Determination of several trace metals in simulated fresh water by

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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978

Determination of Several Trace Metals in Simulated Fresh Water by Graphite Furnace Atomic Emission Spectrometry M. S. Epstein," T. C. Rains, T. J. Brady, J. R. Moody, and

I. L.

Barnes

Institute of Materials Research, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234

Copper, manganese, barium, aluminum, molybdenum, nickel, and beryllium are determined in the part-per-billion concentration range in Standard Reference Material (SRM) 1643 (Trace Elements in Water) using graphite furnace atomic emission spectrometry. The precision and accuracy of results are compared to analysis by graphite furnace atomic absorption spectrometry. The effect of chemical, physical, and spectral interferences on analytical results using both techniques is evaluated.

Graphite furnace atomic emission spectrometry (GFAES) differs from graphite furnace atomic absorption spectrometry (GFAAS) in that the furnace is employed as the spectral excitation source as well as the atomization cell, and light intensity due to radiational deactivation of excited atoms in the furnace is measured, rather than the attenuation of a light beam due to absorption processes. Using conventional tube furnaces designed for GFAAS, several investigators have shown GFAES to be an extremely sensitive and useful technique for the analysis of a considerable number of elements (14). The limit of detection (LOD) using GFAES is significantly lower than GFAAS LOD's for several elements whose resonance lines lie in the visible region of the spectrum (Le., Ba, Na, K). GFAES and GFAAS LOD's for many other elements are similar, as shown in Table I. T h e National Bureau of Standards (NBS) has recently introduced a "trace-elements-in-water" Standard Reference Material (SRM), intended for evaluating the accuracy of trace element determinations in filtered and acidified fresh water. This SRM contains 19 trace metals of ecological or toxicological significance a t concentrations approximating those found in natural fresh water. In addition to these trace metals, gold has been added at a concentration of approximately 10 ng/mL to stabilize mercury and Na (10 pg/mL), K (2 pg/mL), Ca (27 kg/mL), and Mg (7 ,ug/mL) have been added a t concentrations approximating natural fresh water to simulate a natural water matrix. The solution is stabilized with 0.5 M "OB and stored in polyethylene containers. Both GFAES and GFAAS were utilized in the analysis of NBS-SRM 1643 (Trace Elements in Water). These techniques are sufficiently sensitive t o permit the analysis of a large number of the trace elements present in the SRM. In this paper, data relating t o the analytical precision and accuracy of GFAAS and GFAES are presented and the effect of chemical, physical, and spectral interferences on each technique is evaluated. EXPERIMENTAL Instrumentation. The instrumentation used for GFAES, which has been detailed previously ( 3 ) ,consists of an HGA-2100 graphite furnace, quartz optics, a 0.5-m monochromator, and associated electronics. Wavelength modulation ( 7 ) is employed as a background correction system. The monochromator was modified for wavelength modulation by placing a vibrating quartz refractor plate mounted on a torque motor between the entrance This paper not subject to U.S. Copyright,

Table I. Comparison of Furnace Atomic Emission and Absorption Detection Limits (ng/mL)= E le men t GFAAS~ GFAESC Aluminum 0.1 0.04 Barium 2 0.08 Beryllium 0.06 2 Chromium 0.2 0.4 Cobalt 0.8 3 Copper 0.1 0.4 Lead 0.1 60 Magnesium 0.06d 2 Manganese 0.02 0.7 Molybdenum 1 1 Nickel 2 4 Sodium 0.2d 0.002e Titanium 40 10 Potassium O.ld 0.002e a Relative detection limits based on a 50-pL sample volume. Detection limits from this laboratory calculated as the analyte signal equivalent to two times the estimated maximum standard deviation of the background or blank signal during the atomization step. Graphite Furnace Atomic Absorption Spectrometry (26). Graphite Furnace Atomic Emission Spectrometry, HGA-2100 ( 3 ) , except where noted. Sensitivity (ng/mL for 0.0044 absorbance unit). e Carbon Furnace Atomic Emission Spectrometry, HGA-2000 ( 5 , 6). slit and the collimating mirror. The modulation apparatus is driven by the sinusoidal signal from a function generator and audio-amplifier, and the signal from the photodetector is processed by a phase-sensitive synchronous amplifier referenced to the second harmonic of the modulation frequency. Quantitation is performed by peak height measurements of signal tracings from a chart recorder. Analysis by GFAAS was performed using a Perkin-Elmer Model 603 atomic absorption spectrometer and the HGA-2100. Electronic peak height detection and deuterium-arc background correction were used. The instrumental conditions utilized in the analysis of the individual elements are summarized in Table 11. Standardization. Working standards were prepared by serial dilution of aqueous stock solutions, as described by Dean and Rains (8). These were acidified with H N 0 3 to 0.5 M to match the acid concentration of SRM 1643. Standard addition procedures were used to check for the presence of matrix suppression or enhancement, and to correct for these interferences when they were present. The major interfering element was found to be calcium which caused a severe depression of absorption and emission signals for some elements and an enhancement of signals for others. In order to compensate for this interference, calcium was added to all standard solutions at a concentration equal to that found in SRM 1643 (27 Fg/mL). RESULTS AND DISCUSSION Analytical Precision. The results of the analysis of SRM 1643 by GFAES and GFAAS are shown in Table 111. Statistical evaluation of this data using the variance ratio or F test (9) at a significance level of 0.01 (probability level of 99%) indicates that the variability of GFAES results exceeds the variability of GFAAS results for the analysis of Al, Be, Mn, Published 1978 by the American Chemical Society


