Report
Fundamental Research in ICP-OES and ICPMS
E
xciting research progress is beFundamental ing made to quantitatively link i fundamental processes and pracresearch in ICP-OEStical ICP analysis (1-6). The results of such fundamental research are used to and ICPMS can improve practical analysis. Investigation and modeling of space charge effects have result in improved led to the development of ICPMS instruments that are less susceptible to matrix performance, effects. Plasma modeling has resulted in improvements in practical analysis of reliability, accuracy, semiconductor gases and development of helium plasmas for ICPMS. Some manuand ease of use facturers claim that their instrument is better than others on the basis of fundamental plasma properties such as electron number density. Further improvements are unlikely to be gained from purely empirical approaches. One of the biggest dangers in ICP analysis today is the possibility of obtaining inaccurate results without any indication of a problem (3). Results from fundamental research are needed to improve analysis accuracy, develop practical diagnostics and intelligent instruments, and reduce operator skill requirements. Shorter sample washout times, reduced nebulizer-related noise, more efficient, robust ion transport in ICPMS, and better precision are also likely to be achieved. Converting samples into signals
John W. Olesik Ohio State University 0003-2700/96/0368-469A/$12.00/0 © 1996 American Chemical Society
Four sets of processes control ICP signals: plasma dynamics, aerosol generation and transport, production of ions (and atoms), and excitation or ion transport from the ICP to the detector. Inaccuracy in practical analysis can occur when the composition of the sample solvent (acid identity, concentration, organic solvent) and the sample itself (the matrix) affects these processes.
A single drop of sample aerosol undergoes several processes as it travels through the plasma. As the drop is heated, solvent evaporates and leaves behind a particle of analyte. After the particle is sufficiently heated, it begins to vaporize (or perhaps explode). Molecules or atoms are produced; these are then atomized or ionized and diffuse rapidly. A small fraction of the atoms and ions become excited and emit light for ICP-OES. A minuscule fraction of the analyte ions (often less than one in a million) reach the detector of the mass spectrometer. Each of these processes is critically dependent on plasma characteristics, including gas temperature, electron concentration, electron temperature, and energy transport rates. Why is it so difficult to gain an accurate understanding of the fundamental processes? The answer to this question has several components. The processes overlap in time and space. Also, more than 106 polydisperse aerosol drops of sample typically enter the plasma each second. Drops of various sizes complete desolvation at different locations in the plasma. Atoms and ions produced from small drops, together with vaporizing particles and desolvating drops, can all be in the observation volume. Even for a single drop, particle vaporization, atomization, and ionization occur concurrently, although at different rates. The plasma is also spatially heterogeneous, and simple equilibrium-based models are often inadequate to quantitatively describe the ICP. Energy is coupled mainly into a donut-shaped region near the load coil. Heat and chemical species are transported from this "energy addition region" to the center of the plasma, leading to large temperature and concentration gradients. If equilibrium could be assumed in the ICP, a
Analytical Chemistry News & Features, August 1, 1996 4 6 9 A
Report simple set of equations would describe excitation and ionization. Instead, more complex kinetic models are needed. Finally, the effect of the sample on plasma characteristics must be considered. Although less than 50 W of power is typically needed to convert the sample into free atoms and ions, the sample can significantly affect die properties of the plasma. Emission intensity is a function of the number of analyte ions in the observation zone and the fraction excited, and ICPMS signals are a function of the number of analyte ions in die ICP and the fraction of ions that reach the detector. Therefore, single measurements of analyte emission or MS signal alone are insufficient to investigate any one of the fundamental processes involved in ICPMS. Laser-induced fluorescence measurements can provide information on the number of analyte ions in a given volume of the ICP; a combination of emission and fluorescence can be used to determine numbers and excitation of ions. Plasma dynamics. The plasma converts the sample into free ions (for ICPMS) or free, excited atoms or ions (for ICP-OES). Starting from Ar and electrical power (the input ingredients to the plasma), the chemistry of the Ar ICP would seem to be simple, described primarily by the formation of argon ions and electrons from elemental Ar. Most analyte atoms and ions are excited by collisions of electrons, and the probability of producing an excited atom or ion is dependent on the electron concentration and velocity (temperature). The gas temperature (which describes the kinetic energy of the Ar atoms) likely controls the desolvation and vaporization of the sample. The research challenges are to develop fundamental models that relate chemical composition and temperature of the plasma to each other, to develop models that predict the spatial structure of the ICP, and to make reliable experimental measurements to assess and improve the models. If the ICP were in local thermodynamic equilibrium (LTE), a simple expression would relate electron concentration, temperature, and pressure, and the electron temperature and the gas temperature would be the same. Does mis simple LTE approach to relate temperature and chemical composition of the Ar plasma work? Not exactly. Measured electron temperatures 470 A
