.
Processor 220 applies the delay period using the timing of the
actual pulse detected as opposed to the expected timing for the
pulse. Thus, if the actual pulse is detected later by some degree
from the expected timing of the pulse, then the delay period will
begin at the timing of the actual pulse and will also include the
degree of time difference. The time delay can be implemented using
an electronics delay in the detector 230 of LIBS 100. Detector 230
may include any detector known in the art such as a CCD (or other
detector element such as an avalanche photodiode or photomultiplier
tube), that acquires the intensity data during an optimal time
range. In the described example, after the actual laser pulse
generates the super continuum plasma that decays over a delay
period (typically .about.100 ns) plasma signals related to the
chemical elements become prominent and those useful signals are
then acquired with a CCD detector. However, in some cases there is
an effect due to a time difference between the timing of an
expected laser pulse and the timing of an actual laser pulse, the
result is a timing difference of the opening of the acquisition
window where just a matter of nanoseconds can have a substantial
impact on the data which can be particularly importation for
quantification applications. For instance, opening the acquisition
window early means an increase in noise if reading part of the
super continuum, or opening the acquisition window late results in
loss of valuable signal information related to one or more of the
chemical elements.
[0054] In the same or alternative embodiments of the described
invention, LIBS 100 may include variable filter 210 moveably
positioned in the path of beam 250 between laser 205 and the
sample. In the described embodiments filter 210 is under
operational control of processor 220 (e.g. via one or more motors)
and enables precise control of the degree of laser power delivered
to a sample by absorbing a discrete amount of the laser power based
upon the relative position of the variable filter with beam 250.
For example, as those of ordinary skill in the related art
appreciate a variable filter (also sometimes referred to as a
graduated neutral density filter) includes a variety of possible
optical filter arrangements that reduce or modify the intensity of
light based upon a relative position of the variable filter with
respect to the path of beam 250. Variable filters typically have a
plurality of regions each having a particular degree of
attenuation. Variable filters may be configured in a linear,
circular, or other format known in the art some and embodiments may
include various "transition" properties (also sometimes referred to
as "edges") that may include smooth transitions (e.g. a soft edge)
or sharp transitions (e.g. a hard edge) that separate degrees of
the attenuation property of the variable filter.
[0055] In the described embodiments, the variable filers may
include a range from unfiltered (e.g 100% transmission) to
completely filtered (e.g. 0% transmission). Also, typical
embodiments of LIBS 100 would be calibrated for a given laser power
and the attenuator system of filter 210 could be used to maintain
the calibrated level of laser power delivered to a sample in
operation. For example, as described elsewhere detector 215 could
detect the power of beam 250 as it exits variable filter 210 and
adjust the position of the filter relative to beam 250 based upon a
degree of detected laser power. Also, in some embodiments a
temperature measurement may be taken using temperature detector 225
that processor 220 correlates with a known degree of change in
laser power output and subsequently adjusts the position of
variable filter 210 to compensate for the degree of change in laser
power output.
[0056] FIGS. 8-11 provide illustrative examples of embodiments of
an attenuator system that include laser 205 that produces beam 250,
and variable filter 210 (e.g. sliding variable neutral density
filter), 210' (e.g. rotating variable neutral density filter),
210'' (e.g. sliding variable reflector), and 210''' (e.g. sliding
variable edge filter). FIGS. 8 and 9 also illustrate the different
transition properties of the neutral density filters as hard edge
step 210a and soft edge continuous 210b. Also FIGS. 12 and 13
provide illustrative examples of filter spectra and 1064 nm
transmission through edge filter 210''' of FIG. 11.
[0057] FIG. 14 provides yet another illustrative example of an
embodiment of an attenuator system that includes laser 205 that
produces beam 250, and variable filter rotation plate 1420
comprising rotating variable filter 1425 with coupled rotation
plate 1430 comprising beam displacement compensator 1435 (e.g. with
anti-reflective coatings). Also FIGS. 15 and 16 provide
illustrative examples of filter spectra and 1064 nm transmission
through filter 525 and compensator 535 of FIG. 14.
[0058] Having described various embodiments and implementations, it
should be apparent to those skilled in the relevant art that the
foregoing is illustrative only and not limiting, having been
presented by way of example only. Many other schemes for
distributing functions among the various functional elements of the
illustrated embodiments are possible. The functions of any element
may be carried out in various ways in alternative embodiment.
* * * * *