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Table 11. Analysis Conditionsn

a

Wavelength, nm E le men t GFAES GFAAS Charring Aluminum 396.2 309.3 1000 Barium 553.6 553.6 7 00 Beryllium 234.9 234.9 400 Copper 324.8 324.8 7 00 Manganese 403.0 279.5 7 00 Molybdenum 390.0 313.3 1200 Nickel 352.4 232.0 1000 Analysis performed with internal argon gas flow in normal mode on HGA-2100.

Temperature, “C Atomization GFAES GFAAS 2750 2750 2700 2700 2800 2650 2750 2500 2700 2700 2800 2800 2750 2700

Table 111. Results from the Analysis of SRM 1643 Concentration, ng/mL GFAES

GFAAS Certificate Element nn Xb SC n F S valued 5.7 771 1 Aluminum 16 82.1 1.4 21 77.1 Beryllium 10 18.8 21.3 5.5 191 1 0.4 17 Copper 8 14.0 16.2 1.8 16i 1 0.9 18 Manganese 0.7 12 28.0 2.5 29 i 1 10 27.5 Molybdenum 3 5 110 5 105 t 3 8 104 Nickel 7 49.8 51.3 4.2 49i 1 0.8 17 Barium 10 18.7 19.7 1.0 (18)e 0.7 10 Based also on Number of analyses included in the average. Average value of analyses. One standard deviation. several other techniques besides those discussed here (Neutron Activation, Isotope Dilution Mass Spectrometry, and Polarography). e Information value. Barium is not certified because of the large difference between its initial concentration ( 3 9 ng/mL) corresponding to the amount added to water and the stabilized concentration ( 1 8 ng/mL). Ni, and Cu. No significant difference in variability is found between the GFAES and GFAAS analysis results of Ba and Mo at this significance level. T h e apparently poorer precision of the GFAES data compared t o the GFAAS data in these analyses is a result of both the different detection systems used and the differences in the techniques themselves. In order to clarify the differences in precision, the following experiment was performed: Twenty replicate determinations of four different elements were made using exactly the same instrumental and analysis conditions for both techniques. The samples were introduced into the graphite tube with a n automatic pipetting device (Perkin-Elmer AS-1) to minimize pipetting errors. Element concentrations were chosen so that the signal-to-noise ratio (SNR) of the determination by either technique was approximately equivalent, and the SNR was large enough so that variability due t o the background noise level could be neglected. The GFAES data was corrected for nonlinear slope by relating the slope around the data point to a slope of unity and calculating a correction factor to allow a valid comparison of GFAES and GFAAS data. Evaluation of the variability of the data in Table IV indicates that in the case of Al, Cu, and Mn, there is a significant difference in precision (0.01 significance level). A marginally significant (0.05 significance level) difference in precision for the Ba analysis is observed. Because of the conditions of this experiment, the poorer precision of the GFAES analyses must be related to the source of the signal rather than the system noise. T h e absorption signal maximum is only a function of the concentration of the analyte atoms in the light beam, which in turn, depends on the atomization process and the rate of diffusional or convectional removal of the atomic vapor from the tube. On the other hand, the emission signal is affected by both the atomic concentration and the temperature of the graphite tube. This means that the reproducibility of the emission signal will depend on the reproducibility of the

