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.

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 ".

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.