U.S. patent number 3,715,163 [Application Number 05/065,822] was granted by the patent office on 1973-02-06 for apparatus for simultaneous multielement analysis by atomic fluorescence spectroscopy.
This patent grant is currently assigned to Technicon Instruments Corporation. Invention is credited to Douglas Mitchell.
United States Patent |
3,715,163 |
Mitchell |
February 6, 1973 |
APPARATUS FOR SIMULTANEOUS MULTIELEMENT ANALYSIS BY ATOMIC
FLUORESCENCE SPECTROSCOPY
Abstract
Apparatus for processing a plurality of successive electrical
signals of varying magnitudes comprises an amplifier arrangement
whose gain is varied in phase with the signal being then received,
whereby the output is maintained within a predetermined range the
amplifier arrangement is operated under its optimum load
conditions. The amplifier arrangement is designed in junction with
spectroscopic apparatus for effecting simultaneous multielement
analysis.
Inventors: |
Mitchell; Douglas (Tarrytown,
NY) |
Assignee: |
Technicon Instruments
Corporation (Tarrytown, NY)
|
Family
ID: |
10423830 |
Appl.
No.: |
05/065,822 |
Filed: |
August 21, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Oct 25, 1969 [GB] |
|
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42,300/69 |
|
Current U.S.
Class: |
356/317; 356/323;
250/366 |
Current CPC
Class: |
G01N
21/6404 (20130101) |
Current International
Class: |
G01N
21/64 (20060101); G01j 003/34 (); G01n
021/52 () |
Field of
Search: |
;250/71R,218
;356/85-87,93-98,187 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dagnall et al.: Talanta, Vol. 13, June 1966, pages 803-808. .
Dagnall et al.: Analytica Chimica Acta, Vol. 36, November 1966,
pages 269-277. .
Folsom et al.: Applied Spectroscopy, Vol. 22, No. 2, March/April
1968, pages 109-114..
|
Primary Examiner: Wibert; Ronald L.
Assistant Examiner: Evans; F. L.
Claims
What is claimed is:
1. Apparatus for spectroscopic analysis, comprising an atom
reservoir including one or more atomic species, the respective
concentrations of said atomic species being of varying
concentrations, different wavelengths of optical radiation passing
from said reservoir being characteristic of individual atomic
species in said reservoir, means for detecting optical radiation
passing from said reservoir and for converting said detected
radiation into electrical signals corresponding to each of said
different wavelengths, means responsive to said converting means
for amplifying, in turn, said electrical signal corresponding to
said atomic species in said atom reservoir, and means for
controlling said amplifying means in accordance with the electrical
signal being then received by said amplifying means to vary the
characteristics of said amplifying means and maintain the output of
said amplifying means within a predetermined range.
2. Apparatus according to claim 1, wherein said electrical signals
are successively directed to said amplifying means in a
predetermined sequence, said controlling means being operative to
vary the gain of said amplifying means in phase with said
electrical signals received by said amplifying means.
3. Apparatus according to claim 1, further comprising means
associated with said reservoir for producing thermally excited
atomic emission from at least one of said atomic species with said
reservoir.
4. Apparatus according to claim 1 wherein said amplifying means is
a tuned amplifier.
5. Apparatus according to claim 1 wherein said optical radiation is
unmodulated, and said amplifying means includes a tuned amplifier,
and further comprising means for modulating said radiation passing
to said detecting means.
6. Apparatus according to claim 1 wherein said amplifying means is
a d.c. amplifier.
7. Apparatus according to claim 1 wherein portions of said optical
radiation are modulated and unmodulated, respectively, and said
amplifying means includes, at least, a d.c. amplifier and a tuned
amplifier, each of said amplifiers having a corresponding network
arrangement comprising a number of resistive paths for varying the
gain of said associated amplifier, and means for individual
controlling said network arrangements to varying the gain of said
d.c. amplifier and said tuned amplifier in phase with the signals
being then received.
8. Apparatus according to claim 1 wherein said amplifying means
includes a tuned amplifier, and further including a mechanical
chopper interposed between said reservoir and said detecting means
for modulating the radiation from said reservoir and being received
by said detecting and converting means.
9. Apparatus according to claim 1 further including means for
causing selected ones of said species to emit radiation by
thermally excited atomic emission, and means for irradiating said
reservoir for causing others of said species to respond resonantly,
so as to produce radiation emanating from said reservoir
characteristic of atomic species contained therein.
