Instrumentation S. R. Koirtyohann M. L. Kaiser
Deparlment 01 Chemistry and Thm Environmental Trace Substances Research Center University of Missouri Columbia. Mo. 85211
Furnace Atomic AbsorDtion
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A Method Awroaching Maturity The seven ages of an analytical method are conception, verification, instrument development, maturity, applications, broad aceeptance, and senescence, according to H.A. Laitinen (I).He stated that a method matures when “detailed studies of principle and mechanisms are pursued with the aid of improved instrumentation.. . This stage represents the crest of analytical research as distinguished from instrumentation research.” From one perspective this accurately describes the current status of furnace atomic absorption (FAA), but there are two ways in which the method doesn’t act its age. Applications, an age that normally follows maturity, have been quite extensive for well over a decade. Also, one would expect a mature method to have a generally accepted name. Various names are found in the literature, many of which we object to because of negative descriptors (flameless, nonflame) or because they are lengthy and indirect (electrothermal atomizers, heated graphite atom-
.
izers). The name we use here is simple, accurate, and direct. Besides, what is wrong with d i g a furnace a furnace? The first description of a furnace designed for analytical atomic absorption was by L’vov (2)in 1961.Later, Woodriff and Ramelow (3)described a different furnace design. Neither of these led to popular acceptance of the method because L’vov’s work came too early and Woodriff‘s design was complex. M w m a n n ( 4 ) introduced a much simpler furnace in 1968, and modifications of it became the basis for commercial development and popularization of the method. Massmann’s furnace was similar to earlier ones in providing sensitivities in the picogram range, but it differed in that it was heated cyclically rather than having a constant high temperature. Temperature cycling simplified the desinn. reduced nower COWWDtion, ana k a d e the iystem less prone to contamination problems, but at a price. Vaporization,atomization, and
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Electrical Connection
measurement were not separated in time or space. This resulted in severe background absorption problems and matrix-dependent changes in working curve slope. In a sense, it represented a step backward to some of the problems of arc-spark spectroscopy,which had been happily left behind as people converted to flame AA. To make matters worse, furnaces were in the hands of a new generation of users b“an had never seen a spectrograph) who were quite unprepared to cope with these effects. Background problems were handled reasonably effectively by simultaneous continuum source correetors (5,6),though limitations of that approach soon became evident. Many attempts to overcome matrixdependent working curve slope problems were made, but most labs simply lived with them, relying on standard additions for quantitative work. Develooments in several areas have brought dramatic improvements in FAA performance, starting about five years ago and accelerating in the last
mctricel Connection
Light Path
GraphiteTube
Floure 1. Schematic illu8tration of a fwnace for atomic absorption 0003-2700/82/A351-1515$01.0010 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
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two to three years. These changes as well as a look at probable future developments are the topic of this paper. Operating Principles The principles of FAA are quite simple. An electrically heated tubular furnace, shown schematically in Figure 1, is placed in the sample beam of an AA unit. The tube, usually pyrolytically coated graphite, is resistively heated through water-cooled contacts. I t is protected from atmospheric oxidation by an inert gas, usually argon, flowing over the surface or through an enclosing chamber. Typical tubes are 3-8 mm i.d. and 10-30 mm long. Electrical resistance is low, which necessitates a power supply capable of delivering several hundred amps at 10-12 V. Operationally, 5-50 pL of sample solution is placed on the inner tube wall, which is then heated in three or more stages. The “dry” conditions are chosen to evaporate the solvent as quickly as is practical without spattering (110 “C for 30 s). The second “char” stage (350-1200 “C for 45 s) removes volatile sample components at a temperature as high as practical without loss of analyte, which is then quickly vaporized and atomized in the third stage (2000-3000 “C for 5 s). The atoms rapidly diffuse out of the observation zone, and the result is a brief absorption peak, the height or area of which is used for quantitation. The rapid loss of atoms imposes two demands on the system. First, the