are up to 2000 K hotter than gas temperatures (7), and the electron concentration calculated by using the measured electron temperature is 3 to 10timeslower than die measured electron concentration. Because energy is coupled mainly into electrons in the ICP (the much more massive ions respond too slowly to die oscillating magnetic fields), energy must be transferred to Ar atoms via collisions. Models are now being developed to account for the imperfect transfer of energy and dissimilar electron and gas temperatures. Barnes and co-workers (8) have developed fluid dynamics-based models to describe the spatial structure of the ICP starting from input power, torch and load coil geometry, and gas flow rates. These models, although not entirely accurate, are now being used to design practical ICPs and are being modified to include nonequilibrium effects. Aerosol generation and transport. Liquid samples must be converted into aerosol drops that are small enough to be vaporized in the ICP. The total volume of liquid entering the plasma must also be limited (typically 30 uL/min or less). Drift, noise, inefficient use of sample, and matrix effects caused by varia-
Figure 1 . Effects of sample uptake rate on sample aerosol. (a) Primary drop size distribution of sample uptake rates of 0.4, 1.0, and 2.0 mL/min. (b) Ratio of tertiary to primary aerosols showing more extensive aerosol losses as the uptake rate is increased.
Analytical Chemistry News & Features, August 1, 1996
tions in solvent composition (including acid identity and concentration) appear to originate primarily in the aerosol generation and transport processes. For the commonly used pneumatic nebulizers, the key design parameters have been empirically related to aerosol drop size (9). The primary aerosol produced by the nebulizer is modified as it passes through the spray chamber because of evaporation, impact and drop shattering, inertial deposition, and gravitational settling. For a particular pneumatic nebulizer and spray chamber, the gas flow rate and sample uptake rate are the key variables that control drop size and transport efficiency into the ICP. Smaller primary aerosol drops are produced as the gas flow rate is increased, whereas changes in uptake rate have a smaller effect on average drop size. The spray chamber removes most large drops before the aerosol enters the ICP; however, the transport efficiency of even small drops is less than 10% at sample uptake rates of 1 mL/min. As a result, typically less than 2% of the sample aerosol reaches the plasma. It has been suggested that drop collisions and coalescence to form larger droplets reduce the aerosol transport efficiency as the sample uptake rate is increased (10). The amount of sample that enters the plasma barely changes as the liquid uptake rate is increased from 0.4 to 2.0 mL/min. Although the average drop size grows slightly as the liquid flow rate is increased (Figure la), the loss of aerosol of all sizes in the spray chamber rather than production of fewer small primary drops causes the decrease in transport efficiency (Figure lb). Changes in the nebulizer gas flow rate predominately affect the primary aerosol and have less effect on losses in the spray chamber. Analysis of small volumes requires high aerosol transport efficiencies. Fortunately, as rhe liquid flow rate delivered to the nebulizer decreases, transport efficiency into the plasma increases, probably because fewer droplet collisions (and coagulation) occur in the spray chamber. Transport efficiencies of up to 87% have been obtained using pneumatic nebulizers at liquid flow rates of 1 uL/min or less with tertiary to primary aerosol volume ratios > 1 for some drop diameters. The direct injection nebulizer
(DIN) and the thermospray, ultrasonic, and oscillating capillary-based nebulizers can also deliver small volumes of sample into the ICP efficiently. High-efficiency nebulizers that consume only 10-50 uL/min of sample, but produce signals similar to nebulizers using 1 mL/min of sample, have recently been introduced. Production of ions and atoms. Many basic questions concerning the processes that convert a drop (or particle) of sample to free atoms and ions have long been unanswered. How long does it take to completely desolvate a drop in the plasma? How long does it take to vaporize the remaining analyte particle? What are the ideal properties of the sample aerosols for ICP spectrometry? Empirical optimization has led to the use of finer aerosols and lower solution transport rates into the plasma than is typical of flame spectrometry. One group found that emission intensity correlated with the volume of aerosol drops with diameters less than 8 um but not with the total volume of aerosol entering the plasma {11). Some aerosol drops with initial diameters of 10-15 um were found to be incompletely desolvated, even in the analytical zone of the plasma (12). If individual drops of sample could be introduced into the ICP, the desolvation and vaporization processes could be separated in time and space, and the rates of vaporization, atomization, and ionization