10. Apparatus according to claim 9, including means for
illuminating said reservoir in pulsed fashion, whereby radiation
emanating from said reservoir and resonantly acted upon by others
of said atomic species is modulated.
Description
Apparatus for simultaneous multielement analysis by atomic
fluoroescence spectroscopy.
British patent application No. 26250/69, of May 21, 1969 describes
an apparatus for simultaneous multielement analysis by atomic
fluorescence spectroscopy (AFS). In this apparatus, an atom
reservoir is sequentially illuminated with radiation from a number
of light sources L.sub.1, L.sub.2, L.sub.3, etc. each capable of
exciting fluorescent radiation from atomic species A.sub.1,
A.sub.2, A.sub.3, etc. respectively, in the atom reservoir. This
fluoroescent radiation is detected by a photoelectric device,
giving rise to electrical signals S.sub.1, S.sub.2, S.sub.3,
etc.
The switching of light sources and of electrical signals arising
from fluorescent radiation is controlled from a rotating filter
wheel or other device, which causes electrical pulses of
appropriate length to be produced. These pulses then operate
switches in the lamp power supply circuits, and in the signal
processing system.
This technique of pulse control of instrument operation may be used
to control further instrumental functions. The present invention
relates to several improvements in the design of an instrument for
simultaneous multielement analysis by AFS, which are made possible,
or facilitated by, this pulse control technique.
If one of the atomic species, say A.sub.1, is present at a much
higher concentration in the atom reservoir than the other species
A.sub.2, A.sub.3, etc. it may give rise to a very large signal
S.sub.1, which will overload the amplifier or some other part of
the electronic signal processing system. Ideally, all signals
S.sub.1, S.sub.2, S.sub.3, etc. should be of about the same
magnitude, so that the signal processing system can be designed
with all its components operating under optimum load
conditions.
Overloading may be avoided by reducing the magnitude of all signals
e.g. with a shutter in front of the photoelectric device, but this
will reduce the intensity of all signals, making the smaller
signals difficult to measure with adequate precision.
Alternatively, the intensity of light source L.sub.1 only may be
reduced, giving a corresponding weaker signal S.sub.1. However, the
magnitude of electronic noise arising from, for example, other
radiation from the atom reservoir will be unaffected, and the
signal:noise ratio for signal S.sub.1 will be reduced, resulting in
reduced precision of analysis for element A.sub.1.
A better method of optimizing relative signal levels is to
independently vary the amplifier gain for each type of signal, and
this can be readily achieved by pulse control techniques.
Reference is made to FIG. 1 of the accompanying drawings,
illustrating an example of an apparatus incorporating means for
such independent variation of amplifier gain. Certain features are
similar to those of apparatus described and illustrated in the
aforesaid patent application No. 26250/69, to which reference is
directed for details of features in common .
A triggering device 10, which may be controlled from the rotating
wheel 12, (similar to wheel "4" in patent application No. 26250/69
but having four filters F.sub.1 to F.sub.4 corresponding to four
different elements or atomic species) feeds a pulse to each of four
output points 1, 2, 3, 4, in turn, and these pulses cause each
light source L.sub.1, L.sub.2, L.sub.3, L.sub.4 in turn to emit a
pulse of modulated radiation. Thus light source L.sub.1 excites
fluorescent radiation from atomic species A.sub.1 in the atom
reservoir 14. This fluorescent radiation is detected by a
photoelectric device 16, which supplies an electrical signal
S.sub.1 to a preamplifier 18. The pulse from the triggering device
10 is also used to close switch SW.sub.1 in a tuned amplifier 20
for the duration of the pulse, thus bringing the amplifier 20 into
operation. The magnitude of the resistance R.sub.1 determines the
gain of the amplifier 20 and its value may be varied so as to given
an output signal at 22 of the required magnitude for further
processing by means not shown. Light source L.sub.1 is then
switched off, switch SW.sub.1 opens, there is a short rest between
pulses when the amplifier 18 does not function, then light source
L.sub.2 is switched on, switch SW.sub.2 simultaneously closes
bringing the amplifier 20 into operation, with the magnitude of
R.sub.2 determining the gain for signal of type S.sub.2, etc.