heating for atomization must be fast, to form atoms quickly and build up a high population. The furnace power supply must be capable of heating rates of 1000 “C/S or greater. Second, the spectrometer’s electronics must be fast because peak widths as small as 0.1-0.2 s are sometimes obtained. Most older AA units are incapable of following such a peak without severe distortion. Even with distortion, however, the method produces detection limits for most metals in the picogram range and is capable of 1 2 % precision. The improved response time available in many spectrometers today will be very important to researchers who wish to study details of atomization behavior, but routine users need not throw their old instruments away to get good results. Commercial Contributions Improvements in commercial instrumentation have made significant contributions to the current status of FAA. One of the more important, from the operator’s perspective, is automated sample injection. This might seem trivial a t first, but hand injection requires the operator’s attention for perhaps 15 s of a 2-min cycle. Sitting at an instrument doing nothing seven1516A
* ANALYTICAL CHEMISTRY,
eighths of the time is extremely tedious. Automation also improves precision. Furnaces have evolved from relatively open devices-a graphite rod just below the optical path in the extreme case-to enclosed designs with end windows. The sample is surrounded by the furnace, and gas flows are directed to minimize condensation on cool furnace parts. Most power supplies, in addition to providing sufficient power for rapid heating, are now microprocessor-controlled and provide considerably more versatility in selection of the heating cycle than earlier models. Most early models provided constant power within each portion of the cycle. Selection of a low final temperature dictated a slow, nonlinear temperature rise. Lundgren et al. (7) removed this limitation by providing an optical sensor to view the outside of the tube. Full power was applied initially, and sensor output was used to reduce power when a selected temperature had been reached. Though details vary with manufacturer, most current furnaces can be made to heat rapidly to a preselected temperature and then to hold for the remainder of the atomization cycle. Continuum source background correctors fail if background absorption is excessive or if the background is structured within the bandpass of the instrument. In these cases the Zeeman effect can be used to obtain a superior correction. If the source (8) or the atomizer (9) is operated in a magnetic field, the resonance line is split into three or more components that can be separated by their polarization behavior and used to get background-corrected absorbances. In practice, operating the atomizer in the magnetic field causes fewer problems and is the mode chosen for commercial development. We refer the reader to other sources (9,10) for a detailed explanation of the Zeeman effect and its use for background correction in AA. Advantages in furnace work include: A single standard light source is used. Problems due to misalignment of two sources do not exist. No extra source noise is added. 0 Background correction is at the precise wavelength of the line. Background structure is usually not a problem. Background absorbances as high as two can be tolerated in some systems. Atoms outside the magnetic field give no response. Troublesome effects from the furnace ends are less serious. Spatial information can be obtained using a restricted magnetic field. Disadvantages include added cost and complexity, reduced sensitivity for many transitions, and shortened linear range. I t has also been shown (11)that, in flames, systematic errors
VOL. 54, NO. 14, DECEMBER 1982
can be encountered due to Zeeman shifts in the rotational spectra of diatomic molecules. Currently Zeeman systems are available from only two manufacturers, but other companies are interested and are likely to introduce instruments soon. Zeeman and continuum source background correction systems may soon have competition. Smith from Instrumentation Laboratories and Hieftje from the University of Indiana (12) have described an elegantly simple approach that has the potential for good performance. They used high current pulses to the hollow cathode lamp to exploit the long-known fact that resonance lines become broadened and self-reversed a t high lamp current. The lamp was operated at normal current for measurement of atomic plus background absorption. Then a brief high current pulse (typically 100 mA for 300 ps) was used to create a broadened, reversed line. Atomic absorption was greatly reduced because the source line profile was then wider than the absorption line. The line broadening had little if any effect on background absorption, and the difference between low and high current absorbances provided