processes could be determined. A group of rocket scientists has designed a monodisperse dried microparticulate injector (13), or MDMI, that can be used to reproducibly introduce individual, monodisperse drops of user-selectable size into the ICP (14). The rates of production of atoms and ions as well as diffusion can be determined from the time-gated laser-induced fluorescence images of individual atom and ion clouds (Figure 2). Time-integrated measurements as a function of height in the ICP can also be used to investigate these processes, because height in the plasma can be related to time through the use of measured gas velocities. Finally, the initial appearance of emission can be used to indicate when desolvation is complete and analyte particle vaporization begins. Analyte excitation/ionization and ion transport. Only excited atoms or ions produce emission. The mecha-
Figure 2. Images of Sr ion clouds in the ICP showing the production and diffusion of ions detected at different times relative to a reference signal. The time-integrated image of Sr emission is shown at the left.
nisms for excitation and ionization are of great interest because equilibrium-based expressions do not correctly predict experimentally measured emission intensities. If the ICP were in LTE, relative emission intensities and the extent of ionization would be easy to predict. This would have implications on semiquantitative analysis using ICP-OES, on choosing internal standards, and on predicting and correcting spectral interferences. The Ar ICP appears to be in partial LTE (15), close enough for a qualitative understanding but not for quantitatively accurate predictions of excitation and ionization. What could cause deviations from LTE? If emission of light depopulates excited states at a significant rate (compared with nonradiative de-excitation by electron-excited atom or ion collisions), LTE predictions will be erroneous; even simple collisional-radiative models dramatically improve predictions of excitation and ionization (16), and further improvements are being made (17). Mass transport, likely to occur considering the concentration gradients in the ICP, could also produce non-LTE behavior. Although specific reactions such as Penning ionization or ion exchange could also lead to deviations from LTE, except in a few specific cases (18) this does not appear to be happening. Finally, time- and space-dependent fluctuations in plasma conditions caused by cooling by desolvating droplets or vaporizing particles could
lead to biased time-integrated measurements that would appear to deviate from LTE. Refined models for excitation and ionization are being combined with models of sample decomposition and gas dynamics to quantitatively describe ICP signals, and experimental measurements to effectively test and adjust the models—particularly using a combination of laser-induced fluorescence and emission imaging of individual atom and ion clouds produced by the MDMI—are being made. The flow of plasma gas (including analyte ions) through the sampling orifice is controlled by the predominant species in the ICP, neutral Ar atoms. The gas rapidly expands, and all of the species move at nearly the same velocity, independent of mass. Flow through the skimmer is also probably dominated by the flow of neutral Ar atoms, which is consistent with the measured linear dependence of ion kinetic energies on mass downstream of the skimmer (19). Charge separation could be caused by an increase in electron mobility as the gas expands or the effect of the electric field produced by the first ion-optic lens, resulting in a positive ion beam. The location and time required for charge separation could have important implications on ion trajectories and detection efficiency. Once a positive ion beam is formed, the ion trajectories depend on the voltage gradients within the ion optics. The accel-
Analytical Chemistry News & Features, August 1, 1996 471 A
Report which ion-electron charge separation occurs is critically important and needs further theoretical and experimental investigation. Models are needed that can account for the charge separation, its effect on ion trajectories, and the effects of collisions in the region where charge separation occurs. Improved understanding of ion transport should lead to improved sensitivity and reduced matrix effects. Sample effects
the plasma. A combination of modeling and experimental measurements are needed to assess these effects. Changes in die amount of aerosol entering die plasma can have dramatic affects. Vertical emission profiles can shift, and the emission intensity per microgram of analyte entering the plasma can change by as much as a factor of 50,000. Large changes in ion kinetic energy have also been observed as aerosol loading is varied (19). Some aerosol drops survive even in die normal analytical zone of die ICP (22,23). Each drop and vaporizing particle acts as a heat sink that affects a surprisingly large volume of the plasma (12), and average temperatures in this locally cold region can be more than 1500 K cooler than the surrounding plasma (24). Because the volume of plasma affected by a single drop is so large, relatively small numbers of incompletely desolvated drops can affect the number of atoms and ions in the observation volume (both increase, almough atoms increase more dramatically), die fraction tiiat are excited (which decreases), and the fraction that are transported from die plasma to die detector of die mass spectrometer (which also decreases).
