John W. Olesik Department of Chemistry Venable and Kenan Laboratories, CB #3290 University of North Carolina at Chapel Hill Chapel Hill, NC 27599-3290
The inductively coupled plasma (ICP) has become the dominant source for rapid spectroscopic multielement analysis as a result of a set of “legendary” attributes, including low detection limits, a wide linear dynamic range, and high precision (see box). In this REPORT, we will briefly review the fundamental properties of the ICP that lead to its analytical virtues. We will then focus on some of the remaining limitations of ICP-optical emission spectroscopy (ICP-OES) and ICP/mass spectrometry (ICP/MS), including spectral interferences and nonspectral matrix effects (see box). Approaches to deal effectively with these problems will be considered. Operation of the ICP The basic operation of the ICP has been described in a previous REPORT (I) and will be only briefly reviewed here. The ICP is a partially ionized gas (typically Ar, which is less than 1%ionized in the plasma) produced in a quartz torch using a 1-2.5-kW radio frequency power supply. Samples are typically introduced into the center of the plasma as aerosols. Commercial ICP-OES instruments have been available since 1974. Light emitted from a 3- to 5-mm-high region of the plasma is focused onto the entrance slit of a monochromator or polychromator to monitor emission from different elements either sequentially or simultaneously. Spectral lines resulting from singly charged ions are most intense for the majority of elements, and atom lines are most intense for elements with high ionization potentials and the alkali metals. The detected signal depends on both the number of analyte ions (or atoms) in the plasma and the fraction of those ions (or atoms) that are excited. Because most elements exist pre12 A
dominantly as singly charged ions in the ICP, it can be effectively used as an ionization source for MS. ICP/MS instruments became commercially available in 1983. A two- or three-stage, differentially pumped interface is used to extract ions from the atmosphericpressure plasma into the low-pressure (typically 10-5-10-6 Torr) mass spectrometer (Figure 1).Ions pass through a water-cooled sampling cone (often made of Ni or Cu) with a 0.5-1.0-mm orifice that is placed in the plasma. The ions are then placed in the plasma. A small fraction of the expanding gas passes through a second, more sharply angled cone called the skimmer. The pressure in the region between the sampling cone and the skimmer is typically about 1Torr. Ion optics, typically cylinders held at appropriate voltages, are used to focus ions into the mass spectrometer, which is usually a quadrupole system. The analyte signal depends on the number of analyte ions in
the plasma (independent of their state of excitation) and the potentially massdependent transport of ions to the mass spectrometer. There are two key differences in the generation of ICP-OES and ICP/MS signals. First, sample ions must be physically transported from the plasma to the mass spectrometer, whereas the collection of photons in ICP-OES is nonintrusive. Second, emission intensities are strongly dependent on the fraction of ions (atoms) that are excited, whereas mass spectrometric signals are dependent on ionization, but not excitation, conditions within the plasma. Therefore, the effect of changes in experimental conditions (such as power, gas flow rates, location in the plasma from which the signal is acquired, and sample matrix) is quite different in ICP-OES and ICP/MS. Why has the ICP been so successful? To evaluate the remaining analytical
ANALYTICAL CHEMISTRY, VOL. 63, NO. 1, JANUARY 1, 1991
problems and their possible origins, it is useful to consider some of the ICP’s desirable characteristics, including the following. Intense emission is produced from many different spectral lines for almost all elements. Compared with flames or graphite furnaces, the ICP is much more highly excited. Temperatures measured in Ar ICPs range from 5000 to 9000 K, depending on experimental conditions (power, gas flow rates, -and sample transport rate), where in the plasma the measurement is made, and how the temperature is measured. Emission intensities depend on the number of ions (atoms) in the volume from which the signal is collected and the fraction of ions (atoms) that are in an excited state. The population of a particular excited state is proportional to the Boltzmann factor, e-E’kT, where E is the energy of the upper state of the spectroscopic transition (usually between 3 and 7 eV for the most sensitive lines emitted by the ICP), k is the Boltzmann constant, and Tis temperature. The Boltzmann factor for an excitedstate energy of 6.2 eV increases from 3.8 X to 3.3 X more than 7 orders of magnitude, when the temperature is changed from 3000 to 9000 K. Therefore, if the number of ions per unit volume remains constant, the emission intensity will increase by about 7 orders of magnitude. For an excited state with an energy of 3.1 eV, the Boltzmann factor is more than 3 orders of magnitude higher at 9000 K than a t 3000 K. Excited states are so extensively populated in the ICP that intense emission is produced from many lines simultaneously. As a result, rapid simultaneous analysis can be performed. Emission from a flame or graphite furnace is much less intense-characteristic of temperatures between 2000 and 3500 K. Absorption spectrometry is used to detect atoms in flames or graphite furnaces that emit light involving excited states with low energies of < 3 eV (Na, K, Ca, and Li). 0003-2700/91/0363-012A/$02.50/0 @ 1990 American Chemical Society
