Report Walter Savin Perkin-Elmer Corporation
Main Avenue Narwalk, Conn. 06856
mtornic m b s o r p t i o n
This paper provides my personal opinion on the status of atomic ahsorption spectroscopy, its present strengths, and its weaknesses. I also speculate on what the future will provide and indicate where the research opportunities appear to be. The paper is in two sections. First, the status of current techniques and methods is discussed, including limitations. Following this is a discussion of technological opportunities, including how some of these opportunities may be used in atomic absorption spectrosCOPY. I find it very difficult to limit my concern to atomic absorption. Those of us who are working in the field of instrumental analytical chemistry apply physical, chemical, and engineering technology to the service of analytical chemistry. We must always keep in front of us the problem we are trying to solve rather than the application of a particular technology. In that sense, our work is the determination of the elements, particularly the metallic elements, hy spectroscopic means. Atomic absorption speetroscopy must be put into context with emission and fluorescence methods since all are similar and all use essentially the same technologies. In several places in this review specific instruments are mentioned as examples of the points that are being discussed. In such cases, instruments with which I am particularly familiar are used, although of course many other companies also manufacture atomic absorption instrumentation.
Current Status Flame AA. It is now just 26 years since Walsh’s classic publication (I) on atomic absorption spectroscopy, describing flame AA. In that time the 0003-2700/82/0351-685ASO1.OO/O 0 1982 American Chemical Sociely
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n
n
to
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Flgure 1. Detection limits for flame AA and ICP emission. The scale on the left is in micrograms in solution. or parts per billion
technique of flame AA has become the most widely used analytical procedure for the determination of the metallic elements. The principles of the instrumental systems now in use are virtually unchanged from those published in Walsh’s first paper. The engineering technology has greatly improved &sa result of the revolution in electronics and so the new instruments are faster and more convenient to use. The nitrous oxide-acetylene flame ( 2 ) has provided the opportunity to determine the more refractory metals. Electrodeless discharge lamps (3) are
brighter, particularly for the more volatile metals, than hollow cathode lamps recommended hy Walsh. For most metals, flame methods are almost entirely free of interference that depends upon the sample matrix. There are several important exceptions to this generalization hut they are well known and easily controlled. Usually, standards prepared in aqueous solutions are adequate. Typically, quoted detection limits can he reached in solutions that contain as much as several percent total dissolved solids in an aqueous medium. Flame AA detection limits are shown in Figure l. They are important because, most frequently, flame AA is used for trace metal analysis in complex mixtures. In the figure, flame AA is compared with the ICP. The data are complementary, and most of those metals that are insensitively determined hy AA are much more sensitively determined with the ICP. However, flame AA is also easily applied to the determination of major metallic constituents with high precision. With most conventional flame AA instruments, there is no trouble achieving a precision better than 1%. With more expensive instruments it is possible to achieve a precision of 0.2% RSD (4). To translate this precision into accurate analyses requires that the standards he made up very carefully. Typically for such purposes, standards are prepared at concentration levels that bracket the unknown samples. In such a way, flame AA is widely used in the metallurgical and mining industries for buying and selling precious metals, for controlling the content of specialized alloys, and for standardizing alloys that will be used to calibrate faster or more routine techniques. Probably the full potential
ANALYTICAL CI-EMISTRY, VOL. 54. NO. 6. MAY 1982
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I Flgure 2. Detection limits for furnace AA and hydride AA. The scale on the left is in picograms of the element for the furnace and in nanograms for the hydride technique. The furnaceuses p L sample volumes, and the hydride t e c h nique uses mL volumes