Many of the fundamental processes can be affected by the sample in surprisingly dramatic ways. The sample solvent or dissolved solids can affect the plasma temperature and electron concentration. Background species can be formed from reactions between analytes and solvent or plasma species. Figure 3. Effect of 0.11 Ml NaCI on t h e production of Sr ions. Accurate analysis requires that the (a) Measured by laser-induced fluorescence and sensitivity (signal per unit analyte concenSr ion excitation, and (b) determined from the tration) be constant for the sample and ratio of emission-to-laser-induced fluorescence. the calibration standards. However, variaEmission and fluorescence values were determined for an entire ion cloud and integrated tion in the sample matrix, including acid in all directions to obtain a single total value. concentration or identity, the presence of small amounts of organic solvent, and high concentrations of almost any eleeration of ions attributable to the electric ment, can affect sensitivity. fields produced by the ion optics is dependent on kinetic energy, so ions with differOrganic solvents cause their own Effect of sample solvent on the ent energies then move with different veunique set of effects on plasma characterisplasma. The effect of aqueous aerosol on locities. Ion focusing is mass dependent if the plasma is much different than the effect tics and analytical signals. Electron concenion kinetic energies are mass dependent. of water vapor, although both can be signifi- trations and temperatures are reduced, and a 500-W increase in applied power may be cant (21). In general, temperatures and When a large number of positive ions necessary to compensate for solvent-inelectron number densities increase when exist in a small volume, "space charge" can duced cooling. Because organic solvents water vapor is added to a dry Ar ICP. In result in a loss of sensitivity. Positive ions typically have higher volatilities than water, contrast, temperatures and electron num(in the absence of electrons) repel each organic solvent vapor loading may be 10 ber densities decrease when die amount of other and cause defocusing of the ion times larger than that of aqueous samples. water aerosol entering the plasma is inbeam. Alternatively, space charge can be viewed as a radial electric field imposed by creased. Organic solvents generally cause a The maximum tolerable load for many organic solvents is more dependent on volatilthe positive ion beam, which can become as dramatic cooling of the plasma, but their ity than on the nature of the solvent effect depends on die relative amounts of large as the electric field produced by an carbon and oxygen. With a typical nebulizion lens and cause the beam to become The solvent vapor, unlike aerosol, can er/spray chamber, approximately 20 mg of diffuse outside of die center channel of the defocused. The ion current through the water vapor and 5-25 mg (uL) of water first ion lens past the skimmer is 100-1000 plasma, perhaps even before entering die times smaller than predicted by gas dynam- aerosol enter the plasma per minute. plasma, and die distribution of solvent into ics alone, consistent with space-charge efThe power required to convert the wa- different regions of the plasma cantiiusbe fects. Lighter ions (having lower kinetic very important. The carbon-to-oxygen ratio ter vapor and sample aerosol into free atenergies) will move farther out of the cenoms and ions (30-50 W) could be a signifi- of the solvent is important, apparently reter of the beam than will heavy ions. Begardless of the chemical form of oxygen cant fraction of the power available in the cause space-charge effects are mass depen- center channel of the ICP (about 100 W) if and carbon that are introduced into the dent, isotope ratios could be biased toward heat transfer is relatively slow. Although plasma (25). the higher mass ion. hydrogen and oxygen atoms produced Background and spectral overfrom water vapor account for < 2% of the An ion trajectory model that includes laps. In ICP-OES, little can be done to atoms in the plasma, the decomposition space charge has been developed, alreduce continuum background emission products of the water vapor could affect though it is limited to ion currents far bewithout a radical change, for instance, the thermal and electrical conductivity of low those expected (20). The location at viewing emission after expansion into a 472 A