The highly excited nature of the ICP also results in one of the major problems in ICP-OES: spectral overlap. The high temperatures may also lead to an erroneous conclusion that the plasma is unaffected by the sample. Large signals and small background lead to low detection limits. Detection limits are often determined from the relative magnitudes of the signal and the noise in the background. ICP-OES intensities are large, as ex-
pected at high temperatures; and continuum background emission, particularly that attributable to molecular species, is relatively low. The main sources of continuum background in ICP-OES are radiative recombination of electrons and ions (M+ eM hvor M+ e- e-+M e- hv) and the loss or gain of energy by electrons accelerated in a field of ions, called bremsstrahlung radiation. Molecular species are not prevalent in the plasma,
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ICP spectrometry Attributes Rapid, simul, ,eou,, mltielement analysi, Low detection limits (ppb or less for some elements: Relatively small interelementmatrix effects Wide linear dynamic range (up to 5 or 6 orders of magnitude) High precision (0.5 % -5 %) Applicable for analysis of gases, liquids, or solid!
Problems Spectral interferences Matrix effects from concomitant soecies Matrix effects from solvent Difficulty in analyzing solids without dissolution lnefficient sample introduction Detection limits too high for some applicatior Drift and insufficient precision for some applicarions
Figure 1. An ICP/MS instrument.
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and most molecular species are not strong emitters. However, some molecular emission is observed from species produced from aqueous or organic solvents (e.g., OH and CN bands). Furthermore, the most intense continuum background occurs low in the plasma off axis, in the “fireball” region, whereas the maximum analyte ion line intensity peaks about 15 mm higher in the center of the plasma. Low in the plasma, temperatures are cool in the center because of the introduction of aqueous aerosol, but they are hot off axis. The position of maximum emission intensity in the plasma is determined by a combination of temperatures in the center of the plasma and the number of analyte ions (atoms) per unit volume. The rate of sample introduction, desolvation of droplets, vaporization and atomization of the remaining analyte particles, and the diffusion of atoms and ions determines the analyte ion (atom) concentration in the plasma. ICP/MS detection limits are often up to 3 orders of magnitude superior to those in ICP-OES, primarily because there is no fundamental source of continuum background in ICP/MS (i.e., no source of continuum mass to charge ratio ions). Background can be produced by stray ions (those that pass around the quadrupole but reach the detector) or ions that are exactly on axis in the quadrupole (and therefore not mass selected). Photons striking the detector may also produce a signal. Most elements with ionization POtentials < 9 eV exist predominantly as singly charged ions in the plasma. The number of detected ions is limited mainly by poor transmission of ions from the atmospheric-pressure plasma to the low-pressure mass spectrometer. A 10-ppm solution of Mn produces approximately 1.8 X 1013Mn+ per second in the plasma. However, less than 1in 6 X lo7 ions produced in the plasma is detected, leading to a count rate of about 3 X lo5 Hz (2). Detection limits obtained in both ICP-OES and ICP/MS are sample dependent and are degraded if back-