The advantages of this apparatus is that it permits the gain of the
tuned amplifier 20 to be individually set, for each signal S.sub.1,
S.sub.2, S.sub.3 and S.sub.4, to the most likely suitable value for
the above-mentioned further processing, without significantly
affecting the signal/noise ratio. Also, with some light sources it
will not be necessary to alter light source power supply controls.
These can be preset at the optimum operating conditions for each
source.
The apparatus described in patent application No. 26250/69 is
intended for analysis by AFS. For the analysis of some elements,
however, the technique of atomic emission spectroscopy (AES) may be
preferable. In this technique, the intensity of thermally excited
(unmodulated) radiation from atomic species in the atom reservoir
is measured. It may be necessary to simultaneously analyze, say,
three elements in a sample (a) by AFS only, (b) by AES only, or (c)
by a combination of AES and AFS.
The main problem in designing an apparatus for carrying out both
analytical techniques is in designing electronic systems to process
the modulated fluorescent radiation and the unmodulated thermally
excited atomic emission. This can be achieved, for example, by
adding a second filter wheel-photoelectric device unit, with a d.c.
amplifier, or by mechanically chopping the thermally excited
radiation at the appropriate frequency so that the resulting
modulated signal will pass through the tuned amplifier used for
processing fluorescence signals.
A particularly effective method is as follows. The basic electronic
system (FIG. 2d of patent application No. 26250/69, modified in
accordance with FIG. 1 of the accompanying drawings,) is fitted
with a d.c. amplifier-switching unit 24 (FIG. 2 of the accompanying
drawings). A design suitable for the simultaneous analysis of three
elements by AES or AFS as required is shown in FIG. 2. A triggering
device 10' produces pulses to switch on lamps L.sub.1, L.sub.2 and
L.sub.3 (when connected for fluorescence analysis only). These
pulses pass to switches SW.sub.1, E-F, SW.sub.2 E-F and SW.sub.3
E-F respectively, which are set to direct them to switches
SWE.sub.1, SWE.sub.2 and SWE.sub.3 in the d.c. amplifier unit 24,
or SWF.sub.1, SWF.sub.2, SWF.sub.3 in the tuned amplifier unit 20'
as necessary. The gain for both amplifiers is controlled as in FIG.
1, by selecting the magnitude of resistors RE.sub.1, RE.sub.2,
RE.sub.3, or RF.sub.1, RF.sub.2, RF.sub.3.
Consider a user who wishes to analyze elements 1 and 3 by AES, and
element 2 by AFS. The switches SW E-F are set to direct triggering
pulses to the appropriate amplifiers 20' and 24 as shown in FIG. 2.
Appropriate filters F.sub.1, F.sub.2 and F.sub.3 are chosen, and
lamp L.sub.2 is fitted. The filter wheel 12 (FIG. 1) is set in
motion. When filter F.sub.1 reaches a position in front of the
photoelectric device 16' switch SWE.sub.1 closes, bringing the d.c.
amplifier 24 into operation. Signal S.sub.1 arising from thermally
excited radiation in the atom reservoir 14 (FIG. 1) is amplified by
the d.c. amplifier 24 and passes to a gating device 26, which
directs it to an electronic filter and read-out circuit (not shown)
for element 1. Switch SWE.sub.1 opens, there is a pause during
which both amplifiers do not operate, then pulse 2 from the
triggering device switches on lamp L.sub.2, and closes switch
SWF.sub.2, and the resulting modulated signal S.sub.2 is amplified
by the tuned amplifier 20', demodulated at the phase sensitive
detector 28, and passed to the gating device 26.
The advantages of this instrument design are (a) It is an extremely
simple and cheap modification of the basic instrument (as described
in patent application No. 26250/69 ), requiring an additional
amplifier, and some switching circuitry only.
b. It gives simple switch selection of the AES or AFS techniques as
necessary.
Several types of light source may be used in AFS. One of these
light sources, the high intensity hollow cathode lamp (HIL) is
constructed with an anode and a hollow cathode containing the
element (for example, element 1,) whose characteristic radiation is
required. A plasma discharge between these electrodes produces a
cloud of atoms A.sub.1 of element 1, and excites some of these
atoms (primary discharge). A secondary anode and cathode (or a
secondary cathode and a common anode) produce an auxiliary
discharge which gives auxiliary excitation of atoms A.sub.1
(secondary discharge).