Table IV. Precision of Emission and Absorption Measurements Emission Absorption Concn, RSD, Concn, RSD, Element nga %b nga %b Aluminum 1.6 2.8 1.6 0.5 Barium 0.5 4.4 5.0 2.9 5.0 0.2 1.8 Copper 2 5.4 0.2 1.0 Manganese 4 Absolute concentration of element deposited in the furnace. Relative standard deviation (%) of 20 replicate determinations with the automatic sample introduction device.

heating rate of the tube and the tube temperature, as well as the factors which affect the atomic concentration. Long-term signal drift may also be more significant in emission, since changes in the tube resistance may affect its temperature. Analytical Accuracy. Any difference in the accuracy of the two techniques for the analysis of a specific element can be assessed using the “certified” concentration for that element and a statistical analysis of the two data sets. A determination of whether a statistically significant difference exists between the average values of the GFAES and GFAAS analysis results can be performed using the t distribution as described by Natrella (9) for the case of unknown variabilities which cannot be assumed equal. The conclusions will depend on the significance level chosen for the evaluation. In this case, it is necessary that a significance level be chosen which requires a n extremely significant deviation of results before we conclude that an accuracy difference exists between the two techniques. McCutchen (10) interprets the student t test as indicating a difference in averages only when conclusive (Le., when the experimental t value is greater than the theoretical t value obtained from standard tables) a t the 99% probability level. Under these

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conditions, the average GFAES and GFAAS analytical results in Table I11 for copper and aluminum show a significant difference. In these two cases, the GFAES results are almost exactly coincident with the mean certified values of SRM 1643. Even though a statistically significant difference in accuracy occurs in the analysis of two of the seven elements, no specific conclusions can be drawn as to inherent accuracy differences between the techniques. Such differences are a function of analytical conditions and matrix interferences, and will vary with each element, matrix, and analytical instrument. While the precision differences can be related to specific variables in the furnace instrumentation employed, evaluation of any accuracy differences is much more complex, requiring a far more in-depth study. Specific Elements. Barium. The analysis of barium by atomic absorption using a graphite furnace is severely restricted by two factors: the continuum background correction sources found on most commercial instruments are not intense enough at the wavelength of the barium resonance line (553.6 nm) to match line source intensities and the blackbody radiation from the heated graphite severely increases photomultiplier shot-noise. The effect of the blackbody radiation is particularly significant when relatively large spectral bandpasses are employed. Barium, on the other hand, is one of the elements for which furnace emission measurement is extremely sensitive. Background correction in emission is automatic using wavelength modulation ( 3 ) . The effect of ionization on the analytical signal from barium in the graphite furnace appears to be a more significant factor in GFAES than in GFAAS. This is analytically significant since the presence of alkali metals, which easily ionize to produce electrons, will result in a more severe enhancement of the analytical signal in GFAES than in GFAAS. This is discussed in further detail in the section on interferences. A 10% enhancement of barium emission signals was observed in the analysis of SRM 1643. No matrix effects were observed in GFAAS. Memory effects were significant in both techniques, and a “dry” atomization for approximately 5 s was required between each sample introduction. Manganese. The LOD for manganese by GFAAS is more than an order of magnitude lower than the LOD using GFAES, as shown in Table I. However, the concentration of manganese in SRM 1643 was high enough to allow both techniques to be used. The GFAES analysis of SRM 1643 for manganese was straightforward without any apparent matrix interferences. However, the presence of calcium in the matrix introduces the possibility of spectral interference, particularly when emission analysis is performed without wavelength modulation. The emission band structure of CaO is present in the background spectrum from the graphite furnace when calcium is present in significant amounts ( > l o pg/mL) in the sample matrix or when a particular graphite tube has been used previously with samples containing high calcium concentrations (11). The GFAAS analysis of manganese in SRM 1643 is not complicated by spectral interferences. However, background correction was used to eliminate the possibility of molecular absorption or scatter interferences. The effect of calcium and chloride (as HC1) was studied as a possible source of chemical interference in the analysis. While calcium concentrations of 100 pg/mL produced no effect on the absorption signal of 0.03 pg Mn/mL in a 1% HNO, medium, higher calcium concentrations (1000 Fg/mL) produced an enhancement of 10%. In a medium of 1TC HC1, absorption values for manganese were found to be erratic and interferences due to other concomitants (such as Ca) increased.