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REPORT ground increases or spectral overlaps are present (3).For example, if the interference has an intensity that is 10 times the backgkound intensi$y at the analyte peak, the practical detection limit will be degraded by about a factor of 10, assuming that the relative standard deviation (RSD) of the interfering species is equal to that of the background. Recombination of matrix ions and electrons can produce large increases in the ICP-OES continuum background. Intense lines of some elements (e.g., Ca and Al) are strongly broadened so that the wings of the line extend up to several nanometers from the peak center. ICP/MS detection limits are degraded by spectral overlaps caused by isobaric interferences (isotopes of different elements with the same mass) or molecular species. Thus, detection limits determined from one-analyte solutions are often not attainable when samples with complex matrices are analyzed. Some matrix effects in ICP-OES and ICP/MS are smaller than in flames or graphite furnaces. One source of matrix effects in flames and furnaces is incomplete atomization, particularly of refractory species such as oxides. In the ICP, two factors tend to minimize this type of matrix effect: a more “inert” environment and higher temperatures. However, the ICP is not totally inert. Sample water vapor and aerosol produce a concentration of oxygen atoms in the plasma, roughly 2 X 1016cm-3, that is similar to the concentration of oxygen atoms in flames. The formation of most but not all molecular oxides in the ICP appears to be unlikely, based on equilibrium calculations. The ratio of atoms to molecular oxides is a function of the dissociation energy of the molecule, temperature, and oxygen atom concentration in the plasma. At temperatures of 4000 K or greater, calculations predict more than 1000 times more atoms than molecular oxides for compounds with a dissociation energy of about 5 eV (such as SnO and A10). Furthermore, because most elements are highly ionized in the ICP, the ion to molecular oxide ratio should be much larger than the atom to molecular oxide ratio. However, compounds with large dissociation energies of 8 eV or more (such as BO) require temperatures that are higher than 5000 K to produce more atoms than molecular oxides. Although all but the most refractive compounds should be effectively atomized in the ICP according to equilibrium calculations, conclusions must be drawn with great care. Kinetics control the desolvation of droplets and vaporization of sample particles. When va14A
0
detection limits are often up to
3 orders of magnitude superior to those in ICPmOES. porization is complete, few molecules may exist. However, near a vaporizing droplet or particle the plasma temperature and chemical composition may be very different than in the bulk of the plasma (4). Much lower temperatures and higher oxygen concentrations will be found near a desolvating droplet. Easily ionizable elements cause a commonly observed matrix effect in flames. For example, consider the equilibrium between Cu atoms and ions: Cu * Cu+ e-. If Na is added to the flame, more electrons are produced (Na +Na+ e-), driving the equilibrium between Cu and Cu+ toward Cu atoms. As a result, the number of Cu atoms increases, whereas the number of Cu ions decreases. This easily ionizable element effect becomes important when the number of electrons produced by the sample matrix is significant compared with those produced by the flame or plasma gas. In the ICP, the electron concentration is larger (lo1* to 2 x 1015 e-/cm3) than in a flame (lolo to 10l2 e-/cm3). The dominant source of electrons in the ICP is the ionization of Ar. Using equilibrium calculations, the number of electrons produced from an easily ionizable element matrix can be calculated. If a solution of 1000 ppm Na is introduced via a pneumatic nebulizer, about 10 pg of Na enter the plasma per minute. Assuming that the Na is completely ionized and confined to the tenter of the plasma, 1.98 X 1014electrons will be produced in -20 L/min of Ar at about 6000 K, taking gas expansion into account. The number of electrons produced from the ionization of Na (1 X 1013e-/cm3) is much smaller than the number from Ar. Both ICP-OES and ICP/MS signals
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 1, JANUARY 1, 1991
are affected by the presence of < 500 ppm of an easily ionizable element. Therefore, there seems to be an inconsistency between the simple equilibrium-based calculations and experimental observations. Experimental results conflict on whether the electron number density changes when large concentrations of easily ionizable elements are present in the sample. Matrix effects may be smaller in the ICP than in dc arc discharges, partially because the energy addition region is separated from the region where the sample is introduced. About 98%of the power in a 27-MHz ICP is calculated to be dissipated in a hollow cylinder with a wall thickness of 4-6 mm (5).There is direct experimental evidence of energy addition from the region within the load coil to a point 12 mm above it, and the sample tends to be confined to the center of the plasma. If any sample is present in the energy addition region, plasma properties such as thermal or electrical conductivity could be affected, leading to severe matrix effects. Two factors tend to keep the sample in the center of the plasma near the load coil. The Ar gas in the fireball region of the plasma (low and off axis) is rapidly heating and expanding. The sample will have to move against the convective flow produced by the gas expansion to enter the fireball region. If the sample aerosol droplets or