Analytical Chemistry News & Features, August 1, 1996
vacuum. However, automated chemometric means of subtracting background and simultaneous detection of background and analyte signals are improving in accuracy. It is likely that models for ICP-OES will eventually allow spectral overlaps to be quantitatively predicted. The use of solid-state detectors that view multiple emission lines for each element should lead to improvements in handling spectral overlaps, including automated selection of background correction methods and parameters. Although isobaric overlaps (from isotopes of different elements with an isotope at the same nominal mass) in ICPMS are easily predicted, spectral overlaps from molecular ions (such as ArO+, ArH\ refractive oxides, and organic molecular ions) are more problematic. It is not yet clear if molecular ions are formed primarily by ionmolecule reactions or by ionization of neutral molecules. The molecular species could be formed in the ICP (but not seen in ICPOES because molecules are weak emitters and the continuum background is high), near the sampling orifice, or in the expansion region. Molecular oxide ions have lower kinetic energies than do elemental ions of similar mass (19), which is consistent with the formation of molecular oxide ions in locally cool regions of the plasma, such as those near incompletely desolvated droplets or vaporizing particles. Two approaches to overcoming molecular ion spectral overlaps in ICPMS continue to evolve: use of high-resolution mass spectrometers or means to eradicate or reduce polyatomic ion signals by using cryogenic desolvation, addition of xenon or other gases, and a carbon center tube. Solvent composition. The solvent can affect analyte sensitivities because of changes in the analyte transport rate into the plasma resulting from changes in aerosol properties, differences in the chemical composition of the sample and the aerosol that reaches the ICP, or matrix-induced variations in the plasma itself. Large variations in acid concentration or the use of organic rather than aqueous solvent will, not surprisingly, affect analyte signals. However, changes in acid concentration from one sample to the next can cause large, transient variations in analyte signals that may occur in a matter of minutes (26). A large increase in acid concentration (such as from 1% to 20% v/v nitric acid)
reduces the analyte transport rate. Although the drop size distribution of the primary aerosol shifts to only slightly larger diameters as the acid concentration is increased, the average tertiary aerosol drop size decreases, implicating processes within the spray chamber (27). It is not clear why this occurs. Sulfuric and phosphoric acids cause more severe effects than do hydrochloric and nitric acids, although each behaves uniquely. The relative magnitude of the acid effects is critically dependent on the plasma and sample introduction conditions used. Surprisingly small changes in acid concentration can be important when the acid concentration is low (28). Changing the HC1 concentration from 10"4 or less to 10"3 % v/v can result in a 20% increase in signal, and a similar change in HC104 concentration can cause a 30% decrease in signal. Furthermore, the effects can be highly element-dependent (29).
excited when high concentrations of EIEs are present in the sample. This is one reason EIE effects in ICP-OES are often less severe than in ICPMS. There is a shift in the relative number of atoms and ions towards atoms in the presence of high concentrations of EIEs, but the increase in the number of atoms is too small to account for the entire decrease in the number of ions. Matrix effects in ICPMS are caused by a combination of decreases in ion concentration in the plasma and changes in the fraction of ions transported from the ICP to the MS detector. According to theoretical calculations and experimental measurements, there is no mass dependence to EIE effects in the plasma itself (33); rather, the mass-dependent portion of EIE effects in ICPMS appear to be caused by space-charge effects. Instruments designed to reduce space-charge effects show less severe EIE chemical matrix effects with a weaker dependence on mass (34,35). The magnitude of the EIE effect is less severe for matrix elements with high ionization energies, such as Se or As, than for matrix elements with lower ionization energies.