Therefore, the chloride ion should be avoided whenever possible in GFAAS. Molybdenum. The major problem in the analysis of molybdenum by both GFAES and GFAAS is the memory effect caused by retention of molybdenum on the graphite tube surface after atomization. Unlike barium, this memory effect cannot be eliminated by a “dry” burn between samples. I t is therefore necessary to monitor the blank level between every atomization. Aluminum. The detection limits of both GFAES and GFAAS for the analysis of aluminum are similar. However the analytical results reflect a statistically significant difference in accuracy, as well as the difference in analysis precision noted in almost all the analyses. As in the case of manganese, the use of a chloride medium for aluminum results in an increase in chemical interferences as compared to a nitrate medium. Aluminum is volatilized as the molecular chloride and may be partially or completely lost prior to the atomization step. Since aluminum is ubiquitous and extremely sensitive using electrothermal devices, cleanliness in the laboratory is essential. Contamination may be introduced by the simple act of wiping a micropipet tip with a tissue prior to sample introduction into the furnace. Pyrolytic coatings on the graphite tube result in a significant increase in the sensitivity of aluminum analyses. In a nitrate medium, interferences can occur due to the presence of large amounts of calcium (12). T o overcome this difficulty, calcium is added to all standard solutions to match the sample matrix. While some authors have reported an improvement in analysis precision using peak area measurements (13), we found peak height measurements to be superior in the analysis of SRM 1643 by GFAAS. Peak area measurements in GFAES do not appear to be superior to peak height measurements, largely because of the interrelationship of emission intensity and furnace temperature. In GFAAS, the advantage of peak area measurement is that to a first approximation the total integrated absorbance will be proportional to the concentration of analyte, regardless of the rate of atomization. This is valid because the absorbance signal is a measure of only the atom population and the residence time in the furnace. In GFAES, the relationship of integrated emission intensity and concentration is more complex since furnace temperature must be considered. The emission intensity is a function of both atom population and furnace temperature. Because the temperature of the furnace changes during atomization, the excitation energy available for producing emission spectra will change accordingly. Changes in the rate of production of atoms in the furnace will affect the atom population-time relationship (shifting it to higher or lower temperatures) and thus will change the integrated emission intensity. Integration of GFAES peaks is further complicated by the tendency of emission peaks to “tail“ or spread out after the peak maximum during the atomization step. This is most significant when the gas-stop mode is used during atomization. The effect is due to the increasing temperature of the furnace during the atomization step. The dependence of emission intensity on both atom population and temperature results in a greater emission signal (relative to the peak maximum) than absorption signal during the time following the peak maximum since the absorption signal is dependent only on the atom population. The effect of tailing on the integrated signal is twofold. If the emission signal never returns to zero intensity before the end of atomization, an accurate integration measurement cannot be made. Furthermore, the noise level of a GFAES measurement increases greatly toward the end of the


