Leo de Galan Laboratorium vwr Analyiische Scheikunde Technische Hcgeschwl Deln de Vries van Heystplantsoen 2 2628 RZ DELFT, the Netherlands
d WTICAL ATOMIC SPECTROMETRY
Soon after its invention a successful method of analysis goes through a phase of rapid growth and exaggerated expectations before it recedes to a more balanced position in the analytical domain. Flame and furnace atomic absorption spectrometry (AAS)and inductively coupled plasma-atomic emission spectrometry (ICP-AES), as we know them now, were introduced 20 to 30 years ago, developed into commercial instruments within a decade after their first description in the scientific literature, and have now reached a state of developmental equilibrium. It is undeniable that these techniques have continued to develop, but recent advances have been largely technical and cosmetic. The emphasis on automation and software has made life much easier and has significantly reduced the demand for manpower, hut it has not enlarged the analytical scope of the techniques. Many initial promises have been fulfilled, but some shortcomings persist even today. At this point it would be easy to formulate the ideal method that determines all elements from the sub-partsper-billion level to the 100%level, simultaneously, with high precision and accuracy, and a t minimal cost. Clearly, no single method can possibly match such unrealistic expectations. In this article the author bas, therefore, chosen the more modest approach of identifying some weak points in available technology and analyzing possible 0003-2700/8610358-697A$O 1.5010 0 1986 American Chemical Society
remedies. In several cases current developments are reviewed, and novel instruments proposed in the literature are evaluated. In other cases, the problems have hardly been addressed and thus may pose a challenge for future research. MultielementAAS Textbooks on AAS stress that it is a single-element technique, because a separate hollow-cathode lamp is needed for each element. It is interesting to determine whether this is a fundamental restriction or the result of technical limitations. Provisions for automatic repositioning of up to 10 lamps offer only a partial solution, as the method remains sequential. It demonstrates, however, that the technical problems relating to the light source are surmountable. Multielement lamps can be constructed, or the radiation of many lamps can be combined with appropriate optics. Naturally, the monochromator can and must be replaced by a multichannel detector. A more fundamental objection can be raised to the use of hollow-cathode lamps for multielement AAS, either simultaneously or sequentially. Given the logarithmic relation between intensity and concentration, the dynamic range of the absorption process a t a given wavelength is restricted to a t most two decades (0.01 to 1au) and usually less. With a paucity of useful absorption transitions for some ele-
ments, this means that sample dilution must correspond to the sample composition. As a result, different samples or even different elements in one sample may require different dilutions. The problem can be alleviated if we can shift the primary wavelength from its usual position a t the peak of the atomic absorption line profile to the wings of this profile. One possibility is the use of the Zeeman technique, better known for its capabilities for hackground correction. If we use a more moderate field, we can shift the line partly from its peak position to obtain an analytical curve with lower sensitivity (slope). An elegant solution is offered by an ac-driven magnet around the furnace atomizer. When data are collected a t three rather than two field strengths, two backgroundcorrected signals of different sensitivity are obtained and the dynamic range of AAS can be extended by another decade. A more far-reaching alternative is offered by a continuum lamp in combination with a high-resolution monochromator (Figure 1).The extensive work of @Haver has amply demonstrated that simultaneous, multielement AAS is technically feasible and that the effective dynamic range can be extended to five decades by shifting the wavelength to the wings of the absorption line profile (I). A limitation remains in the far-UV region, where the intensity of current contin-
ANALYTICAL CHEMISTRY, VOL. 58. NO. 6, MAY 1986
697A
7 Furnace?
To mo
EIMAC
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Flgure 1. Two approaches to simultaneous determinations with the graphite furnace (a) SlmuI1BImw6 muniiementatomic sbsuplion w h a continuum source (7). EIMAC Mntlnum xenon arc souroe; L lens; W: q u a plate; EG echelle grating: P PTiSm; M: minu. (b) FANES (a. Relerence 4. courtesy of Kmbon Spekbalanalytik. Eohing. F.R.G.)
