1240
Anal. Chem. 1983, 55, 1240-1245
of 0.0105. The detection limit (for a signal-to-noise ratio of 2) is 69 pM. Implications. The results presented demonstrate the use of single coulostatic decay curves as a viable technique for obtaining concentrations and El!z values in the 2C-100 ms time scale. On such a time scale this technique could be applied as a detection method in liquid chromatography or flow injection analysis.
LITERATURE CITED
.
-0.3 +0,40
.
S
I
I
I
+0,30
+o.zo
Potential ( V I
Figure 7. Voltammogram constructed from the slope of the log-log decay curve for ferrocyanide at CPE.
to concentration and the change in slope occurs at a potential near Ell2. For the plot in Figure 6, the two linear sections intersect a t log E = -0.544 or E = 0.29 V. A plot of the slope of the log E vs. log t plot is shown in Figure 7. As with Figures 3 and 4, it resembles a normal voltammogram. Decay curves were collected for five solutions of ferrocyanide, ranging in concentration from 100 to 500 pM. The Ellz values obtained were all in the range from 0.28 to 0.30 V. These values agree very well with the literature value for Ellz and the differential pulse results given earlier. For these five solutions, the initial slope of the log-log decay curve was plotted as a function of concentration. The working curve obtained was linear with a correlation coefficient of 0.985 and a standard error estimate
(1) (2) (3) (4) (5) (6)
(7) (8) (9)
(IO) (11) (12) (13) (14)
(15) (16) (17)
Delahay, P. J. Phys. Chem. 1962, 66, 2204-2207. Delahay, P. Anal. Chem. 1982, 3 4 , 1267-1271. Delahay, P. Anal. Chim. Acta 1062, 2 7 , 90-93. Relnmuth, W. H. Anal. Chem. 1982, 3 4 , 1272-1278. Kudlrka, J. M.; Abel, R.;Enke, C. G.Anal. Chem. 1972, 44, 425-427. Hahn, B. K. Ph.D. Thesls, Mlchigan State University, East Lanslng, MI, 1974. Katzenberger, J. M.; Daum, P. H. Anal. Chem. 1075, 47, 1887-1893. Daum, P. H.; McHalsky, M. L. Anal. Chem. 1980, 5 2 , 340-344. Schrelber, M. A.; Last, T. A. Anal. Chem. 1981, 5 3 , 2095-2100. Last, T. A. Anal. Chem. 1982, 5 4 , 2327-2332. Barnes, A. C.; Nleman, T. A,, submitted for publication In Anal. Chem. van Leeuwen, H. P. I n "Electroanalytical Chemlstry"; Bard, A. J., Ed.; Marcel Dekker: New York, 1982; Vol 12, pp 159-238. Astruc, M.; Bonastre, J.; Royer, R. J. Nectroanal. Chem. 1072, 3 4 , 211-225. Avery, J.; Lovse, D. "The ADD8080 Microprocessor Manual"; Unlversity of Illlnols, Urbana, IL, 1977. Ide, Y. Mem. Fac. Sci., Kyushu Unlv., Ser. C 1972, 8 , 121-129. Kudlrka, J. M.; D a m , P. H.; Enke, C. G. Anal. Chem. 1972, 4 4 , 309-314. Barnes, A. C.; Nieman, T. A,, Urbana, IL, unpublished work, 1982.
RECEIVED for review February 7,1983. Accepted April 4,1983. This work was supported in part by the National Science Foundation (CHE-78-01614) and a NIH Biomedical Research Support Grant (RR07030).
Interferences in Electrothermal Atomization Metastable Transfer Emission Spectrometry H. C. Na and T. M. Nlemczyk" Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 87131
The range of appllcablilty of any analytlcal technique is often limited by the susceptlbllity of the method to Interferences. Metastable transfer emlsslon spectrometry (MTES) Is an atomic spectrometric technique and Is thus subject to those Interference processes that can affect technlques of this general type. I n the paper we assess the problems of Interferences that might posslbly cause a lack of preclslon. I t is shown that there Is a fundamental ilmlt to the amount of material, analyte and matrlx comblned, that can be introduced to the MTES plasma before the lntenslty vs. analyte mass relationship breaks down. I n addltlon, there can be interferences In the atomization step. These Interference processes are very slmliar to those seen In electrothermal atomlzatlon atomic absorption spectrometry. The results obtained here are compared to those obtalned In an atomlc absorptlon system. One major difference seen Is that the reactive nature of the nitrogen plasma can contribute to the ellmlnation of Interferences due to the vaporization or formation of molecular species lnvolvlng the analyte.