atomization step because of the increase in blackbody radiation striking the photomultiplier tube and generating shot-noise. An integrated GFAES measurement will include more of this noise and will therefore result in a decrease in analysis precision and poorer detection limits. Structured background emission spectra from the furnace (such as C2 band emission) may also result in significant baseline offset during the last part of the atomization step. This offset will be included in an integration measurement but not in a peak height measurement. The result on the integrated signal will be a long term drift component which is dependent on the intensity of the background signal and the wavelength stability of the monochromator. We have compared integration and peak height measurements in GFAES using the HGA-2100 graphite furnace, and have found the latter to be more precise and satisfactory for analytical measurements (14). Beryllium. The significant difference in analysis results between emission and absorption for beryllium is primarily due to the much lower LOD for atomic absorption, as shown in Table I. Beryllium reacts with carbon to form a refractory carbide which inhibits its atomization. This problem can be overcome to some extent by the use of pyrolytic graphite tubes. The effect on beryllium absorption signals in the graphite furnace of the major cations found in natural waters was investigated. Sodium exhibited no interferences, while calcium concentrations of 25 to 100 pg/mL enhanced the absorption signal of 20 ng/mL by 200% in 0.5 M HN03. Addition of calcium to all standards compensated for the interference. Copper and Nickel. A comparison of analysis results for these elements shows a statistically significant difference in accuracy in the case of copper. The precision differences are a function of the difference in detection limits as well as other factors mentioned previously. The reason for the difference of the copper GFAAS analysis results from the certified value has not yet been explained. The absorption signals of copper and nickel did not appear to be appreciably affected by the normal concentrations of sodium and calcium found in natural waters. Calcium was added to all standards as a precautionary measure. Interferences. Since the accuracy and to some extent the precision of an analytical technique will be a function of the number and kind of interferences involved in its implementation, a study of the similarities and differences between interferences encountered in GFAES and GFAAS is of great significance. The major differences between the techniques may be summarized as follows: Shape of the Analytical Curve. While the analytical curve in GFAAS may be linear over several orders of magnitude, GFAES analytical curves exhibit linearity over an order of magnitude or more for some elements while showing little or no linear relationship to concentration for other elements. This is primarily due to self-absorption and self-reversal phenomena which result from the shape of the atomization cells utilized and the thermal gradient between the center and the end of the atomization cell (15). The magnitude of this effect will depend on the atomization characteristics of the analyte and the emission intensity of the element relative to its atomic absorption coefficient. The effect may also be modified by interaction with the wavelength modulation detection system. When the analyte emission line width becomes greater than the spectral bandwidth of the detection system, anomalies in the line shape will distort the analyte signal from the actual intensity-concentration relationship. An example is shown in Figure 1. This figure shows the intensity-time plot of aluminum atomic emission a t a very high concentration (10 000 pg/mL). The high concentration

Slit

877

Function

@,?

nm

1-

396 3

396.1

W a v e I e n g t h ( n in)

Figure 1. Spectral line distributions at various points on the intensity-time plot from the atomization of 10 000 p g h L aluminum in the HGA-2 100 (analysis wavelength = 396.2 nm; spectral bandpass = 0.1 nm; atomization at 2800 O C for 10 s). (a) Complete self-reversal at the start of atomization. (b) and (c)Partially self-reversed and asymmetrical distributions. (d) Broadened distribution at the intensity maximum

is necessary to bring the effect to within the resolution capability of the 0.5-m monochromator used in the study. The line profiles were obtained from an oscilloscope synchronized to the modulation frequency of the refractor plate (7). The slit function of the monochromator at a spectral bandpass of 0.1 nm is shown in the lower right corner of Figure 1. At the beginning of the atomization step (a), the line is extremely broadened and self-reversed. A negative signal is generated a t this point because the intensity distribution within the wavelength modulation interval is peaked in the negative direction. The second harmonic detection system will see the largest intensity component from the self-reversal dip while only a small positive component is generated from the line itself. A t (b), the positive and negative second-harmonic intensity components are equal and the net output intensity is zero. A t (c), self-reversal has diminished and the positive intensity component becomes predominant. Finally, a t (d), the signal maximum, the effect of self-reversal on the line shape has disappeared, although the line is still severely broadened.

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5

CALCIUM ADDED

50

( IN

1I

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Flgure 2. Effect of calclum on manganese atomic emlsslon and atomlc absorption uslng the HGA-2100. Analysls condltlons: 403.1 nm emlsslon; 279.8 nm absorptlon; 2.5 ng of manganese atomlzed at 2800 OC uslng gas-stop durlng atomlzatlon