the background-corrected data, which indicated very efficient background compensation. Potential advantages of the Smith-Hieftje method include simplicity, no source alignment problems, background correction at a wavelength very close to the absorption line, and applicability to any atomizer. Disadvantages are potentially shortened lamp life (which has been addressed by lamp redesign), somewhat reduced sensitivity, and the fact that electronic constraints will probably preclude retrofits on existing instruments. Methods for Reducing Matrix Effects As mentioned earlier, the Massmann-type furnaces have been plagued by troublesome-to-severe matrix-related problems in the atomization process. The sensitivity was needed, and no better furnaces of acceptable simplicity were available. Therefore, researchers have been busy over the past 8 to 10 years developing techniques for reducing chemical interferences. Matrix Modification. One major advance was the introduction of matrix modifiers by Ediger (13)in 1974. This technique involves the use of chemical agents which, when added to the sample aliquot, alter the charring and atomization process of sample components in one of two basic ways. First, compounds such as NH4N03 have been used to form more volatile species with matrix constituents, par-
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Figure 2. (a) The modified L‘vov plat-
form. SMe view of the platform position within the graphite tube. Tube dimensions, 28 X 6 (i.d.) mm. Platform dimensions, 7 X 5 mm. (b) The modified L‘vov platform, end view ticularly the halides, thus helping to e l i n a t e interfering agents during the char step. Second, the addition of specific modifiers, including phosphate in the form of HsPO, and NH&P04, allows the use of higher
charring temperatures by reducing analyte volatility through the formation of more thermally stable analyte species. Organic modifiers have also been utilized to improve analyte response, presumably by providing an intimate mixture of carbonaceous reducing agents with the sample. In addition, chemically active gases have been introduced inm the internal gas flow of the system. Oxygen has been used to aid in the charring process (14). This causes a delay of analyte atomization due to chemisorbed 02 on the active carbon sites of the graphite furnace as determined by Holcomhe’s group (15).Chemical modification has helped to minimize matrix effects, but not to the extent that the need for standard additions has been removed. Atomizer Surfaces. Surface effects are important in atom formation. Sample losses can occur due to diffusion through the porous graphite walls of the furnace. This deficiency has been minimized by coating the tube with a thin layer of pyrolytic graphite, which not only reduces porosity hut improves performance in other ways. Recent improvements in the quality of graphite used for the furnace tubes and the pyrolytic coating have greatly improved FAA performance (16). Carbon plays an important role in the reduction of metal oxides by the furnace. Unfortunately, some analytes form refractory carbides that reduce sensitivity. Metal carbide-coated surfaces have been explored to improve the atomization efficiency of carbide formers. Surfaces of Mo,Ta, Zr, and La carbide have been successfully applied to the tube interior, reducing physical contact between the analyte and graphite wall. Several investigators have also reported marked im-
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L Figure 3. Ratio of standard additions curve slope to working curve slope in dilute H N 4 for lead in selected NBS materials. Atomization was from the tube wall with out matrix modification and from the modified L‘vov plalform with NH4H2P0. modi-
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ANALYTICAL CHEMISTRY, VOL. 54. NO. 14, DECEMBER 1982
provements in sensitivity and performance for selected elements when utilizing metal furnaces of Mo (17),W ( I @ , and Ta (19). The “Pseudo” Constant-Tempera t u r e Furnace. The constant-temperature fumace designed by Woodriff (3)does not show severe matxix effects. Consequently, commercial furnaces have been modified to better approximate constant temperature performance without loss in convenience. In L’vov’s initial design ( Z ) , the sample was dried on the end of a graphite rod and then introduced into a preheated graphite tube that was nearly isothermal in time and space. For commercial furnaces he suggested a modification (20)in which the sample aliquot is placed on a tungsten wire and brought near a warm furnace for drying and charring. The wire and dried sample are then inserted into the graphite tube after a high temperature has been reached. Significant reductions of matrix-related effects on analyte response have been reported (16,21).Difficulties of the method include loss of operational