resulting particles survive long enough (get high enough in the plasma), diffusion of analyte vapor into the fireball region is unlikely. This normally occurs when pneumatic nebulizers are used, but not if the sample is introduced as a vapor or dry aerosol (6). Matrix effects are not always smaller in the ICP than in other sources such as flames and dc arcs. Spectral interferences in both ICP-OES and ICP/MS can be severe. Furthermore, some nonspectral matrix effects, particularly those attributable to differences in solvent composition, can be more severe in an Ar ICP than in a flame ionization or dc plasma. Remainingsources of potential errors Spectral interferences. The high temperatures consistent with intense emission and minimal vaporizationtype matrix effects also produce one of the biggest problems in JCP-OES: spectral overlap. Based on Wohlers’ ICP wavelength tables (7),an average of 294 spectral lines are emitted per element, ranging from eight for H to 2532 for Cs at wavelengthsbetween 183 and 850 nm. Rare-earth elements, which were not listed in Wohlers’ tables, produce notoriously cluttered spectra.
The wide linear dynamic range characteristic of ICP-OES allows measurement of trace quantities of one analyte and major quantities of another simultaneously without sample dilution, separation, or preconcentration. Very weak emission intensities, however, must be measured in the presence of intense emission from other elements. Furthermore, lines from different elements have widely varying sensitivities. For example, sensitivities for the most intense Ca and Sn lines differ by a factor of 5000. If a sample contains 1ppm Sn and 1000 ppm Ca, the Sn line intensity is 5 x 106 times less intense than the Ca line. Direct line overlaps, broad wings of spectral lines, and stray light can also produce overlap problems. However, ICP-OES is very selective when interference-free spectral lines can be found. For example, 1 ppb Mg can be detected even though the ratio of Ar to Mg species in the plasma is more than 10l2to 1. It is also possible to determine sub-part-per-billion levels of some elements in the presence of up to 10000 ppm of other elements. Spectral interferences in OES can be minimized (but not eliminated) by using high-resolution spectrometers. The most advantageous resolution is limited by the width of spectral lines emitted from the plasma (typically between 1.4and 5 pm) (8).Therefore, a t 200 nm a resolution (A/AA) of approximately 150 000 is needed to match the physical linewidths. Increasing resolution to values greater than about 300 000 does not further reduce spectral overlap. Three different tactics can be used if spectral overlap occurs. First, the operator can choose interference-free lines for a particular sample. Alternatively, the source of the spectral interference can be identified and its magnitude determined and subtracted from the measured signal. Finally, a multi-line detection scheme can be used with factor analysis. The primary difficulty in choosing an appropriate emission line is determining how to make the decision rapidly. If the identity and approximate concentration of elements in the sample are known, spectral databases (7,9,10) can be used to choose appropriate analysis lines. Expert systems can also be used to choose appropriate lines depending on the nature of the sample (11).Transferring data from tables to a particular instrument is difficult because the extent of spectral overlap depends on the resolution of the spectrometer. Spectral simulations can be developed that would allow the operator to estimate sensitivity factors for the par-
etettion
often not attainable in samples with complex matrices. ticular spectrometer being used. When high-resolution spectra are available, the extent of spectral overlap using a particular spectrometer can be quantitatively predicted (12). Alternatively, spectral simulations based on plasma conditions and spectrometer characteristics can be calculated in an ab initio manner (13).For the latter approach to work, models that correctly predict spectral line intensities are needed. In some cases, primarily because polychromators have a limited number of analysis lines for each element, an ‘interference-free analysis line cannot be used. In such cases, the contribution of the interfering lines can be subtracted from the measured intensity after determining the sensitivities (emission/concentration) of each of the interfering lines a t the analyte wavelength. The concentration of each element (or the intensity of another line for the interfering element) producing an interference must then be measured, and previously calibrated or relative sensitivities can be used to calculate the intensity of the interfering line at the analyte wavelength. Manufacturers of polychromators typically provide software for spectral interference correction. However, the precision of the analysis suffers as the analyte to interfering line(s) intensity ratio decreases. If the analytical precision using the interference correction is unusually poor (> 10%RSD), the interfering line intensity is probably large compared with analyte intensity. The choice of an appropriate line depends on the desired detection limit and precision (3). Determination of the interfering