uum sources is too low to provide adequate detection limits (Table I). However, a remaining problem is the atomizer. If elements are to be determined simultaneously, they must be atomized under a single set of uniform conditions. Possibly, such compromise conditions can he formulated for high-temperature flames. However, in terms of analytical performance, such an instrument would meet stiff competition from ICP-AES, especially when the cost of the latter dimin-
ishes, as will be discussed later. For their superior sensitivity electrother. mal atomizers (ETAS) are a better choice for multielement AAS, but compromise conditions are much more difficult to formulate here. Until this critical issue has been decided, the fate of multielement AAS remains uncertain. l"Wovedelectrotherma' Even though a uniform temperature program for all elements may be a re-
Table 1. Clerecrion limits (ng/rnL) of estamsnea ana novel metnoas Element
A"
ICP-AES
ICP-MS
20
0.2
v
ETA-AM
SIMAAC
0.6
Zn
SIMAAC: Simultaneous multielement atomic absorption using a continu
furnace atomization ( 1 ) FANES Furnace atomization nonthermai emission spectrometry (4)
6WA
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
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mote prospect, there are other aspects of electrothermal atomization that warrant improvement. A wider range of elements, the ability to cope with a large excess of matrix, and, in general, more trust in the result of the analysis are clearly desired. No doubt, all three aspects relate to the construction of the furnace-its geometry and base material. Important steps toward this goal have already been made (Figure 2). Interestingly, the introduction of the platform technique and tubes coated with pyrolytic graphite resulted from fundamental studies on the volatilization and the dissociation processes in the furnace. Similarly, improved instrumentation and performance may be expected from a better insight into background correction techniques and the temperature distribution along the furnace. These examples demonstrate the benefits to be gained from well-executed basic research. There can be little doubt that the deuterium lamp will soon be obsolete and will be replaced by the more powerful Zeeman and pulsed lamp (Smith-Hieftje) techniques for background correction. Similarly, one can expect that future instruments dedicated to ETA-AAS will use controlled heating based on true furnace temperature, preferably referring to the inside wall. Even then, there will still be a need for an inert furnace material that can he used up to 3300 K with constant performance during 500 firings. As glassy carbon has not lived up to initial expectations, hope is now centered
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Figure 2. The Zeeman background correction technique and the inserted platform have contributed much to improve the reliability of graphite furnace AAS in the future such furnaces may be produced from total pyrolytic graphite
on total pyrolytic graphite (2). Meanwhile, there is ample room for research into high-temperature chemistry phenomena occurring a t the surface and in the atmosphere of the furnace. The necessarily limited and often ambiguous thermodynamic studies executed up to now could benefit tremendously from mass-spectrometric probing and surface analysis techniques. Unfortunately, these are expensive and rather fundamental experiments that cannot be easily financed. Perhaps this is also the place to speculate on the possibility of ahsolute (i.e., calibration-free) analysis. As indicated by L’vov, ETA-AAS is in the best position to achieve this goal. Indeed, characteristic concentrations measured with a good instrument conform to predictions based on pure physics to within 20%. This is certainly an impressive scientific accomplishment and demonstrates the simplicity of the furnace as a model source of atoms. Nevertheless, the practical significance should not be overrated. Ohviously, a similar or better accuracy can be obtained with a single calihration. More important is the question of whether the characteristic concentration (i.e., the analytical sensitivity) remains the same in the presence of a matrix. Indeed, improved resistance to interferences is one of the most desirable properties expected from future furnace materials, such as pyrolytic graphite. A real disadvantage of electrothermal atomization is the long cycle time. The drying, ashing, atomization, cleaning, and consecutive cooling of the furnace take between two and three minutes. Omission of the ashing 700A
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step brings some relief hut does not appear to be generally applicable. A drastic reduction in analysis time is possible only when the desolvated sample is flash volatilized upon introduction into a preheated furnace. Past proposals such as aspiration of sample solutions or inserted probes do not meet these requirements. Surely, the excess water is unacceptable if we are to retain analytical sensitivity, and the heating of even a small probe takes time. A principally better approach is the hanging-droplet technique, whereby sample droplets are suspended on a carbon thread that is pulled through the hot furnace. If the droplets are dried before they enter the furnace, the cycle time is indeed reduced. Unfortunately, the technique is far from practical, but the physical considerations were correct. It demonstrates that the problem is solvable, in principle. It just needs a fresh look to develop a system that is practical to use.