The great potential of metastable transfer emission spectrometry (MTES) as an analytical technique has been pointed
out in several recent publications (1-5). The technique involves using an active nitrogen plasma as an energy donor which, when brought into contact with a substrate, can transfer the stored energy to the substrate. When the substrate is an atomic vapor the energy transfer process, a collision of the second kind, results in excitation of the atomic vapor and the resulting luminescence is used as the basis of the analytical technique. The history of active nitrogen is long and full of conflicting claims. Much of the earlier work has been summarized in two monographs (6,7).Although there have been many controversies concerning various facets of active nitrogen chemistry, it is now generally agreed that the constituent most responsible for excitation of metal atoms is the metastable species, N2(A3Cu+) (8-14). Thus, it has been recognized that the concentration of N2(A3C,+) is one of the most important active nitrogen source parameters. The use of different active nitrogen sources has been prevalent even in the analytical literature. In several reports the source of active nitrogen was a microwave discharge (1-3). The generally accepted mechanism for the production of Nz(A3Cu+)in a low pressure microwave discharge is a recombination of nitrogen atoms. Although the nitrogen atoms are responsible for the production of N2(A3Cu+)via recom-
0 1983 Amerlcan Chemical Society 0003-2700/83/0355-1240$01,50/0
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
bination, they are also rather efficient quenchers of N2(A3C,+) and thus the nitrogen atom concentration might limit the maximum Nz(A3C,+) concentration. Dodge and Alllen used a low-pressure dielectric discharge as an active nitrogen source so that quenching due to nitrogen atoms would be avoided (4). D'Silva, Rice, and Fassel used a similar type di,scharge, but at atmospheric premure, to producle the active nitrogen used in their apparatuu (5). In any application of the MTES technique the relationship between the emission intensity, I , the energy donor concentration, [N2*],and the analyte concentration, [A], is expressed by the following equatiion I := klK[Nz*][A] (1) where kl is the rate constant for the excitation reaction and K is a constant vvhich consolidates factors such as the transition probability, atomization efficiency, collection efficiency of the optical system, etc. Under conditions of constant N2* production and eq 1 can be assumed to be first order and I = KIA] (3) Thus, a plot of I vs. [A] should be linear rmd serve as the basis of analytical determinations. It should be pointed out, however, that anything that changes any factor in K'cari lead to an interference. An especially vulnerable point is thie atomization efficiency for it can be affected by perturbations to the chemical nature of the active nitrogen plasma or to changes in the performance of ithe sample introduction system. As in most atomic spectroscopic techniques, MTES requires that the analyte be in the gas phase. For this reason much of the early work has been in analyst1 of gaseous species. Capelle and Sutton have determined bismuth and germanium by monitoring the atomic emission ]produced wlhen trimethylbismuth and germane were mixed with active nitrogen (2, 3). They also have employed their MTES system as a general purpose (15)as well as functional group specific (16, 17) gas chromatographic detector. In similar applications, Rice et al. used an atmospheric pressure active nitrogen system as a gas chromatograph detector (18) and also determined the concentration of several. elements in aqueous sampleis (5). In the latter case they were limited to mercury and elements that form volatile hydrides. Ramsey and Nelson used a MTES system to detect carbonic impurities in argon gas samples (19). In all these applications the analytes are limited to gaseous species or elements that readily form volatile compounds. There certainly can be interferences in such analyses, especially in those that require a hydride forming step, but they are of a different nature than many that occur when the analyte must be forced into the gaseous state by some sort of atomizer. In all applications involving the determination of transition metals in aqueous samples, an electrotlhermal atomizer has been employed to dry rind atomize the samples. Melzer et al. determined lead in aqueous samples by using a tantalum boat (20). Dodge and Allen determined zinc and mercury by using a tungsten wire atomizer (4). We recently reported the determination of' seven elements by using a tantalum boat atomizer as well as the effects of various experimental parameters on the MTES system (1). In all cases the results were obtained by using laboratory samples so matrix or other interferences were not discussed. It is well established that atomic absorption spectrometers using electrothermal atomization are subject to matrix effects and one would expect that a MTES system employing an electrothermal atomizer would be no different. In an atomic absorption system interferences may be the result of several effects. One common problem is the incom-
1241
plete dissociation of' analyte containing molecular species. It might a t first be assumed that this would lead to an interference in a MTES system (decreased atomization efficiency), but the reactive nature of the active nitrogen plasma can, in some cases, overcome this problem. MTES does have, however, other interference effects that are unique. In this paper we present a discu,ssion of the interferences that can be a problem in a MTES system employing an electrothermal atomizer. The results from this study will be frequently compared to those from furnace atomic absorption spectrometry (AAS) due to the many good studies that have been performed in this area and the lack of data for MTES systems.