can be a long and involved process. Botto (14)has determined 84 interferences of Cr, Mo, U, Ti, and V. To use the same interfering line sensitivity factors over long periods of time, plasma and sample introduction conditions must remain constant from day to day. One successful approach is based on adjustment of the center gas flow rate to maintain a constant Cu/Mn+ intensity ratio (14). Traditionally, one spectral line is used for each element of interest. Selectivity, determined by the uniqueness of the signals for each element, can be improved by applying factor analysis to multi-line spectra (15).However, a wide spectral range must be rapidly (and preferably simultaneously) detected with good resolution. Although two-dimensional spectrometers using charge-transfer detectors can provide this capability, no commercial instruments of this type are available. ICP/MS spectra generally are simpler than ICP-OES spectra. Rather than producing hundreds or even thousands of lines for each element, they produce only a few peaks. Matrices that contain rare earths are likely to produce severe spectral overlaps with ICP-OES but not with ICP/MS. However, when quadrupole mass spectrometers are used, mass spectral overlap can be a major problem (2). Overlap can be caused by isobaric interferences and atomic and molecular species produced from the plasma gas (Ar+, Arz), entrained atmosphere or impurities in the Ar supply (CO;, CO+, N l , N+), acids used in sample preparation (SH+, SO+,NOH+), and combinations of these (ArH+, ArOH+, ArCl+, C10+,ArS+, etc.). A number of isobaric interferences can occur. For example, the major isotope of Ar+ has a mass of 39.948 whereas the most abundant isotope of Ca+ has a mass of 40.08. The second most abundant Ca isotope, 44Ca(2.08%), suffers from an interference with l2Cl60;, and the third most abundant Ca isotope, 42Ca(0.64%), occurs at the mass of 40ArH;. The most abundant Ni isotope, 58Ni, suffers from an interference from 58Fe.Such isobaric interferences can be corrected by monitoring more than one isotope of the interfering species and then using the known isotope abundance9 to correct the interference, if the analyte to interfering species signal ratio is high enough. Commercial instruments typically provide such software. Alternatively, another analyte isotope may be used. Molecular oxides can also cause spectral interferences. For example, the oxides of the five isotopes of Ti (46Ti160+,47Ti160+,48Til60+,49Ti160+,
ANALYTICAL CHEMISTRY, VOL. 63, NO. 1, JANUARY 1, 1991
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REPOR7 50Ti160+)cause mass spectral interferences with 62Ni+,63Cu+,64Zn+,65Cu+, and 66Zn+,respectively. It is difficult to determine small concentrations of Cu in the presence of large concentrations of Ti. It is unclear if molecular species are present in the plasma or formed in the sampling system of the mass spectrometer. However, even if the molecular species were present in the plasma, their emission intensities would be small compared with those of atodic species. Therefore, molecular spectral interferences are more of a problem in ICP/MS than in ICP-OES. Because different matrices pose problems in ICPJMS and ICP-OES, optimum sample preparation procedures may differ for each technique. For example, HC1 is commonly used for sample dissolution for ICP-OES. However, species including C10+, ClN+, ClOH+,Cll, and ArCl+ are detected in ICPJMS when C1 is present in large concentrations. The most abundant V isotope (97.76%), 51V+,suffers from interference with 35C1160+;the second most abundant isotope (0.24%),50V+, overlaps with S5Cl15N+as well as‘50Ti+ and 50Cr+.Therefore, ICPJMS cannot be used to measure small concentrations of V in the presence of a large chlorine concentration. The major Cr and Ga isotopes, 52Cr+and 69Ga+,suffer from interferences with 35C1160H+ and 37C1160160+, respectively. Use of nonchlorinated acids, such as HN03, can reduce the problem if chlorine-containing species are not present at high concentrations in the sample. Argon, rather than being “inert,” combines with many species. For example, ArP+ and ArS+ are observed when phosphoric and sulfuric acids are used in sample preparation, and Ar combines with matrix elements to form species such as ArNa+, ArNi+, ArGa+, and ArSe+. Count rates from these species are significant when matrix concentrations are high (1000 ppm). Many spectral interferences in ICP/ MS could be eliminated by using highresolution mass spectrometers, because the peak width is instrumentally rather than physically limited. Table I lists some isobaric and molecular interferences and the resolution necessary to separate peaks of equal magnitude. Many molecular interferences requiring resolution of about 4000 could be overcome with double-focusing instruments (16). However, some isobaric interferences probably cannot be separated even by double-focusing instruments because resolution greater than 300 000 is needed. High-resolution, double-sector ICP/ MS systems are uncommon, mainly because of their high cost. Furthermore, 16A