convenience, increased potential for contamination, and increased potential for error due to blockage of the light beam by the wire. In 1978, L’vov (22) suggested placing the sample aliquot on a small graphite platform within the Massmann furnace. The temperature of this platform, heated primarily by radiation, lags behind that of the tube wall. Hence, vaporization is delayed until the atmosphere reaches a high and nearly constant temperature. The high temperature leads to efficient decomposition of molecules and reduced v a p o r - p h e interferences. The constant-atmosphere temperature reduces the effects of matrix-dependent variations in atomization temperature. Slavin and associates (16) a t Perkin-Elmer Corporation have presented successful results in the application of platform FAA in reducing matrix interferences for the more volatile analytes, Pb, Cd, and TI. Limited success was reported for Cu, Mn, and Sn until matrix modification was also employed. Work in our laboratory has resulted in a modified platform made from furnace tubes (Figure 2). Matrix modification and these platforms were used to virtually eliminate matrix effects for nine different elements in a variety of sample types (23).An example of data obtained for lead in selected NBS standard reference materials is presented in Figure 3. The ratio of the slope of the standard additions plot for the sample to the working curve slope for standards in dilute HN03 is used as an index of matrix effects. A ratio of less than 1.00 indicates suppression and more than 1.00
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Table 1. Precision lor Lead Using Pyrolytically Coated Graphite Tubes * " ~ U M
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indicates enhancement; near 1.00 metrix effects are adequately controlled. With the platform and matrix modifi. cation, the slope ratio is near 1.00 for all samples. Similar success has been obtained for Cd, As, Cr, Co, Ni, AI, Sn, and TI in fresh waters, NBS materials, clinical samples (blood, liver, and urine), sludge, and sediment samples. Also, a dramatic improvement in sample precision, as presented in Table I, was observed by this group with platform FAA. The apparent reason for the precision improvement is purely physical. The sample cannot spread toward the cooler furnace ends when placed on the platform. An interesting approach that is similar to the L'vov platform is wed by the researchers at Instrumentation Laboratories (24). They deposit sample solution as an aerosol onta a microboat held at slightly over 100 'C in the furnace tube. The aerosol dries on contact. restricting the opportunity for chemical interaction and large crystal formation. Generally, matrix interference control is similar to that with the platform but there are interesting differences that have not yet been investigated. The data do indicate that the physical method of samd e deDosition can be imnortant in Lrtaii cases. Holcombe (25)has described a variation of the platform idea in which a rather massive piece of graphite extends into the furnace through a slot cut in the rear of the tube. The analyte and other condensable species of the sample are vaporized from the tube wall during a high-temperature char step and allowed to condense onto the cooler second graphite surface. During the atomization stage, the analyte is revaporized from this surface into a hot and nearly isothermal environment. The procedure allows for extremely high char temperatures to be utilized, thus increasing the opportunities for separating the analyte from gaseous thermal decomposition products of the matrix. Holcombe re-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
ports improved peak height sensitivities and reduced interferences from sulfate matrices on Sn response. Further work is needed to develop the method, but results to date look extremely promising. Woodriff and Lawson (26)have described modifications of an older model Varian furnace. The graphite tube is heated from the ends. The center portion with the sample is heated by conduction. Furnace performance similar to that with the L'vov platform was obtained for the samples tested. In our opinion, the combination of the L'vov platform and matrix modification gives the greatest, readily usable improvement in FAA to date. Platforms are easily placed in most furnaces and once installed cause no significant inconvenience. On our instrument the platform causes no light loss from either the sample or background correction beam. The platform has little effect on background absorption. The maximum sample size is somewhat restricted, depending on furnace and platform design. In most of our work peak heights were used for quantitation. Slavin (16) reports further improvement in troublesome cases by using tubes with high-quality pyrolytic coating and integration of the absorption signal.