Fundamental processes can be affected by the sample Measurements to separately assess the effect of matrix on plasma properties and in surprisingly conversion of drops into free atoms and ions are now being made. Some initial redramatic ways. sults using the MDMI are shown in Figure Small amounts of organic solvent in an aqueous sample can have a significant effect on analyte signals. For example, one drop of acetone added to 50 mL of an aqueous sample produced a 15% decrease in analyte signal, because the fraction of ions that were excited decreased more than the 11% increase in analyte transport rate (30). Further investigation of aerosol generation, modification, and transport, as well as plasma-sample interactions, is essential. Dissolved solids. Analyte signals can also be affected by the presence of high concentrations (> 10 mM for ICPOES, > 1 mM for ICPMS) of any element that is efficiently ionized, as most are, in the ICP. Efficiently ionized elements (EIEs) generally cause a decrease in the number of analyte ions in the analytical zone of the ICP as measured by laser-induced fluorescence (31,32). At the same time, a larger fraction of analyte ions is
3. The entire ion cloud is within the laser beam and optical collection volume so that the total number of ions produced can be calculated as a function of the location of the center of the cloud. The presence of Na in the sample causes an earlier appearance of ions (lower in the ICP), probably because of faster vaporization, but it takes longer from the initial appearance of ions until the analyte is completely ionized (reaching a steady value). Excitation is dramatically affected by the presence of NaCl in the sample, as determined from emission-to-fluorescence ratios. This is consistent with the observation of increases in electron concentration, electron temperature, and in some cases, gas temperature by using Thomson scattering (36). When these experiments are combined with atom emission, fluorescence imaging, and laser light scattering, the effect of matrices on each fundamental process can be assessed.
Analytical Chemistry News & Features, August 1, 1996 4 7 3 A
Must Reading! The Internet: A Guide for Chemists
This timely book presents the basics of the Internet along with instructions on becoming an information provider on the Internet. In a stepwise and thorough manner it describes how to use the major Internet resources, including e-mail, electronic lists, Gopher, FTP, and the World Wide Web and gives useful information specific for chemists, such as Web addresses, electronic mailing lists, Gopher sites, and FTP sites. Also includes a brief history and overview of the Internet, and describes how a chemical company has integrated the Internet into its business regimen. Contents History of the Internet Electronic Mail The Berkeley Mail Program Electronic Lists Gopher The World Wide Web Designing an Internet-Accessible Database Electronic Conferencing Electronic Lists for Chemists Managing the Computational Chemistry List Chemistry and Gopher Chemistry and the World Wide Web Steven M. Bachrach, Editor, N o r t h e r n Illinois University 360 pages (1996) Clothbound: ISBN 0-8412-3223-7, $29.95 (plus $3.00 handling) Paperbound: ISBN 0-8412-3224-5, $23.95 (plus $3.00 handling)
ORDER FROM American Chemical Society 1155 Sixteenth Street, NW Washington, DC 20036 Or CALL TOLL FREE 1-800-227-5558 (in Washington, DC 872-4363) and use your credit card! FAX: 202-872-6067. ACS Publications Catalog now available on Internet: gopher acsinfo.acs.org or URL: http://pubs.acs.org
474 A