as resolution increases, the ion transport efficiency through the mass spectrometer to the detector tends to fall (16). Fourier transform ion cyclotron resonance (FT-ICR) systems are capable of high resolution but operate at very low pressures, and efficient coupling of atmospheric-pressure ICPs with FT-ICR spectrometers would be difficult. To date, no ICP/FT-ICR instruments have been reported. Nonspectral matrix effects. Although many matrix effects in ICPOES and ICP/MS are in general less severe than those encountered in flames or graphite furnaces, the severity of the problem is strongly dependent on its origin. Formation of refractive molecular species is less severe in ICPs than flames. However, at high concentrations almost every matrix can cause a change in sensitivity, as shown in Figure 2 (17). Errors can be
produced by dissolved solids as well as solvents (acids or organic solvents) (18-20),and matrix effects can originate in the aerosol generation and transport processes or in the plasma itself. The most severe errors resulting from high concentrations of dissolved solids are from elements that are highly ionized in the ICP. However, according to equilibrium calculations, all elements with ionization potentials < 8 eV should be more than 90%ionized in the analytical zone of the ICP. The nature of the matrix-induced error (including whether it enhances or depresses the signal) is highly dependent on experimental conditions (gas flow rates, applied power, etc.) and where in the plasma signals are measured. Low in the plasma, concomitant species produce increases in emission intensities; high in the plasma, depressions are
able I
Figure 2. Change in Zn sensitivity in the pres,,,ce of 0.05 M Na, Fe, Mg, Ba, ana Ti.
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PE Nelson’s Model 1020 Personal Integrator with’screendriven 100 ppm) can cause errors in ICP/MS. The matrix effects can often be minimized by reducing the nebulizer (center) gas flow rate, but at the expense of sensitivity. For example, under conditions where the optimum signal count rate was obtained, the Zn+ count rate decreased by about 80% when 1000 ppm Na was present (2). When the nebulizer gas flow rate was decreased from 1.16 L/min to 0.98 L/min, 1000ppm Na depressed the Zn+ count rate by < 30%.However, the Zn+ count rate fell by a factor of 10 for the Na-free Zn sample when the nebulizer flow rate was reduced to 0.98 L/min. In general, similar behavior is observed for other analytes and matrices (Figure 5). The signal depression is more severe for more massive matrix ions and less massive analyte ions. Matrix effects also depend on ion optics and the sampling conehkimmer interface design. The number of ions entering the mass spectrometer may decrease when the nebulizer gas flow rate is decreased so that the space charge effect is mitigated. Alternatively, plasma conditions may be less sensitive to the presence of matrix when the nebulizer gas flow rate is lowered (e.g., electron and Ar+ number densities and temperatures in the center of the plasma become higher). Also, the matrix concentration in the plasma should be lower when the nebulizer gas flow rate is reduced because the amount of sample aerosol entering the plasma falls. Solvent-related matrix effects. Less than 3%of the power coupled into the plasma is consumed by the sample. Less than 10W of power are required to vaporize, atomize, and ionize the 10 p L of sample aerosol that typically enter the plasma each second. About 20 W are needed for atomization and ionization of the water vapor carried from the spray chamber. Therefore, it is tempting to conclude that the sample will have little effect on the plasma. However, the desolvating droplets, and subsequently the vaporizing particles of sample, are confined to a narrow region in the center of the plasma, and local effects attributable to the sample can be large (4).There may be vaporizing droplets and particles as high as 20 mm above the load coil in a 1-kW Ar ICP. Furthermore, solvent vaporization begins low in the plasma so that the vaporized and atomized solvent can diffuse outward into the current-carrying region of the plasma, affecting both
tempting to conclude that the sample will have little effect on the from the sample-containing central channel. The presence of acids used to dissolve solid samples can lead to errors if the standards and the sample do not have similar acid concentrations (Figure 6, p. 20 A). The acid type and concentration are known to affect the aerosol droplet size distribution and transport rate, even when a pump is used to maintain a constant flow of sample to the nebulizer. Fortunately, increases in