Other Recent Developments The need for rapid heating to atomize the sample quickly has already been mentioned. Cbahabarti and coworkers (27).using suggestions made earlier by L'vov, have carried this concept further by heating the furnace tube with capacitive discharge. The tube used for their experiments was made from anisotropic pyrolytic graphite, which has the advantage of high electrical resistance perpendicular to the planes. Heating rates as high as 100 OOO OC were employed with the goal of making the time required for atomization short relative to the time for atoms to escape. The absorbance then becomea independent of the at-
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omization rate. The furnace atmosphere also heats quickly, enhancing the decomposition of gas-phase molecules. The result is slightly increased sensitivity and a drastic reduction in both matrix interferences and background absorption problems. In fact, the authors say that there is no need for background or slope corrections except in high-solid-content samples such as seawater. A potential disadvantage of capacitive heating is the need for extremely fast electronics to accurately record the signals. Holcombe and Salmon (28) have described an instrument that gives both time (1ms) and space (0.3 mm) resolution of absorbances within tl 3-mm diameter furnace. The system is being used to study the effects of vaporization, desorption, oxidation, etc. on atom populations and the detailed distribution within the atomizer. Holcombe’s studies represent the “detailed studies of principle and mechanisms” to which Laitinen referred. They are more likely to have an impact on the design of the next generation of furnaces than to dramatically change the way people use the present ones. Simultaneous multielement analysis, which is so common in emission spectroscopy, has not been significantly employed in atomic absorption. This may change based on work by Harnley et al. (29).They use a continuum source, a multichannel high-resolution echelle spectrometer, and wavelength modulation to get backgroundcorrected absorption data for 16 elements. Detection limits are comparable to conventional AA a t wavelengths greater than about 280 nm. There is a gradual deterioration toward shorter wavelengths, with zinc (214 nm) being about an order of magnitude poorer than with a hollow cathode lamp source. The linear range is also limited with their system because the effective bandpass is about the same as the line width-not much narrower, as required in the simple equations that predict linearity. They have developed a novel method of automatically shifting the wavelength of measurement to the wings of the line when high absorbances are encountered. In this way, the measured absorbance can be kept low enough to be in the linear range, even at relatively high analyte concentrations. The system seems ripe for commercial development, because it provides high-quality background correction plus multielement capability. In most laboratories furnaces are used exclusively for atomic absorption, but Ottaway and co-workers (30) have demonstrated that the furnace atmosphere is hot enough to excite useful emission from some of the more
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easily excited elements. The results look promising in spite of the fact that weak line emission must be measured in the vicinity of intense black body radiation. Also, one is attempting to measure atomic emission-a very temperature-sensitive phenomenon-in a system with a rapidly changing temperature. Platforms have a similar and perhaps more profound effect in emission than in AA because the hotter atmosphere affects both atomization and excitation. Furnaces are frequently used to vaporize samples for introduction into other emission sources, such as inductively coupled or microwave plasmas. In a different approach, Falk et al. (31) have described a furnace with provision for operating a low-pressure hollow cathode discharge inside the tube for nonequilihrium excitation of the vaporized sample. Currently, sensitivities are comparable with FAA, the linear range is much better (l@lo"), and the potential for multielement analysis is apparent. It remains to be seen if these advantages will offset the inconvenience of using a vacuum system, especially since atmospheric pressure is used for part of each cycle and 1-5 torr for atomization and excitation. A completely new way to use furnaces, magneto-optical rotation spectroscopy (MOR), is currently being studied (32,33).Space does not allow an in-depth discussion of MOR here. Briefly, MOR depends on Zeeman splitting of atomic lines and takes advantage of changes in polarization when resonance radiation passes through atomic vapor in a magnetic field. The polarization change is used to generate the signal, not to correct it as in Zeeman AA. Currently, the sensitivity reported for furnaces using MOR is comparable to FAA, and improvements seem likely. Those interested in furnace applications should be watching the developments carefully. Fundamental investigations into furnace processes are being pursued in perhaps a dozen laboratories around the world. The efforts range from mathematical modeling, to use of classical physical chemistry (thermodynamics and kinetics), to application of sophisticated instruments such as mass spectrometers and ESCA units, to the study of furnaces. Current knowledge of atomization mechanisms indicates great complexity involving physical and chemical properties of surfaces as well as analyte compounds, adsorption and revaporization, diffusion limited by constraints of graphite structure, and gas-phase interactions. As these processes are better understood, wiser furnace designs and use should become possible.