of flame AA has not yet heen reached in the application to major metal constituents in solutions. , The automation of flame AA is highly developed. For example, the Perkin-Elmer Model 5OOO coupled with its automated sample-handling system can determine six elements in 50 samples in less than 35 min, complete with standardization, etc. This calculates to he about 8.5 quantitated determinations/min or about 500/h.These modern instruments are easily coupled to elaborate computer systems for handling the massive amounts of jnformation that the instrumentation can generate. Most modern flame AA instruments use microprocessors (5).Because of the microprocessor, modern instrumentation is versatile and relatively easy to use. But we have by no means reached the full potential provided by microprocessors. The optimized choice of experimental conditions, detection of errors, and assurance of analytical reliability are all potentials that have yet to be fully exploited. This will be discussed more later. Hydride AA. A group of metals and metalloids forms metallic hydrides (6) that are gaseous at room temperature. Since many of these metals are of interest at very low concentration levels, hydride analysis has become an important analytical technique for As, 666A
Se, Bi, Sh, and several other metals or metalloids. Detection limits in nanograms are shown in Figure 2, although concentration levela are usually reported. The field has heen reviewed very competently hy Godden and Thomerson (7). While widely used, the hydride method has not yet been fully explored. Many analytical constituents inhibit the formation of metallic hydrides and produce interferences. Interference-free sample preparation systems have yet to be developed and accepted as being broadly applicable. The sample-handling and hydride generation steps are not yet fully automated. The advantage of the technique is that relatively large volumes of sample can he used, thus producing low relative detection limits. The detection limits are attractive on a mass/volume basis although they are poor compared to furnace methods on an absolute or mass basis. In a 10-mL sample, the 1-ng detection limit for As, Bi, and Se is 0.1 pg/L (pph), and larger samples can be used. Furnace AA. Probably the most sensitive analytical technique for the determination of trace metals, furnace AA was developed several decades ago by L’vov (8), a Russian physical chemist. He observed that Walsh had adapted atomic absorption to the spectrophotometric instrumentation available at that time. Walsh used a flame that provided a steady-state ahsorption signal. Spectrophotometers that had been developed for UV ahsorption analysis measured the
steady-state signal. In contrast L’vov took a very small sample and converted it completely to an atomic vapor, integrating the absorbance pulse that was thereby generated. This technique has yielded some of the best detection limits in terms of absolute amounts, picograms, shown in Figure 2. Few techniques can successfully compete with those levels. However, the pulsed nature of the L’vov furnace puts different reqnirements on the instrumentation. Massmann (9)in Germany and West (10) in England modified the L’vov furnace technique so that it would fit, more or less, on conventional AA instruments. The importance of the graphite furnace has caused the instrument manufacturers to gradually modify instrumental designs to he more nearly optimum for fumace work. The stabilized temperature platform furnace (11) more fully utilizes the potential that was earlier predicted by L’vov. Figure 3 shows the typical furnace beating program for the Massmann adaptation of the L’vov furnace where the sample is applied on the wall. When the wall of the furnace tube reaches a temperature at which the analyte will vaporize, the metal is driven from the surface into the gas phase. This temperature will vary depending upon the matrix constituents. Also, the rate at which the metal comes off will depend upon the quantity and the specific nature of the matrix constituents. No wonder, then, that the furnace technique has been characterized in the literature by numerous and complex interferences. When thii tech-
rigure a. hawing of the heating profile for the furnacetube (upper curve). The analyte signal is evolved as the wall passes through an appropriate temperature. The analyte signal is delayed (on the right) when the sample is deposited on the platform
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Figure 4. The L‘vov platform and its podtion in the graphite tube
