Report Walter Slavin Perkin-Elmer Corporation Main Avenue Norwalk, Conn. 06856
atomic absorption spectroscopy The present a n d Future This paper provides my personal opinion on the status of atomic ab sorption spectroscopy, its present strengths, and its weaknesses. I also speculate on what the future will pro vide 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. Fol lowing this is a discussion of techno logical opportunities, including how some of these opportunities may be used in atomic absorption spectros copy. 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 engi neering 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 appli cation of a particular technology. In that sense, our work is the determina tion of the elements, particularly the metallic elements, by spectroscopic means. Atomic absorption spectrosco py must be put into context with emission and fluorescence methods since all are similar and all use essen tially the same technologies. In several places in this review spe cific instruments are mentioned as ex amples 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 (1) on atomic absorption spectroscopy, describing flame AA. In that time the 0003-2700/82/0351 -685 A$01.00/0 © 1982 American Chemical Society
Flame 0.1
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100 L Figure 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 instru mental systems now in use are virtual ly unchanged from those published in Walsh's first paper. The engineering technology has greatly improved as a result of the revolution in electronics and so the new instruments are faster and more convenient to use. The ni trous oxide-acetylene flame (2) has provided the opportunity to deter mine the more refractory metals. Electrodeless discharge lamps (3) are
brighter, particularly for the more vol atile metals, than hollow cathode lamps recommended by Walsh. For most metals, flame methods are almost entirely free of interference that depends upon the sample matrix. There are several important excep tions to this generalization but they are well known and easily controlled. Usually, standards prepared in aque ous solutions are adequate. Typically, quoted detection limits can be reached in solutions that contain as much as several percent total dissolved solids in an aqueous medium. Flame AA de tection limits are shown in Figure 1. They are important because, most fre quently, 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 in sensitively determined by AA are much more sensitively determined with the ICP. However, flame A A is also easily ap plied to the determination of major metallic constituents with high preci sion. With most conventional flame A A 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 be made up very care fully. Typically for such purposes, standards are prepared at concentra tion levels that bracket the unknown samples. In such a way, flame AA is widely used in the metallurgical and mining industries for buying and sell ing 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 CHEMISTRY, VOL. 54, NO. 6, MAY 1982 · 685 A
Furnace (pg)
Hydride (ng) Hg
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Ba
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CaPb
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100
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Figure 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 furnace uses μL· sample volumes, and the hydride tech nique uses mL volumes
of flame AA has not yet been reached in the application to major metal con stituents in solutions. The automation of flame AA is highly developed. For example, the Perkin-Elmer Model 5000 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 be 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 information that the instrumentation can generate. Most modern flame AA instruments use microprocessors (5). Because of the microprocessor, modern instru mentation 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, de tection of errors, and assurance of an alytical 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 in terest at very low concentration levels, hydride analysis has become an im portant analytical technique for As,
Se, Bi, Sb, and several other metals or metalloids. Detection limits in nano grams are shown in Figure 2, although concentration levels are usually re ported. The field has been reviewed very competently by Godden and Thomerson (7). While widely used, the hydride method has not yet been fully ex plored. Many analytical constituents inhibit the formation of metallic hy drides and produce interferences. In terference-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 auto mated. The advantage of the technique is that relatively large volumes of sample can be used, thus producing low rela tive 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 μg/L (ppb), 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 chem ist. 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 ab sorption signal. Spectrophotometers that had been developed for UV ab sorption analysis measured the
steady-state signal. In contrast L'vov took a very small sample and convert ed 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 require ments 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 fur nace has caused the instrument manu facturers to gradually modify instru mental designs to be more nearly opti mum for furnace 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 heating program for the Massmann adapta tion 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 tem perature will vary depending upon the matrix constituents. Also, the rate at which the metal comes off will depend upon the quantity and the specific na ture of the matrix constituents. No wonder, then, that the furnace technique has been characterized in the literature by numerous and com plex interferences. When this tech-
Tube Temperature
i
From Platform
From Wall
Time Figure 3. Drawing of the heating profile for the furnace tube (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
686 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
Graphite Tube
Platform
Figure 4. The L'vov platform and its position in the graphite tube
nique is modified to add a thin graph ite 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 gas eous vapor within the tube. Instead of volatilizing the atomic species as the temperature is changing, appropriate conditions can be 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 atom ic vapor is generated at a constant temperature, L'vov's original theory is applicable. If the absorbance pulse is now integrated, the resulting integrat ed absorbance signal is directly pro portional to the number of atoms present in the sample. This is inde pendent 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 sam ples, where a more refractory matrix is present, it will come off as a lower, broader peak. Either situation can be 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 be very fast since these peaks are gen erated quite rapidly and disappear quite rapidly. The electronic system used for the furnace on which we have done most of our work makes a com plete absorbance reading in less than 10 ms, and it does this every 20 ms. There are many papers in the litera ture (12,13) that show that the older, analog electronic circuits developed for flame AA, which have relatively long time constants, will yield analyti cal errors when used for the graphite furnace. To operate as a stabilized tempera ture platform furnace, the tube must
be heated very rapidly so that the platform, which is heated by radia tion, 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 °C seems necessary to achieve the degree of interference freedom that we expe rience. 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 remark ably low temperature, thus limiting the temperature at which the matrix components can be charred away. When the appropriate atomization temperature is then applied to the furnace, the wide temperature differ ence that must be traversed is too large for the system to accommodate in a short enough time to permit accu rate quantitation. In very early work, Ediger {16) developed the concept of matrix modification. Particular mate rials could be added to reduce the vol atility of the analyte and thus permit the sample to be ashed at a higher temperature. This concept has been extended to most metals. It is neces sary to find an appropriate matrix modifier for each element so that the char temperature may be raised to a high enough level to rid the sample of a larger proportion of the matrix. Fig ure 6 shows the temperature profile of the tube and absorbance profiles for Mn solutions containing increasing amounts of Mg(N0 3 ) 2 (11). The Mg(N0 3 ) 2 solution delays the Mn sig nal until the tube is more nearly at constant temperature. And finally, 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 inherent in ordinary graph ite materials. Thus a wide range of instrumental characteristics must be 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 by the mo lecular 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. Differ ent companies will argue that their background corrector will compensate for a particular absorbance signal. In our experience, the S/N ratio is con tinuously degraded as the background signal increases. Thus there is consid erable promise of analytical improve ment with the availability of the Zeeman effect background correction sys tem (18), which works on a very dif ferent principle from that of the con tinuum source background correctors that have been used up until recent times. In the presence of an intense mag netic 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 mag netic field alternately on and off.
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