The asymmetry of the self-reversed spectral distributions shown in (a), (b), and (c) of Figure 1 is real and is not an anomaly of the wavelength modulation detection system. It has been observed in GFAES for several other elements, including calcium, sodium, magnesium, and copper. The asymmetry appears as the self-reversed line with a more intense long wavelength component and can be explained by a wavelength shift of the absorption line and/or the emission line. Because of the poor resolution of the oscilloscope trace and the variations in the signal, it is difficult to estimate the magnitude of the shift, although it appears to be on the order of 0.01 nm. The asymmetry of self-reversed spectral lines has been reported by several authors, including Braun et al. (16) for Lyman-cu line profiles generated from a microwave-powered lamp, by Human and Scott (17) for high concentrations of several elements in an inductively-coupled argon plasma, and by Piepmeier and deGalan (18) for pulsed hollow cathode lamps. Several explanations can be proposed t o account for the shift of the absorption or emission lines in GFAES. Piepmeier and deGalan (18) proposed that the asymmetry in line profiles observed from pulsed hollow cathode lamps was caused by Doppler wavelength shifts of the absorbing and emitting species, dependent on their velocities relative to the detector. In GFAES, during the atomization cycle, when the vaporization or decomposition temperature of the analyte on the surface of the graphite tube is reached, a large part of the analyte may be physically expelled from the surface into the furnace chamber because of the rapid heating rate and the high analyte concentration. The net directional velocity of these atoms relative to the line of sight of the detector will be zero. However, the atoms in the absorbing layer closest to the detector (i.e., those expelled into the cooler portion of the furnace closest to the detection system and responsible for self-reversal) will have a directional velocity toward the detector, which will result in a Doppler shift toward shorter wavelengths of the absorbing line relative to the emission line. Because of the uncertainty in the measurement of the wavelength shift, it is difficult to relate it to the velocity required to produce a Doppler shift. However, it can be said qualitatively that the required velocity (105-106 cm/s) is similar to the predicted thermal velocities (104-105cm/s) of

atoms a t the temperature of the furnace. Further investigations are being presently performed using a high-resolution echelle spectrometer and will be reported in a future publication. Another explanation that should be considered is a shift of the emission line to longer wavelengths relative to the absorption line due to resonance (Holtzmark) interactions in the presence of a high density of analyte atoms. Shifts reported by Braun et al. (16) in a microwave discharge were explained in this manner. Such an explanation requires the assumption of a difference in the furnace environment between emitting and absorbing species, which is not unreasonable, since temperature and pressure gradients in the HGA-2100 have been reported. Although the interaction of the wavelength modulation system with line shapes may result in the distortion of analytical curves, the effect is only significant at very high concentrations where broadening and reversal effects become resolved. Self-absorption and reversal, although unresolved by the spectrometer, are primarily responsible for the nonlinearity of GFAES curves a t low concentrations. The result of the nonlinear intensity vs. concentration relationships of some elements in GFAES is that analysis requires either calibration by close bracketing with standards or an exact definition of the shape of the analytical curve. In either case, accuracy may be poorer than in the case of a linear analytical curve. Chemical Interferences. A t the present time, there is no significant evidence that chemical interferences differ in GFAES and GFAAS, although such a difference might be expected due to the different mechanisms of the two techniques. In Figure 2, a close correlation of signal suppression between emission and absorption measurements is observed. Calcium causes a reduction of manganese emission and absorption signals in an HC1 medium. This interference may be attributed to the formation of a vapor-phase compound of manganese and chlorine, which reduces the population of manganese atoms, therefore affecting both techniques similarly. Similar effects have been observed by Ottaway (19) for magnesium and Czobik and Matousek (20) for several transition metals. Since the mechanism of both techniques is the same up to the atom formation process, it would be expected that

ANALYTICAL CHEMISTRY, VOL. 50. NO. 7, JUNE 1978

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Figure 3. Emission or absorption-time distributions illustrating the relationships of atomic (-) and ionic ( - - - ) species in the HGA-2100 (The ion signals have been scale-expanded to permit easy comparison with the atomic signals). (A) Emission signals from 1 ng of barium at 553.5 nm (atomic) and 455.4 nm (ionic) under gas-stop atomization conditions, 0.3-s time constant, at 2800 OC. (B) Absorption signals from 10 ng of barium under conditions identical to emission measurements

chemical interferences, particularly those involved in enhancing or suppressing the atomization process, would be similar for both techniques. The magnitude of these interferences could be different since a change in the atomization rate might affect the emission signal in a completely different manner from the absorption signal. For example, an extreme case might be that a decrease in the rate of atomization might decrease the absorption signal while the increased temperature a t which the emission maximum occurs might increase the emission signal. Chemical interferences occurring in the vapor phase might also differ because of the difference in the time of absorption and emission signal maxima, although this does not seem to be a significant effect in Figure 2. We did not observe significant differences in chemical interferences in the analysis of SRM-1643, although some minor effects could be responsible for the accuracy differences observed. Ionization interference, specifically in the case of barium, is a more significant problem in GFAES than in GFAAS. Figure 3 presents a comparison of the barium emission-time and absorption-time relationships of furnace measurements a t a maximum furnace temperature of 2800 “C. The actual vapor temperature experienced by the atomic vapor cloud depends on the time at which the analytical measurement is made, but is significantly less than the maximum furnace temperature (21)which is the temperature of the graphite tube walls (measured with a pyrometer) at the end of the atomization step. The degree of ionization in GFAAS appears to be small a t the absorption maximum, and a t the time of the emission maximum, the ionization in GFAAS is still much less than in GFAES. This is probably because absorption measurements characterize the entire atomic vapor cloud