nique is modified to add a thin graphite plate at the bottom of the graphite tube on which to deposit the sample, (Figure 4), the situation becomes more controllable. This graphite plate is heated by radiation from the walls so that the temperature of the sample on the plate is delayed relative to the wall of the tube and therefore to the gaseous vapor within the tube. Instead of volatilizing the atomic species as the temperature is changing, appropriate conditions can he found to volatilize the sample after the wall and the gas phase have reached a more stable or steady-state condition. In this situation, in which the atomic vapor is generated at a constant temperature, L’vov’s original theory is applicable. If the absorbance pulse is now integrated, the resulting integrated absorbance signal is directly proportional to the number of atoms present in the sample. This is independent of the rate at which the atoms are generated. In some samples (see Figure 5 ) the pulse will come off as a narrow, high peak and in other samples, where a more refractory matrix is present, it will come off as a lower, broader peak. Either situation can he calibrated with standards that do not depend upon the matrix. However, to take advantage of this technique, the electronics that record the integrated absorbance signal must he very fast since these peaks are generated quite rapidly and disappear quite rapidly. The electronic system used for the furnace on which we have done most of our work makes a complete absorbance reading in less than 10ms, and it does this every 20 ms. There are many papers in the literature (12,13)that show that the older, analog electronic circuits developed for flame AA, which have relatively long time constants, will yield analytical errors when used for the graphite furnace. To operate as a stabilized temperature platform furnace, the tube must
he heated very rapidly so that the platform, which is heated by radiation, will have lagged the heating cycle sufficiently for the tube and the gas to come to a steady-state temperature prior to the evolution of the analyte of interest. In practice this requires the use of a very fast heating cycle that is not available on some of the early graphite furnace systems (14,15).A heating rate in excess of 1000 O C seems necessary to achieve the degree of interference freedom that we experience. However, recent experience has shown that even this arrangement does not provide satisfactory results in all cases. Many metals in the presence of certain analyte constituents are vaporized slowly, starting at a remarkahly low temperature, thus limiting the temperature at which the matrix components can he charred away. When the appropriate atomization temperature is then applied to the furnace, the wide temperature difference that must be traversed is too large for the system to accommodate in a short enough time to permit accurate quantitation. In very early work, Ediger (16) developed the concept of matrix modification. Particular materials could be added to reduce the volatility of the analyte and thus permit the sample to he ashed at a higher temperature. This concept has been extended to most metals. It is necessary to find an appropriate matrix modifier for each element so that the char temperature may he raised to a high enough level to rid the sample of a larger proportion of the matrix. Figure 6 shows the temperature profile of the tube and ahsorhance profiles for Mn solutions containing increasing amounts of Mg(N03)z (11).The Mg(N03)~solution delays the Mn signal until the tube is more nearly at constant temperature. And fmally, for the more refractory metals particularly, the quality of the graphite will control the degree of in-
terference that is found (17).For many years graphite tubes have been coated with a thin layer of pyrolytic graphite. This is a form of graphite in which the graphite sheets are laid down layer upon layer in an extremely homogeneous way. Pyrolytic graphite layers seal the sample from the porous structure idherent in ordinary graphite materials. Thus a wide range of instrumental characteristics must he provided to obtain graphite furnace performance that is inherent in the technique. Even with such an optimized system, the amount of sample that can be handled is, in many cases, limited hy the molecular absorption and scattered light background caused by that portion of the sample matrix that could not be destroyed during the ashing step prior to atomization of the analyte. Different companies will argue that their background corrector will compensate for a particular ahsorhance signal. In our experience, the S i l l ratio is continuously degraded as the background signal increases. Thus there is considerable promise of analytical improvement with the availability of the Zeeman effect background correction system ( I S ) , which works on a very different principle from that of the continuum source background correctors that have been used np until recent times. In the presence of an intense magnetic field, the energy levels in the atom that define the wavelength of the emitted radiation are shifted slightly. This is the Zeeman effect. It is convenient to use the Zeeman effect on the analyte and to pulse the magnetic field alternately on and off.
Figure 5. The change in peak shape with increasing tlrings. The sample is 2
ng AI from a platform in a pyrolytically coated tube. While the maximum absorbance varies, the integrated absorbance (abs-s) is reasonably constant
ANALYTICAL CHEMISTRY, VOL. 54. NO. 6, MAY 1982
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are alreadv doine this. or nart of this.