Future Developments For the many reasons cited above. FAA will avoid the seventh age of anaa lytical method-senescence-for number of years. Furnaces are inherently simple and, if a way can be developed commercially to make them multielement devices, they should remain competitive. Based on findings of fundamental research now under way, we expect development of a new generation of furnaces that will solve most remaining atomization problems. Furnaces will remain tools for ultratrace metal determinations. Flames and plasmas will be the atomizers of choice for higher concentrations.
Hamed. H. A,; Bertels,P. C. Anal. Chem. 1981.53.444. (28) Holcombe, J. A.; Salmon, S. A. Anal. Chem. 1919.51.648. (29) Harnley. J. M.;O'Haver. T. C.;Golden, B.; Wolfe. W. R. Anal. Chem. 1979, 51,2001. (30) Ottaway. J. M.; Bezur, L.;Marshall, J.,Anolyst 1980,105,1130. (31) Falk, H.; Hoffman.E.; Ludke. Ch. !prochim. Acta. Part B 1981,36B, I",.
(32) Kitagawa, K.;Shigeyesu, T.; Takeuchi, T. Analyst 1978.103,1021. (33) Hirokawa, K. Anal. Chem. 1980.52, 1966.
Acknowledgment We appreciate the use of facilities of the Columbia National Fisheries Research Laboratory, where one of us (SRK) was on leave during preparation of this paper. References (1) Laitinen. H. A. Anal. Chem. 1913.45. 2305. (2) L'vov, B. V.Spectrochim.Acta 1961.
.
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(3) Woodriff, R.;b e l o w . G. Speclrochim. Acta, Part B 1968,238,665. (4) Massmann, H. Spectrochim.Acta. Part R 1968,238,215. (5) Koirtyohann. S.R.; Pickett. E. E. Anal. Chem. 1965,37,601. (6). Kahn. H. L. At. Absorpt. Newsl. 1968.
1269 A. (11) Massmann, H. Talanta,in press. (12) Smith. S.;Schleicher, R. G.; Hieftje, G. M. Paper Number 442,Pittsburgh Conference,Atlantic City, N.J., 1982. (13) Ediger, R. D.; Peterson.G. E.; Kerber. J.D. At. Absor t Newsl. 1974.13,61. (14) Beaty, M.; farnett, W. At. Spectrosc. ioun ., I 77 (15) Salmon, S.G.; Holcombe. J. A. Anal. Chem. 1982,54,630. (16) Slavin, W. Anal. Chem. 1982,54,685
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Suzuki, M.; Ohta, K:; Yamakitn, T.; Kat". T. Soeetroehrm. Acto. Part B
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(18) Puschel, P.; Formanek, 2.;Hlavac. R.;
Kblihova. D.; Sychra,V. Anal. Chim. Acto, 1981.127,109. (19) Styris. D. L.; Kaye, J. H. Anal. Chem. l982,54.864. (20) L'vov, B. V.; Pelieva, L. A. Zh. Anal. Khim. 1978.33,1572. (21)Cedergren, A,; Frech, W.; Lundberg, E.; Persson,J. And. Chim. Acto 1981, 128.1.
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1524A * ANALYTICAL CHEMISTRY, VOL. 54. NO. 14. DECEMBER 1982
S.R. Koirtyohann (top) started workin with atomic absorption spectroscopy in 1961 while employedat the Oak Ridge National Laboratory. In 1963, he accepted a n instructorship at the University of Missouri, where he remained after completing his PhD in 1966. He has been active since that time in research on atomic absorption methods and their application to problems of biological and enuironmental interest. He is currently professor of chemistry and research professor in the Environmental Trace Substances Research Center at the Uniuersity of Missouri.
M.L. Kaiser began work as a n analytical chemist in 1972 while employed by Analytical Biochemistry Laboratory in Columbia, Mo. She returned to the Uniuersity of Missouri in 1978 t o work under S.R. Koirtyohann for a master's degree in instrumental analytical chemistry. After completion of the MS program in 1980, she began work a t the Enuironmental Trace Substances Research Center as a research scientist. Current research interests include furnace atomic absorption improuements, inductiuely coupled plasma emission spectroscopy, and computerized data handling.