Furnace emlsskn specbometry The long cycle time of current electrothermal atomizers emphasizes the importance of simultaneous, multielement furnace analysis. Given the difficulty of simultaneous AAS, it is natural that efforts have been made to combine the furnace with emission techniques. Although the registration of emission signals from the furnace itself is certainly the simplest approach, the low temperature during the release of volatile elements leads to poor detection limits for high-energy transitions (e.g., Cd, As). Naturally, the literature abounds with proposals to sweep the vapor released from an electrothermal atomizer into a plasma
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
source. Both microwave-induced plasmas (MIPS) and ICPs have been used for this purpose. Unfortunately, the gain in sensitivity over straightforward ICP-AES is less than hoped for, and the long transport lines create their own prohlems. Better results can he expected from the two-furnace design of Frech (3). The sample is volatilized from one furnace into a second furnace preheated to its maximum temperature. Although designed to separate volatilization and dissociation processes in furnace absorption, the combined furnace could have potential for furnace emission. The same principle underlies the approach taken by Falk (4). In this approach, the vapor released from a graphite furnace is postexcited in a radio frequency hollowcathode discharge. The entire assembly operates a t a low pressure of a few torr, which accounts for a high effective excitation temperature in the nonthermal discharge. This phenomenon is expressed by the acronym FANES, coined by Falk for his technique-furnace atomization nonthermal emission spectrometry (Figure 1). Impressive detection limits have heen reported for many elements including some nonmetals (Table I), but much more research is needed to establish this technique’s simultaneous multielement potential. As in multielement AAS, uniform atomization conditions are a prerequisite. Also, the fear of enhanced interferences arising from the nonthermal hollow-cathode discharge should be alleviated. High-resdution ICP Spectral interferences are still the major source of error in analyses with the ICP when it is used as a source of optical radiation in AES. Surprisingly, in this age of automation, we must still rely on human judgment to recognize and avoid them. Fancy software will help by projecting spectra and calculating corrections, but we are still a long way from the situation in which a novel sample can he safely trusted to automated initiation and optimization of the analysis. At first sight, higher resolution seems the answer to spectral interference prohlems. Indeed, echelle spectrometers are available, and a Fourier transform UV spectrometer has been described for this purpose. Unfortunately, spectral interferences do not diminish in proportion to the vastly superior resolution offered by these instruments. In fact, soon after we surpass the resolution of 0.01 nm provided by conventional grating instruments, we meet the natural boundary set by the physical width of spectral lines emitted by the ICP, which varies from 0.003 to 0.02 nm. It is therefore
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doubtful whether high-resolution instruments warrant the additional expense. It would appear that a much simpler solution is possible. Overlapping spectral curves are not unique to ICP-AES but are well-known in IR spectrometry, neutron activation analysis, and energy-dispersive X-ray fluorescence spectrometry, to name only a few. Curve resolution techniques developed successfully in these areas should be equally profitable in ICP-AES. Improved data processing could also solve the related problem of background correction, which is still performed either on-line or off-line at discrete wavelengths selected by the operator. Recently, Taylor has adapted software routines from neutron activation analysis to assess the spectrum around an analysis line in each individual sample (5).Superior background correction and removal of interfering lines were achieved at the cost of a longer time needed to scan the wavelength region of interest. However, Taylor’s suggestion that multichannel (e.g., photodiode array) detectors could overcome the time disadvantage seems premature. Current photodiodes are inferior to the photomultiplier tube in dynamic range and in sensitivity, certainly in the far UV. A more fundamental objection is the pixel size of 25 pm, which corresponds nicely to the width of the entrance slit but does not allow collection of many intensity data across the spectral line as is needed for curve resolution. If we are to collect spectral data with 0.001-nm step size, we need either a very high dispersion monochromator or much smaller pixels of 1pm. The latter are not yet available. ICP-AFS and ICP-MS
When we realize that spectral problems in ICP-AES arise from the density of emission spectra, then we should look at alternative registration systems. Despite their high selectivity, absorption techniques appear useless because the current weak primary sources cannot overcome the intense radiation of the ICP. Also, we would give up multielement capability. Atomic fluorescence spectroscopic (AFS) techniques retain multielement capability and offer remarkably empty spectra. Unfortunately, the primary source of radiation is again their weak point. Continuum sources would be preferable because they stimulate all elements simultaneously, but even the brightest continuum source available is much too weak. Hollow-cathode lamps are cheap enough to arrange a collection of 20 lamps around the discharge as is done in the only commercia1 ICP-AFS available. Unfortunate702A