EXPERIMENTAL SECTION The experimental apparatus has been previously described in detail (1).All samples were introduced to the tantalum boat with a 5.0-bL Eppendorf pipet. The samples were dried at approximately 100 "C and then atomized. The ashing stage of the programmable poweir supply was intentionally skipped in order that the vaporization of molecular species would be enhanced (21). This was done so thiat any interference effects seen would be magnified. In the studies involving matrix effects the concentrations of the cadmium, copper, and lead were 1.0 ppm, 1.0 ppm, and 2.0 ppm, respectively. 'The wavelengths monitored for the determinations were as follows: Cd, 326.1 nm; Cu, 324.7 nm; and Pb, 405.8 nm. All chemicals used to prepare samples or matrix materials were reagent grade. Stock solutions and all dilutions were prepared with distilled, deionized water. The argon and nitrogen used were "high purity" and wed without further purification. RESULTS AND DISCUSSION Anion Effect. It is well-known that, in furnace AAS, the magnitude of analyte signal intensity depends strongly on sample composition. For example, cadmium and lead chdorides (22-24) give reduced sensitivity when compared to their nitrate counterpartag, while cupric chloride has been foundl to give the same or even higher sensitivity than the nitrate (25, 26). In another report, the presence of hydrogen chloride has been shown to suppress the copper signal, presumably clue to the formation and volatilization of cuprous chloride (27). The simplest test to investigate the anion effect in this system is to run calibration curves for each metal starting with different salts. For cadmium and copper, nitrate, sulfate, and chloride compounds were compared. For lead, only nitrate and chloride salts were used due to the very small solubility product of lead sulfate. The results indicate that, for all three elements investigated, the signal produced from the chloride salt was not depressed when compared to the oxyanion salts. A plot of the signal intensity vs. mass of analyte is shown in Figure 1 for cadmium. Plots of the copper and lead data look essentially the same. The working curvet3 do show small but measurable changes in sensitivity when a peak height measurement is used, but the peak areas produced are, in all cases, identical. This deviation from the furnace AAS results, even when using atomization conditions designed to promote molecular vaporization, is most likely due to the reactivity of active nitrogen. Active nitrogen may decompose any undissociated molecules in the vapor phase. The signal vs. time profiles of cupric chloride are particularly interesting. As Figure 2 shows, a small signal emerges long before the main peak begins to appear. This suggests that two compoundei having distinct boiling points are formed during the vaporization process. Also, the size of the smlall initial peak grows as the total mass of cupric chloride is increased. This indicates that the phenomenon is probalbly surface related. Since the volume used is constant (5 bL), a solid sample is likely to spread out over a constant area. For higher mass, this will result in a multilayer deposit. To further
1242
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983 100 A 0
h
c .-
5
+a
75
A 0
v
I 0
ep
0
I
I
2.5
5.0
I
7.5
I
10.0
Mass o f Cd (ng) Figure 1. Signal Intensify vs. mass of analyte for cadmium deposited as different salts: circle, SO,-,; trlangle, NO,-; square, CR peak height indlcated by solid symbols; peak area Indicated by open symbols.