Report (12) Olesik, J. W.; Fister, J. C. Spectrochim. Acta 1991,46B, 851-68. Improvements in ICP-OES and ICPMS (13) French, J. B.; Etkin, B.; Jong, R. Anal. performance, reliability, accuracy, and Chem. 1994, 66, 685-91. (14) Olesik, J. W.; Hobbs, S. E. Anal. Chem. ease of use depend on further fundamen1994, 66,3371-78. tal research. Sample introduction systems (15) Blades, M. W.; Caughlin, B. L.; Walker, are likely to be dramatically improved by Z. H.; Burton, L. L. Prog. Analyt. Spectrosc. 1987,10, 57-109. investigation of aerosol generation pro(16) Burton, L. L.; Blades, M. W. Spectrochim. cesses and sample-plasma interaction. Acta 1990,45B, 139. Intelligent instruments based on diag(17) Fey, F.H.AG.; Benoy, D. A; de Regt, J. M.; van der Mullen, JAM.; Schram, nostic signals could warn the operator, D. C. Spectrochim. Acta B 1993,48B, not only if the instrument is malfunction1579. ing but also if a particular sample is likely (18) Farnsworth, P. B.; Smith, B. W.; Omenetto, N. Spectrochim. Acta 1991,46B, to suffer from an analysis error, by identi843-50. fying changes in the aerosol transport rate (19) Tanner, S. D.J. Anal. At. Spectrom. 1993, or the extent of ionization and excitation 8,891-97. in the plasma. Problem samples could (20) Tanner, S. D. Spectrochim. Acta 1992, 47B, 809-23. then be reanalyzed by using matrix(21) Long, S. E.; Browner, R. F. Spectrochim. matched standards or standard additions. Acta 1988,12,1461-71. Alternatively, the instrument could force (22) Cicerone, M. T; Farnsworth, P. B. Spectrochim. Acta 1989,44B, 897. plasma conditions to be the same, regard(23) Olesik, J. W.; Smith, L. J.; Williamsen, less of the sample matrix, to eliminate the E.J. Anal. Chem. 1989, 61,2000-08. potential error. (24) Hobbs, S. E.; Olesik, J. W. Spectrochim. Acta 1993, 48B, 817-33. Progress has already been made to(25) Weir, D. G.; Blades, M. W.J. Anal. At. ward this goal. For example, Mermet (37) Spectrom. 1994, 9,1311-22. (26) Maessen, F.J.M.J. Spectrochim. Acta has described a set of diagnostic signals 1982,37B, 517. that can be used to determine if a nebu(27) Canals, A; Hernandis, V.; Todoli, J. L.; lizer is likely clogged, if the applied power Browner, R. F. Spectrochim. Acta 1995, 50B, 305-21. has drifted, if the spectrometer is drifting, (28) Marichy, M.; Mermet, M.; Mermet, J. M. or if the window to the spectrometer is Spectrochim. Acta B 1990,45B, 1195becoming fogged. As pointed out by 1201. Blades and Weir (1), "Eventually plasma (29) Brenner, I. B.; Mermet, J. M.; Segal, I.; Long, G. I. Spectrochim. Acta B 1995, diagnostics will probably be used interac50B, 323-31. tively to provide on-line feedback control (30) Olesik, J. W.; Moore, A. W., Jr. Anal. to compensate for drift, matrix effects, Chem. 1990, 62, 840-45. (31) Gillson, G.; Horlick, G. Spectrochim. Acta solvent loading effects, and noise." 1986,41B, 619. (32) Olesik, J. W.; Williamsen, E. J. Appl. Spectrosc. 1989,43,1223-32. References (33) Hobbs, S. E.; Olesik, J. W. Appl. Spectrosc. (1) Blades, M. W.; Weir, D. G. Spectroscopy 1991,45,1395-1407. 1994, 9(8), 14-21. (34) Turner, P. J. In Applications ofPlasma (2) Hieftje, G. M.; Galley, P. J.; Glick, M.; Source Mass Spectrometry, Holland, G., Hanselman, D. S.J. Anal. At. Spectrom. Eaton, A. N., Eds.; Royal Society of Chem1992, 7, 69-73. istry: Cambridge, 1991; p. 71. (3) Hieftje, G. M. Spectrochim. Acta 1992, (35) Tanner, S. D.; Cousins, L. M.; Douglas, 47B, 3-25. D.J.Appl. Spectrosc. 1994,45,1367-72. (4) Houk, R. S.; Shum, S.C.K.; Wiederin, D. R. (36) Hanselman, D. S.; Sesi, N. N.; Huang, M.; Anal. Chim. Acta 1991, 250, 61-70. Hieftje, G. M. Spectrochim. Acta B 1994, (5) Montaser, A. In Inductively Coupled Plas49B, 495-526. mas in Analytical Atomic Spectrometry, (37) Poussel, E.; Mermet, J. M. Spectrochim. 2nd ed.; Golightly, D. W., Ed.; VCH PubActa 1993,48B, 743. lishers: New York, 1992. (6) OlesikJ.W.^Mfl/. Chem. 1991,63, In addition to performingfundamental re12 A-21 A. (7) Huang, M.; Hanselman, D. S.; Yang, P.; search on plasmas, John W. Olesik does reHieftje, G. M. Spectrochim. Acta 1992, search on CE/ICP, LC/ICP, and ion spray 47B, 765. and electrospray MS. Address correspondence (8) Gaillat, A.; Barnes, R. M.; Boulos, M. I. about this article to Olesik at Laboratory for J. Anal. At. Spectrom. 1995,10, 935. (9) Canals, A; Hernandis, V.; Browner, R. F. Plasma Spectrochemistry, Laser Spectroscopy /. Anal. At. Spectrom. 1990,5, 61-66. and Mass Spectrometry, Dept. of Geological (10) Olesik, J. W.; Bates, L. C. Spectrochim. Sciences, 275 Mendenhall Laboratory, 125 Acta 1995,50B, 285-303. (11) Olsen, S. D.; Strasheim, A. Spectrochim. S. Oval Mall, The Ohio State University, Columbus, OH 43210 Acta 1983,38B, 973.
Future progress
Analytical Chemistry News & Features, August 1, 1996