ly, hollow-cathode lamps have limited intensity, especially for the ion lines that are most useful in ICP analysis. Indeed, when used for AFS, the ICP discharge is run under uncommon conditions to promote the atom fraction over the ion species. Even then, detection limits are inferior to ICPAES for many elements. Lasers are the obvious answer to our prayers, but a dependable, tunable, low-cost laser that reaches the far UV is still not available. Given the high degree of ionization of most elements in the ICP, it is not surprising that mass spectrometric detection receives much attention. The work of Gray in the United Kingdom and Houk in the United States (6) has been remarkably rapidly transformed into commercial instruments. Potentially, the mass spectrometer promises cleaner spectra and a higher and more uniform sensitivity (Table I). Inevitably, the first applications currently reported in conferences present a more sobering view, but there is still cautious optimism that the expectations will be borne out. Undoubtedly, the interface between the atmosphericpressure, high-temperature ICP and the low-pressure quadrupole mass spectrometer plays a crucial part. Apparently, the efficiency by which the small pinhole extracts ions from the ICP is influenced by the sample composition, which gives rise to chemical interferences not known in ICP-AES. This problem seems surmountable. Molecular fragments may pose more difficulties, as a simple calculation shows. If matrix constituents are atomized to 99.99%, then the 0.01% fraction of molecules still forms a large excess in comparison with the trace constituents determined at the parts-permillion level. Because molecules are very weak emitters of radiation, their presence goes unnoted in ICP-AES. However, in TCP-MS the corresponding molecular ions are detected with equal sensitivity and may thus give rise to interfering mass overlap in the ICP-MS spectrum. Consequently, both the interface and the ICP itself must be optimized for virtually complete dissociation of matrix constituents. ICP-MS is still under rapid development, and it is far too early for a definitive assessment. For one thing, the currently high price of the instrument must come down before ICP-MS becomes a real alternative to ICP-AES. Excitation sources for AES
Despite the popularity of the ICP, other sources forthe atomization and excitation of dissolved samples still receive attention, although a true alternative has not yet emerged. Apparent-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
ly, it is difficult to match, let alone beat the two major features of the ICP: low detection limits for many elements and high resistance against matrix effects. Incidentally, these favorable properties of the ICP have evolved empirically and are gratefully accepted rather than satisfactorily explained. The many, and often repetitive, studies of spatial distributions of temperature, electron concentration, and sample constituents contribute little to our theoretical understanding. Consequently it would be naive to expect a superior discharge to emerge from such studies. Curiously, potential alternatives to the ICP are generally recommended for cost-effectiveness. Of these, the dc arc discharge had the longest history before it evolved to its present configuration of three electrodes arranged as a Y. Detection power seems adequate, but interferences remain a weak point requiring the addition of ionization buffers. An interesting new proposal is an arc discharge generated inside a silica tube by three to six externally placed electrodes (7). Further development of this design must be awaited. When run on argon at atmospheric pressure, the MIP can accept a nebulized solution and detect metals. However, detection limits are barely adequate, and it is difficult to see how matrix effects can become tolerable unless the power is raised from the current 100 W to appreciably higher values. The difference with the ICP would then become marginal, in performance and in price. Perhaps this is also the place to mention the nonspectroscopic but analytically related technique of laserenhanced ionization (LEI). When an analyte is atomized in a flame and irradiated by an intense beam of resonance radiation from a tunable laser, its ionization is greatly enhanced and a current can be detected with electrodes placed around the flame. Again, detection limits are good, but interferences from other ions in the flame pose problems. More important from a practical point of view is the laser source, which increases the cost and makes the technique sequential rather than simultaneous. If cost-effectiveness is indeed a major concern, then more can be expected from novel torch designs for the ICP proper (8). By carefully restricting the annular spacing between the two outer tubes, the so-called high-efficiency torches can be operated on 1 kW power and less than 10 L/min of total argon. When the torch is cooled externally with water or air, the argon demand can be reduced to a value as low as 1L/min (Figure 3). In either
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case, the analytical performance matches that of currently available torches. In the future, external cooling may be obviated when a suitable hightemperature ceramic is found to replace the silica outer tube. It is expected that these designs will soon be incorporated into commercial low-cost ICP instruments.