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which experiences a lower overall vapor temperature than the atoms which result in emission signals. The emission measurements characterize the ionic population of the portion of the atomic vapor which experiences the highest vapor temperatures (Le., in the hot central portion of the furnace). The true “total” ionization is measured by absorption while an “apparent” ionization is measured in emission. Of further interest are the relative shapes of the emission-time profiles of the ionic and resonance lines. The emission due to the ion population appears after the appearance of the emission due to the atomic population, but disappears before the atomic population does. The ionization mechanism in GFAES, like the excitation mechanism, has not been adequately defined. However, it is obviously related to the temperature of the furnace. No matter what causes the ionization, the shorter lifetime of the ionic population can probably be explained by the segregation of ionic species in the hot center of the furnace. The formation of ions will occur after the formation of atoms since the former requires a greater energy transfer. As determined by Sturgeon and Chakrabarti (22), a major cause of analyte loss in an HGA-2100 furnace using gas-stop conditions is diffusion in the cooler ends of the graphite tube where condensation occurs. The atomic population would therefore tend to increase relative to the ionic population as the ions diffused into the cooler portion of the furnace where recombination phenomena would be more likely to occur than re-ionization. The ionic emission intensity would therefore decrease a t a faster rate than the atomic emission intensity. This effect is not apparent under gas-flow conditions, where convectional loss of the analyte is significant. Spectral Interference and N o n - A t o m i c Absorption. In GFAAS, non-atomic absorption due to scatter or molecular absorption is often a significant interference, as illustrated in Figure 2. The interference is additive in nature and is usually corrected for by the use of a continuum radiation source, such as a hydrogen hollow cathode or deuterium-arc lamp, time-shared with the line source. In analysis with a graphite furnace, the use of background correction is a requirement to obtain the highest level of accuracy. Scatter and molecular absorption do not appear to be very significant interferences in GFAES. The effect of the interference should be multiplicative and, similar to a signal suppression by a chemical interference, correctable by standard addition procedures. Spectral interferences in GFAAS may be classified into two groups: direct spectral overlap and spectral overlap with the background correction radiation. The former occurs rarely, as discussed by Fassel et al. (23),and involves the overlap of matrix component absorption lines with the analyte emission line or other emission lines in the spectrum of the line source passed by the spectrometer. Two examples of this type of interference are the zinc (213.856 nm) and iron (213.859 nm) pair and the zinc (213.856 nm) and copper (213.853 nm) pair, which cause severe problems in the determination of zinc in specific matrices, such as steel or high-purity copper (24). The latter type of interference, caused by non-analyte absorption lines in the spectral bandpass, is particularly significant in GFAAS since background correction is commonly employed. The interference involves the atomic absorption of the continuum background-correction radiation by a matrix component line which may or may not overlap the analyte line. The result is generally an overcorrection by the system and a negative deviation of analytical results. Direct spectral overlap interferences in GFAES are more severe than in GFAAS. However, the relatively low temperature of the source compared to analytical flames and the possible application of temporal separation of analyte and