1
ked% the commercial instrumenta-
I
the analysis was made accurately or, conversely, had a certain potential error. For instance, diagnostic signals can be generated that can indicate that a furnace determination might he
1
nals and provide wamingsk, prefeiablv. make annronriate corrections. This phase b i microprocessor use can accommodate instrument-based servicing of the equipment. Some computers 'Ie commercially available a'ready in which 'Iitica1 'Ie redundantly available. When the instrument diagnostic microprocessor programs sense potential failure of such a unit, the standby unit is switched in. The operator is warned to remove and service the defective component while the analytical work proceeds unimpeded. In addition, the design engineer can build monitoring circuits that will provide very valuable diagnostic information on potential instrumentation failure modes and guide the operator in their correction. These potential opportunities will occupy the minds of young computertrained analytical chemists for the next five to 10 years. Commercial implementation will absorb the largest portion of instrument company development budgets over that same period of time. Continuum AA. For many years, @Haver at the University of Maryland has been using a continuum source for AA to avoid the need for element-specific light sources. An important advantage of such a technique is that it provides the opportunity for multielement automation very much along the lines of the direct reader emission instruments. Separate detector'channels are provided for each of the elements to be determined. To get good analytical results with a continuum source, the spectral bandpass of the monochromator must be smaller than is now used in most commercial AA instrumentation, and @Haver uses the echelle monochromator. By modulating the wavelength at relatively high frequency over a narrow spectral range, a signal is generated that permits background measurements to be made. In recent work, Hamly and @Haver (20) showed that a very wide range of analytical concentrations can be handled automatically
Jure 6. The thermal profile and absorbance profiles for 0.4-ng Mn and various amounts nf Mg(NO& matrix modifier on a platform in a pyrolytically coated tube. The matrix modifier delays the Mn peak
When the magnetic field is on, the absorbance wavelength ofthe analyte is shifted slightly away from the resonance line in the source, providing a signal for background only. The difference between the two signals is background-corrected atomic absorbance. The inherent sensitivity of furnace AA is degraded only by the inability, in the case of a few elements, to move the wavelength absorbed by the analyte beyond the wavelength emitted by the source. Typically this reduces analytical sensitivity by a small amount. This arrangement provides the attractive characteristic that background correction has been accomplished at precisely the same wavelength as has been used for the analytical measurement. Thus, in certain situations where the background is structured or metals have resonance lines quite close to each other, this Zeeman effect background correction technique will eliminate the potential error.
Technological Opportunities Automation. The 500 determinationsh inherent in flame AA appear to be adequate for almost all laboratory requirements. Reliable data are generated at such a rate that the principal stumbling block is the utilization of all that information. However, hydride and furnace methods produce much slower results. Typically, the furnace can be used at a rate of 2 min/determination, or about 20 calibrated analytical resultah. Rowley et al. (19)at Colorado State University have used photodiode arrays so that several AA analyses can be done simultaneously on the same sample. In 690A
their case, they could determine six elements either by hydride oifurnace methods, increasing the sample throughput by a factor of six. This, and faster sample handling, are still needed to provide an increase in analytical throughput of at least a factor of 10 for furnace and hydride work. Optimization. The application of modern microprocessors in analytical instrumentation has followed a series of stages. In the first stage the microprocessors simply replaced conventional electronic systems that were used for processing the data. This produced more reliable and faster data. It was followed quite soon by use of the microprocessor to control logic sequences required in instrumentation. Thus the microprocessor replaced programming devices as well as manual programming. As a result of this development, all of the functions of the analytical instrument have been designed to be controlled by digital electronic signals. We are deeply within this second stage of the microprocessor revolution. The third stage involves taking advantage of the very powerful microprocessor and large data storage capacity to optimize the analytical conditions as the analysis is proceeding. First of all, this can be used to replace the complicated instructional procedures that are now required for flame and furnace AA. Instead of looking in the instruction manual for a particular analysis, operators can tell the instrument what kind of samples they have and for what elements and the expected analytical range. The remainder of the steps will be taken correctly, automatically, and rapidly. Some instruments
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by making the absorbance measurement in the wings of the absorption band a t reduced sensitivity. Thus, @Haver’s technique is being suggested as a way of automating AA measurements, particularly furnace measurements. The chief limitation of his technique is that the S/N ratio is reduced by the requirements of the continuum source, especially if the system is to be used for very fast analytical signals as in the case of the furnace. In practice, this is most serious for the elements that are determined in the far UV where the very bright continuum sources (for example, the xenon arc) are not very useful. O’Haver’s results for As, Se, and probably Zn and several other metals are not particularly attractive. Atomization Improvements. Remembering the admonition a t the beginning of the paper that our mission is the determination of metals by spectroscopic means, we must always concern ourselves with procedures for determining those metals that cannot now be determined. One class of metals presently not determinable by atomic absorption is metals requiring higher energies for atomizing the sample than are available with flames or furnaces. Of course, one alternative is to recommend the