sample lnboduction into the ICP If users, manufacturers, and experts of the ICP agree on one thing, it is on the need for better introduction devices for dissolved samples. The exist-
ing pneumatic nebulizer-chamher combinations are wasteful of sample and noisy. The unfortunate reappearance of the internal standard technique in atomic emission is an ominous sign. Its success in improving the analytical precision of ICP analysis from 2% to below 0.5% identifies the sample introduction step aa the major source of imprecision, hut it accommodates rather than solves the problem. To be sure, alternatives to the inadequate cross-flow and concentric nehulizers have been proposed. However, the V-groove Babington nebulizer (Figure 4) offers no better precision or efficiency, although it is superior in some other respects. The ultrasonic nebulizer has probably generated more frustrated scientists than any other piece of atomic spectrometric equipment. There are, however, alternative nebulization principles to he tested and better aspiration chambers to he constructed. If anything, this would appear to be a fruitful topic for research. On the more general theme of sample transfer to an atomic spectrometer, wide perspectives are opened by the use of flow injection techniques. In their simplest form these techniques guarantee a regular flow of carrier liquid of constant viscosity. With only slightly more complex manifolds we can provide for the addition of matrix modifiers, ionization buffers, and internal standards. By including an exponential dilutor we can provide alternative ways of calibration.
SolM samples The prospect of being released from the tedious and time-consumingsample dissolution step in an analysis recurrently induces studies into the possibility of direct introduction of solid samples. It is really not surprising that success has been limited. If, as we have seen above, we already have
problems in introducing liquid samples into the ETA and the ICP, how can we expect to cope with the much greater problems of solid samples? It is instructive to recall the hy now almost forgotten carbon arc, where a few milligrams of a coarsely ground solid sample were heated to 4000 K i n the arc electrode to evaporate freely-and erratically-over a period of several minutes. Current atomizers are lacking in either the temperature or the residence time or both. As a result, volatilization and, hence, atomization remain incomplete, depending on the composition and the size of the particles. Calibration against carefully matched standards becomes the rule, unless the particle size is reduced to about 1pm. For most samples grinding to such small dimensions is more arduous and time-consuming than dissolution. Indeed, electrothermal atomizers can cope with only a few milligrams of the most volatile, usually organic samples. Even then, sample inhomogeneity and dispensing problems account for poor precision. In the ICP, slurry techniques in conjunction with Bahington nebulizers have occasionally been used with some success, but have limited applicability. Free evaporation from carbon cups lifted into the discharge region of a high-power ICP gives rise to spectacular pictures but doubtful analytical results. A better approach is to separate the evaporation of solids from their excitation. Lasers and high-current sparks have been used to generate a vapor that is swept into an ICP for consecutive excitation and detection. Although an already available ICP can be exploited in this way, it is a little foolish to buy one for this purpose. Sputtering techniques are used much more successfully in specially designed instruments, such as the glow discharge and, of course, the spark spec-
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Figure 4. Nebulizers for the ICP have evolved from cross-flow to Concentric and V-groove models, but further improvement is needed 704,.
ANALYnCAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
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trometer itself. Harrison and, more recently, Caroli (9) have reported interesting results for solid samples excited in a low-pressure hollow-cathode discharge. This approach warrants further attention. And let us not forget that X-ray fluorescence spectrometry is an excellent method for analyzing solid samples.
w#nnaclls All optical atomic spectrometric methods of analysis primarily address metallic elements. Again, it is interesting to analyze whether the exclusion of nonmetals is attributable to fundamental restrictions or to technical limitations. Atomization should not be difficult. If metals can be dissociated from their chemical environment, then the halogens, phosphorus, and sulfur should also be atomized to a high degree. Carbon, hydrogen, nitrogen, and oxygen associate mutually more strongly. However, for these common elements the background levels from impurities in the feed gas and from the solvents usually prohibit low limits of detection. Invariably, spectral transitions from nonmetals have a high excitation energy. Excitation temperatures of 6000 K, reached in (approximately) thermal sources operated a t atmospheric pressure, are too low to generate sufficient emission intensities. Indeed, the detection limits for I, P, and S in the ICP are substantially higher than the nanograms-per-milliliter values cited for most metals. Incidentally, ICP-MS offers no respite, because efficient ionization of nonmetals is prevented hy their ionization energies, which are also high. Much higher electronic excitation temperatures can he reached in nonthermal sources, especially when operated at reduced pressure. For example, effective excitation temperatures an high as 100,OOO K have been reported for MIPS operated in a few torr of He at low power (50 W). Unfortunately, the kinetic temperature of this and similar sources is very low (