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interferent signals by volatilization using temperature programming make the interferences less severe than in a comparison of the flame techniques. Wavelength modulation can be used to reduce or eliminate additive errors caused by spectral overlaps of continuum or broad-band spectral distributions, such as the blackbody emission from the heated graphite. When the spectral overlap is well characterized, wavelength modulation can also be used to reduce or eliminate the effect of structured overlaps such as sharp molecular bands or atomic lines ( 7 ) . Spectral interferences cause a problem in GFAES in situations where the analytical curve is nonlinear. When a blank signal is observed, it must be identified as either a spectral interference or contamination since data treatment for one will be different from the other. The intensity from a spectral interference must be subtracted from the analyte intensity while the concentration of a contamination blank must be subtracted from the analyte concentration. The magnitude of the effect of all these interferences will depend on the element and matrix involved. In some cases, emission may be preferred; in others, absorption. Furnace atomic emission appears to be a viable alternative to furnace atomic absorption. For some elements, it would be preferred because of greater sensitivity and/or reduced background-correction problems. A comparison of GFAES and GFAAS in the analysis of SRM 1643 demonstrates that there may be some differences in the accuracy of the two techniques, but further investigation is required to adequately explain them. Differences in precision have been discussed. Since the analyses described were performed with a system designed for atomic absorption, it would be reasonable to expect an improvement in analytical results with changes in furnace design. Ottaway and Shaw (6) and Ottaway and Hutton ( 5 ) have described changes in the graphite tube shape to reduce self-absorption and increase analytical curve linearity. The possibility of increasing sensitivity by increasing the temperature experienced by the analyte at its maximum atomic concentration in the furnace makes GFAES a promising technique for the future. Steps in this direction have been taken by Littlejohn and Ottaway ( 2 5 ) ,who have employed rapid furnace heating to improve detection limits for several elements. This technique may also reduce some of the problems associated with integration measurements in

GFAES. Increased control of furnace parameters may also improve the precision to values competitive with GFAAS.

ACKNOWLEDGMENT The authors express their gratitude to W. Braun and J. Mandel of the National Bureau of Standards for their help in the research involved in this paper. LITERATURE CITED J. M. Ottaway and F. Shaw, Anaiyst(London), 100, 438 (1975). J. M. Ottaway and F. Shaw, Anal. Lett., 8, 911 (1975). M. S.Epstein, T. C. Rains, and T. C. O'Haver, Appi. Spectrosc., 30, 324 (1976). J. M. Ottaway and F. Shaw, Anaiyst(iondon), 101, 582 (1976). J. M. Ottaway and R . C. Hutton, Anaiyst (London), 101, 683 (1976). J. M. Ottaway and F. Shaw, Appl. Spectrosc., 31, 12 (1977). M. S. Epstein and T. C. O'Haver, Spectrochim. Acta, Part 8,30, 135 (1975). J. A. Dean and T. C. Rains, Chapter in "Flame Emission and Atomic Absorption Spectrometry", Vol. 2, J. A. Dean and T. C. Rains, Ed.. Marcel Dekker, New York, N.Y., 1971, p 327. M. G. Natrelia, "Experimental Statistics'', National Bureau of Standards Handbook 91, Washington D.C., 1966. R. L. McCutchen, "Statistical Treatment of Data", ORNL Master Analytical Manual, Oakridge, Tenn., 1954. R . C. Hutton, J. M. Ottaway, M. S. Epstein, and T. C. Rains, Anaiyst (London). 102. 658 11977). K. C. Thompson, R. G.Gcdden, and D. R. Tomerson, Anal. Chim. Acta, 74, 289 (1975). P. Schramel, Anal. Chim. Acta, 72, 414 (1974). M. S. Epstein, P h D Thesis, University of Maryland, College Park, Md., 1976. M. S.Epstein, 28th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1977, Paper 155. W. Braun, A. M. Bass, and D. D. Davis, J . Opt. Soc. Am., 60, 166 (1970). H. G. C. Human and R. H. Scott, Spectrochim. Acta, Part 8 ,31, 459 (1976). E. H. Piepmeier and L. deGalan, Spectrochim. Acta, Part 8 ,30, 263 (1975). J. M. Ottaway, Proc. Anal. Div. Chem. Soc., 13. 185 (1976). E. J. Czobik and J. P. Matousek, Anal. Chem., 50, 2 (1978). R. E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, Part 8,32, 231 (1977). R. E. Sturgeon and C. L. Chakrabarti, Anal. Chem., 49, 1100 (1977). V. A. Fassel. J. 0. Rasmuson. and T. G. Cowiev. Acta. , Soectrochim. . Part 8 ,23, 579 (1968). A. T. Zander, T. C. O'Haver. and P. N. Keliher, Anal. Chem., 49, 838 (1977) \ . - . . I .

D. Littiejohn and J. M. Ottaway, unpublished work. Perkin-Elmer Instrument Literature, HGA-2100, February 1974, March 1977.

RECEIVED for review September 30, 1977. Accepted February 24, 1978. The specification of commerical products does not imply endorsement by the National Bureau of Standards.

Determination of several trace metals in simulated fresh water by

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