ICP for many of these more refractory metals. It is precisely thils motivation that led to the development of instrumentation that combined the ICP and atomic absorption. But the opportunity of atomizing some of these more refractory metals by simpler procedures that might be more compatible with the other advantages of atomic absorption still must be addressed. The technique of sputtering will produce an atomic vapor from solid conducting metal. Thus the atomic vapor over an alloy sputtered a t high potential is reasonably representative of the concentration of the alloy, even for metals that are very refractory such as Zr, U, W, etc. Walsh’s colleagues a t CSIRO (21) in Australia and Butler’s a t CSIR (22)in South Africa continue to explore sputtering as a sampling procedure for atomic absorption and emission. The various arcs have the capability of atomizing the very refractory metals. The cost for the components of some versions of the arcs is very small. More thought should probably be given to the possibility of atomizing metals in this way while quantitating the atomic concentration by absorption or fluorescence. Using AA will avoid the requirement for the highresolution spectrometer necessary for plasma emission spectrometry. Atomic Fluorescence and Resonance Detection. Alkemade (23) showed that the fluorescence of metals
could be stimulated in a flame using an appropriate light source. Winefordner (24) then developed atomic fluorescence as an analytical technique for determining metals. The method continues to have considerable potential and to be underutilized. The advantage of fluorescence is that small quantities of a fluorescing material produce a very small signal above a background which is, theoretically, no signal a t all. In contrast with this, trace analysis by AA requires the measurement of a very small difference between two large signals. It is often easier to develop instrumentation that will detect the small signal than it is to make a very stable measurement of a very small difference. Atomic fluorescence in a flame has been frequently demonstrated as providing greater sensitivity than AA for such far UV metals as Cd and Zn and probably for several others. A few workers including Walsh have recognized that the elements determined by the hydride technique happen to be elements that are particularly suitable for atomic fluorescence because their resonance lines lie in the far UV. A nondispersive multielement atomic fluorescence hydride instrument has been described (25).Electrodeless discharge lamps are imaged upon a heated cell in which the hydride is destroyed, producing the atomic vapor. Fluorescence radiation is observed through a simple filter and a solar blind photomultiplier. Detection limits are about one order of magnitude better than those that have been reported by AA hydride methods, shown in Figure 2. The principal limitation of atomic fluorescence or resonance detection systems is the need for very bright light sources. When the day arrives that bright tunable lasers are available in the far UV, atomic fluorescence will have a new lease on life and may become a very powerful analytical technique. Hydride Analyses. Better detection limits and more rapid throughput for hydride analyses are available using nondispersive atomic fluorescence. Automation of the full procedure is obviously feasible to increase the analytical throughput. Any transient or pulse technique is better handled by integrating the signal, just as a continuous signal, like flame AA, is better handled with a highly damped readout circuit. Thus, hydride work would benefit in greater freedom from matrix effects by using integration methods. There are reports in the literature that describe trapping methods to collect the hydride from larger samples and thus improve the concentration detection limits. Furnace. I suppose that all of us
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
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feel that the technique with which we are working is inherently more valuable than most others. Otherwise, we would presumably stop working on it and choose to apply our efforts to another goal. Thus, I see a lot of opportunity ahead for graphite furnace AA. One such AA furnace is called a probe furnace (26),which stems originally from work done by L'vov. In the conventional furnace, fast heating and the platform technique have made it possible to separate the functions of heating the furnace and vaporizing the sample. However, the separation remains somewhat tenuous. A more direct approach involves putting the sample on a surface outside the furnace. The sample is dried and charrec just outside the furnace, thus avoiding contamination of the analytical furnace itself by the char products. The furnace is then brought to the desired atomization temperature. When that temperature has heen achieved, the probe with the charred sample on it is plunged rapidly into the furnace. In our experience we can do every analysis in the probe furnace that we have been able to do with the platform. In general, we can do it with fewer constraints with the prohe furnace. Why then is such a device not yet commercially available? We must he sure that the results are sufficiently better than a well-designed platform furnace to make the considerable cost of new instrumentation affordable. Designing probe furnaces is not easy, and there are some formidable engineering problems to he solved. But, in principle, such a system has attractive possibilities. The furnace, both in its probe and platform versions, is presently limited from determining several important refractory metals because metals such as W, Ta, and Nb cannot be atomized. Conventional graphite begins to decompose rapidly to gaseous carhon at temperatures above 3000 "C. Probably with some care in the handling of the graphite tubes and some sacrifice of tube life, many of these metals will eventually be determined in a welldesigned furnace system. Already strong absorbance signals have been shown for B, U, and Ce. These opportunities must be explored. There is a slim hut not impossible chance that materiaJs other than conventional graphite could be used for high-temperature furnace work. Chakrabarti (27) has applied anotber L'vov idea using a capacitive discharge to heat the furnace. He achieved a heating rate of several thousand "C/s, bringing the wall to temperature before the atomic vapor was produced. His results achieved the same freedom from interference that were achieved with the stahilized694A
(4) Gliksman, J. E.; Gibson, J. E.; Kandetski, P. E. At. Speetrose. 1980, I , 66-61.