I 0
I
10
20
30
Time (sec) Figure 2. Signal vs. time profiles produced for the determination of varlous amounts of copper deposited as CuCI,: (A) 10.0 ng of Cu; (6) 7.5 ng of Cu; (C) 5.0 ng of Cu; (D) 2.5 ng of Cu. confirm this trend, samples containing 50 ng of cupric chloride were deposited on the atomizer and the peak profiles observed. In this case the small early-time peak of Figure 2 grows to the same relative magnitude as the main peak. When the experiment was run in the dark, a beautiful blue-green flame was observed, followed by a smokelike colorless flow. The blue-green flame is probably due to emission from cuprous chloride (bp = 545 "C at 2 torr). This phenomenon was first observed by Strutt when he mixed cuprous chloride directly with active nitrogen (28). In addition, the formation of cuprous chloride is reasonable since cupric chloride transforms to cuprous chloride upon heating. The dissociation energy of cuprous chloride is 83 kcal/mol (3.59 eV), which is well below the energy level of Nz(A3C,+) (6.17 eV at the zero vibrational level); thus, the breakup of cuprous chloride via energy transfer from Nz(A3C,+)is entirely possible, but more likely the breakup is due to attack by nitrogen atoms. This experiment confirms that some cuprous chloride was introduced into the vapor phase, but it must be decomposed in the flow of active nitrogen because the copper analyte was detected. Indeed, the integrated signals produced when 20 ng of copper are atomized as the chloride or the nitrate are identical and both still within the linear range of the technique. In an atomic absorption system vaporization of the chloride would certainly produce a negative interference, especially in a system such as this where the vaporized material is imme-
diately swept into a low-temperature region. The second of the two peaks in the signal vs. time profile is most likely due to the atomization of copper atoms (bp 1756 "C at 2 torr). This would be the case for the salt layer in contact with the tantalum, for it would likely undergo a reduction reaction analogous to the carbon reduction of oxides, or it might undergo a thermal decomposition of the oxide (29). In either case copper atoms would be released from the surface. Obviously, this signal would have been lost if the active nitrogen had not been present, i.e., if any absorption measurement mode had been used. On the other hand, the bottom layer of sample is in intimate contact with the tantalum surface and is directly decomposed to form copper (bp = 1757 "C at 2 torr). The great difference in the boiling point between cuprous chloride and copper accounts for the big difference in the appearance times. The data presented here show that the chemical activity of active nitrogen is important to the overall atomization process. Elimination of the ashing cycle and vaporization into a low-temperatureenvironment should promote interferences; yet when compared to results of furnace AAS the interferences seem to be somewhat less. The chemical activity responsible for the gas phase atomization process is most likely due to the presence of a significant concentration of nitrogen atoms in the active nitrogen flow. This means that careful consideration must be made when choosing, or designing, a source of active nitrogen. It has been shown that nitrogen atoms are quenchers of the Nz(A3C,+) state and that the ultimate concentration of this metastable is limited by the presence of nitrogen atoms (30,31). Sources of active nitrogen designed to eliminate the nitrogen atom concentration in order to achieve higher metastable concentrations might do so at the expense of increased interferences when real samples are encountered. Matrix Effects. Cadmium, copper, and lead were again chosen as analytes for this study and sodium, magnesium, and ammonium salts, both as chlorides and nitrates, were chosen to be the interferents. Sodium and magnesium were selected because they are examples of commonly encountered matrix materials. Ammonium salts were adopted because of their distinctly different properties when compared to metal salts, The effect of each interferent on each particular element was investigated separately. For each analyte-interferent pair, a series of solutions were prepared of constant analyte concentration but varying interferent concentrations. The concentrations of cadmium, copper, and lead were 1ppm, 1ppm, and 2 ppm, respectively. It has been noted (32) that incomplete conclusions may be reached from interference studies at only one or two concentration levels over a small range. Therefore, magnesium and ammonium salt concentrations from 1to 1000 ppm and sodium salt concentrations from 1 to 5000 ppm were used. The necessary number of data points over the concentration range studied was determined for each individual analyte-interferent pair. In most cases, a 10-fold step in concentration was found fiie enough to offer conclusive results while lengthy experimental time was not involved. Relative signal changes are all referenced to the signal at zero interferent level. Integrated peak area was used to report all results. Signal changes less than 5% were considered insignificant and were recorded as zero change. General comments on the results will be given followed by a separate discussion of the data obtained for each element. It should be recognized that there are several important but basically unknown parameters which complicate the nature of this matrix study. First, the physical condition of the atomizer surface can affect the magnitude of the interference. It has been shown that the effects of a potential interference vary significantly depending on the type of graphite used to