sample transport rate are accompanied by a decrease in the fraction of atoms or ions that are excited, and the effect on emission intensities is less than expected if the ICP was an ideal source, simply reporting the amount of analyte entering the plasma (26). The addition of organic solvents to aqueous solutions tends to decrease the mean aerosol droplet diameter, leading to an increase in the sample transport rate. This phenomenon has been used to advantage in flame ionization spectrometry to enhance emission intensities. However, organic solvents have a deleterious effect on ICPs. For example, the power applied to the plasma must be increased by about 500 W when xylene is introduced into the plasma to match excitation conditions to those when aqueous aerosols are introduced (27). Small amounts of organic solvent can affect ICP emission intensities (18). For example, the addition of one drop of acetone to 50 mL of aqueous solution can produce a 10% decrease in Sr ion emission intensities. Ar and H emission intensities can fall by more than 30% when 0.1% (v/v) acetone is added. Although excitation conditions in the plasma are adversely affected by even small amounts of organic solvents, the situation may be quite different for ICP/MS. Laser-induced fluorescence intensities increase by about 20% when
Figure 4. Na atom emission intensity versus height above the load coil for alkali sence of easily ionizable matrices.
0.4% acetone is added to the sample solution, in contrast to the 40% de-
crease observed in emission intensities (Figure 7). The effect of 1%ethanol on ICP/MS signals depends on nebulizer flow rate (28). Strategies to minimize errors Clearly some important analytical problems remain in ICP-OES and ICPI MS. Five ways to deal with the potential errors are listed in the box on the next page. The appropriate procedure depends on the desired accuracy and reliability. It is often acceptable to use the analysis results provided by the instrument without further steps, but there is a risk of not knowing when an error has occurred. Internal standards can sometimes be used to improve analytical precision, particularly in ICP/MS. However, they may not overcome matrix-induced errors resulting from changes in plasma excitation conditions in ICP-OES. A change in plasma temperature affects spectral lines with different upper state energies dissimilarly. Therefore, internal standard lines must be chosen with characteristics similar to each analyte line of interest. Under certain plasma conditions, internal standards can produce significant improvements (29). Unlike ICP-OES, a 10%change in center gas flow rate can result in a 500% change in ICP/MS signal magnitude. Internal standards or isotope ratio methods often dramatically improve ICP/MS precision. Short- and long-term precision can also be improved using internal reference procedures. The response of each analyte line is calibrated as a function of power, central gas flow rate, and intermediate gas flow rate in the Generalized Internal Reference Method (GIRM) (30).Five internal standards are used to monitor these parameters over time, and the effect of changes in each parameter on the analytical results is removed via quadratic calibration curves. Two internal standards are used in the Parameter-Related Internal Standard Method (PRISM) (31). Both GIRM and PRISM require the addition of internal standards and extra time to determine the effect of each experimental parameter on each analyte line of interest. It is possible to remove matrix effects by closely matching the standards to the sample or by using the method of standard additions, presuming prior knowledge of the sample matrix or an interactive procedure. An extension of the one -component -at-a-time stan dard addition method, the Generalized Standard Addition Method (GSAM) (32),can be used. However, GSAM re-
quires a larger number of samples to be made and analyzed. For some samples it may be best to separate the matrix from the analytes of interest; however, this procedure may be very time consuming. Ion chromatography can be used to separate analyte and matrix species as well as to gain speciation information. Clever means to reduce aqueous and organic solvent loading of the plasma have also been reported. For example, a membrane-based device has been described to minimize organic solvent loading
(33) and a desolvation system including cryogenic cooling has been built to completely remove solvent from liquid aerosols (34). The selection of the appropriate strategy to minimize matrix-induced errors is highly dependent on the Sample type as well as the desired accuracy and precision of the analysis. Deciding when the matrix is a problem ultimately depends on how severely the matrix affects the sample aerosol generation, transport into the plasma, and plasma conditions.