(5) Barnard, T.W. Anal. Chem. 1979,5J, 1172-78 A. (6) Holak, W. Anal. Chem. 1969.41, 1712-13. (7) Godden, R. G.; Thomerson, D. R Analyst 1980.105,1137-57. (8) L'vov, B. V. Speetrochim. Acta 1961, 17,761-70. (9) Massmann, H.Spectrochim. Acta 2. 5 1
20
temperature platform furnace. Houever, the signd is generated so rapidly that the electronica must be much faster than at present. This reduces the S/N ratio and complicates the instrumentation. But it remains an interesting technique. Ottaway (%), and to a lesser extent several other workers, continue to explore the option of furnace emission analysis. Detection limits from their recent emission work are compared to furnace AA in Table I and have become quite useful. The big advantage, potentially, of furnace emiasion is that it permits these very sensitive measurements to be made on a multielement basis, using very small samples. Using only microgram quantities of sample, analyses for several elements in a multichannel spectrometer were made down to picogram detection limits. That is probably about as micro as any analytical technique will be for the next several years. Speciation. There is increasing interest in the separate measurement of the different chemical forms of the metala in natural samples. In environmental chemistry and in biology it is often more important to know the species present than to know the total metal content. Metal-specific detection for gas and liquid chromatography has used plasma emission and atomic absorption systems. It is important to find a better coupling between LC and furnace AA because these coupled systems usually require the best sensitivity that can he found. A really good coupling between LC and the furnace has not yet heen reported. References (1) Walsb, A. Speetrochim. Ada 1955.7,
ioe-17. (2) Amos, M. D.; Willis, J. B. Spectrochim.
Acta 1966.22,1325-43.
(3) Barnett, W. B.; Volhner, J. M.; De Nuzzo, S. M. At. Absorpt. Newsl. 1976; 15.33-37.
ANALYTICAL CHEMISTRY. VOL. 54, NO. 6, MAY 1982
1968,23B,215-26. (16) West,T. S.; Williams, X. K.Anal. Chim. Acta 1969,45,2741. (11) Slavin, W.;Carnriek, G. R.; Manning, D. C. A n d Chem. 1982.54,621-24. (12) Siemer, D. D.; Baldwin, J. M. Anal. Chern. 1980,52,29E-300. (13) Lundberg, E.; Frech, W . Anal, Chem. 1981.53,1437-42. (14) Lundgren, G.; Lundmark, L.; Johansson, G. Anal. Chem. 1974,46,102&31. (15) Fernandez,F. J.; Iannarone, J. At. Absorpt. Newsl. 1978,17,117-20. (16) Ediger, R. D. At. Absorpt. Newsl. 1975.14,127-30. (17) Slavin, W.; Manning, D. C.; Carnrick, G. R. Anal. Chem. 1981,53,150&9. (18) Fernandez, F. J.; Myers, S. A.; Slavin, W. Anal. Chem. 1980,52,741-46. (19) Rowley, P.G.; Beaulieu, P. R.; Maglaty, J. L.; O'Keefe, K.R. Pittsburgh Conference,Cleveland,Ohio, March 1979
(zij'Hmiy, J. M.;@Haver, T. c. Chem. 1981,53,1291-98.
mi.
(21) Gough, D. S.;Meldrum, J. R. Anal.
Chem. 1980,52,642-46.
(22) Butler, L.R. P.;Kroger, K.;Wwt,
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Walter Slauin graduated in 1949 from the Uniuersity of Maryland, where he receiued his training i n physics and mathematics. He has spent all of his career in the deuelopment and application of instrumentation for chemist r y and physics and is currently working on improued instrumentation and applications in AA, LC, and fluorescence in the research department of Perkin-Elmer. S h i n served on the Aduisory Board of ANALYTICAL CHEMISTRY from 1979-81.