The instrument described in patent application No. 26250/69
requires pulse modulated light sources. HIL's may be pulse
modulated by:
a. operating a d.c. secondary discharge and a pulse modulated
primary discharge, FIG. 3a diagrammatically illustrating the
voltages applied to the primary electrodes (at (ii)) and the
secondary electrodes (at (i)) in this case;
b. operating a d.c. primary discharge and a pulse modulated
secondary discharge, the primary and secondary voltages for this
being diagrammatically shown in FIG. 3b at (ii) and (i)
respectively; and
c. pulse modulating both discharges, probably with the primary
discharge maintained at a low d.c. level between pulses, the
primary and secondary voltages for this being diagrammatically
shown in FIG. 3c at (ii) and (i) respectively.
All of these modes will give pulse modulated radiation. However,
method (a) will give a low degree of modulation, since the atomic
cloud will not have time to decay away between successive waves and
will be continuously excited by the secondary discharge.
Method (b) will give a high degree of modulation, but the pulse
light intensity will be limited by the need to operate the primary
discharge at reasonably low currents to avoid overheating the
hollow cathode.
Method (c) has the advantage of permitting both primary and
secondary discharges to be operated at high pulse currents at
reasonably low mean currents. The primary discharge will produce a
high concentration of atoms A.sub.1 in the vapor phase without
overheating the hollow cathode, and the secondary discharge can
provide auxiliary excitation, without overheating the lamp.
This mode of operation can be developed further. The atoms in the
vapor phase produced by the primary discharge decay away very
slowly between successive waves of the primary discharge. The
atomic concentration produced is limited by the need to maintain
the hollow cathode at a low mean temperature. It is advantageous,
therefore, to pulse modulate the primary discharge, not with square
waves (that is, waves with unity "mark/space ratio",) as
illustrated in FIG. 3c, but with the wave form shown at (ii) in
FIG. 3d, in which the primary electrode voltage has a low
mark/space ratio, that is, with the durations of voltage pulses
less than the durations of intervals between pulses. This will
permit very high pulse currents for the primary discharge at low
mean current levels. The secondary discharge is square wave
modulated as before, as shown at (i).
It is advantageous, even with hollow cathode lamps which do not
have an auxiliary discharge, to use the voltage waveform shown at
(ii) in FIG. 3d. These lamps can be operated at very high pulse
currents, and the additional time between successive waves permits
light intensity to decay to lower levels than with square wave
modulation, giving a higher degree of modulation.
All types of light source are subject to drift in intensity value
which, if not corrected for, will result in analytical error. This
correction can be carried out in several ways; for example by using
a double beam system with a monochromator, which is inconvenient,
or by frequent calibration, which is time-consuming.
A particularly simple technique well suited for correcting drift in
the instrument for simultaneous multielement analysis by AFS is
illustrated in FIG. 4.
FIG. 4 shows a four channel instrument using light sources L.sub.1,
L.sub.2, L.sub.3, and L.sub.4. Small photodiodes or other simple
small photoelectric devices D.sub.1, D.sub.2, D.sub.3 and D.sub.4
are placed as shown to receive radiation from the lamps. They are
connected in parallel and to a tuned amplifier 30 as shown.
When lamp L.sub.1 is switched, it emits a pulse of modulated
radiation and device D.sub.1 gives an output signal So proportional
to the intensity of the radiation it receives. Fluorescent
radiation excited from the atom reservoir 14" passes through filter
F.sub.1 in filter wheel 12" and is detected by a photoelectric
device 16" usually a photomultiplier, giving a signal S.sub.1. Both
signals are amplified (by amplifiers 30 and 18"/20" respectively)
and processed as necessary, fed to a ratioing circuit 32 which
gives an output signal proportional to S.sub.1 /So and then to a
gating device 26". Alternatively, the signal So may be used to
directly control the intensity of the light sources, using a
feedback system. For example, signal So could be used to control a
lamp current limiting device, such that an increase in So would
cause a compensating decrease in lamp current and hence lamp
intensity. Signal So could also be used to vary amplifier gain in a
similar manner. Pulses of radiation from lamps L.sub.2, L.sub.3 and
L.sub.4 are similarly treated.
The advantages of this instrument design are: (a) THE RATIO S.sub.1
/So is approximately independent of drift in lamp intensity. (b)
the design illustrated in FIG. 4 with photodiodes connected in
parallel is cheap and simple.
In the description and drawings, like devices have like references
numerals, distinguished (except for the lamps) by indices ' and
".
* * * * *