Flgure 5. Sc+ signal as a function of added concomitant species and nebulizer flow rate. he two sets of conditions. Values are normatized to 100 without added matrix species (a) Optimized nebulizer flow rate (1.15 Lfmin) for maximum signal; 0.075 M concentrationsof matrix specie Lf ations of matrix specie
ANALYTICAL CHEMISTRY, VOL. 63, NO. 1, JANUARY 1, 1991
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REPORT
Flgure 6. Effect of sulfuric acid concentration on Fe2+ emission intensity.
Figure 7. Effect of acetone on Sr+ emission and fluorescence intensities.
The key for accurate analysis is to match the sample aerosol generation, transport, and plasma conditions when the sample is analyzed to those used when the standards were measured. Therefore, diagnostic signals that monitor plasma excitation conditions and sample transport into the plasma can be used to detect potential analysis errors. Furthermore, if the plasma and sample transport conditions can be controlled (to force constant conditions regardless of sample type), matrix effects can be eliminated. For example, if a particular matrix causes a reduction in the plasma temperature it may be possible to overcome this problem by increasing the power applied to the plasma. The combination of diagnostic signals and instrument parameter control would lead to truly intelligent instruments. However, sound understanding of the sample aerosol generation, transport, plasma, and plasmahample interactions is needed first. The robustness of the diagnostics and instrument intelligence must then be demonstrated with a wide range of sample types before being used for analysis. The final possibility is to design a totally new system for elemental analysis. Because many of the fundamental processes leading to emitting atoms and ions in the ICP are not fully understood, it is difficult to predict which conditions are optimum. For example, should the plasma be in local thermodynamic equilibrium? What should the temperature be? Should the sample be introduced as a liquid aerosol or should the solvent be removed? What is the optimum droplet size distribution? Should separate vaporization and ionization/excitation sources be used? What number of ions should be collected by mass spectrometer interfaces? The answers to these questions will be difficult to predict until we understand the key processes controlling vaporization, ionization, and excitation of analfie in plasmas. A future REPORT will focus on the recent progress and current understanding of fundamental processes controlling ICP emission and mass spectrometric signals. References (1) Meyer, G.A. Anal. Chem. 1987, 59, 1345 A-1354 A. (2) Horlick, G.; Tan, S. H.; Vaughan, M. A.; Shao, Y. In Inductively Coupled Plasmas in Analytical Atomic Spectrometry; Montaser, A; Golightly, D. W., Eds.; VCH Publishers: New York, 1987. (3) Boumans, P.W.J.M.; Vrakking, J.J.A.M. Spectrochim. Acta, Part B 1987, 42B. (4)819-40. Olesik, J. W.; Smith, L. J.; Williamsen,
E. J. Anal. Chem. 1989,61,2002-08. 20 A
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( 5 ) Boumans, P.W.J.M.; Hieftje, G. M. In
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John W. Olesik received his B.S. degree from the University of Rochester in 1977 and his Ph.D. from the University of Wisconsin-Madison in 1982.He was a postdoctoral research associate at Indiana University before joining the faculty of UNC-CH in 1984. His research interests include optical spectroscopy, spectrochemical analysis, fundamental processes in plasmas used for elemental analysis and materials processing, plasma diagnostic techniques (using emission, laser-induced fluorescence, and MS), spectroscopic imaging and detectors, and development of intelligent spectrochemical instruments.
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