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
--
___.
Table I. Percent Changes in the Analyte Signal Intensity for Various Levels of Matrix Concentration analyte signal change, % concn, Pb cu Cd interferent ppm "$1
1 5 10 50
0
0
0
0
100
0
0
-8 0 0 0
0 0 0 0 0
500 1000 1 10
",NO,
100 1000 1 5 10 50 100
NaCl
0
+19
5 50
0 0 0 0 0 0 0 -1 8 0 0
500 50100
-7 -29
1 5 10
0
-66 0
0
0
0
0
1000
-34
1000
0 0 -1 0 -1 7
-1 0 0 0 0
500
1000 5000
0 0
-14 0 0
-20
- 80 -86
-4,8 0 0 -62
-81 0
0
500
--
0 0 0 0 0 0 0 0 0 0 0
-2;2 0
50
100
0 0 0
0 0 0 -41.4
-54 0 0 0
-1.1
make the atomizer or, for a given type of graphite the effects vary during the lifetime of an individual graphite tube, due to changes in surface conditions (21,33,34).Our experience with a tantalum boat atomizer suggesits that an analogous situation exists when a tantalum atomizer surface is used. After many atomization cycles the tantadum surface becomes coated with an oxide layer, which gives a different performance than a clean tantalum surface. Therefore, for every set of solutions, a new tantalum boat was used imd experimentswere run starting with the least concentrated solution and progressing to the highest concentration to minimize the effect of atomizer surface condition changes as well as the possibility of any memory effects. This procedure was repeated1 at least three times for each set of solutions. We have attempted to be cautious in our observations based on the data presented here. The above-outlined procedure nearly eliminated the effects due to tho age of the atomizer surface, and the minor effects seen thait could be attributed to this phenomenon will not be discussed. Although the flow cell was designed to eliminate all active nitrogen from the atomizationregion, some back-diffusion of active nitrogen into this region was observed to occur. No effects due to its presence could be detected, but it may play some m a l l role in the atomization of some analytes. The results of the matrix study are presented in Table I. Several conclusions can be immediately drawn by examining the data presented in the table. First, several anal@-matrix pairs show a negative ]interference a t high matrix material concentration levels. In these cases, the level of interference increases as the concentrationof the matrix material increases, but the increase in interference does riot seem to be functionally related to the increase in matrix material. This would indicate that physical effects, such as occlusion and covola-
1243
tilization, are dominating the interference rather than a stoichiometrically (dependent chemical effect. Second, no positive interference is seen except for the analyte cadmium in a 1000 ppm ammonium nitrate solution. Third, there were no observable interferences for low concentrations of matrix material. All analytes were free from interference to a matrix material concentrationof 50 ppm or higher. For some of them, e.g., sodium chloride on cadmium, ammonium chloride, ammonium nitrate, and magnesium nitrate on copper, and ammonium chloride and ammonium nitrate on lead, the analyks are immune from interference up to 1000 ppm of interferent concentration. The interferent concentrationis thus 1000-fold times (500-fold for lead) the analyte concentration. This can be contrasted to much of the published results of furnace AAS interference studies, where signal changes due to relatively small amounts of matrix are not Uncommon. This is especially amazing consideringthat an ashing cycle was not used in the experimental procedure. Fourth, ammonium salts cause the least negative interference among the interferenta tested. This is not surprising due to the high volatility of the ammonium salts, although they have been shown to cause interferences in furnace AAS (3,s). Most, or all, of the ammonium cialt probably was vaporized before the release of the analyte atoms. The cadmium signal was suppressed at lo00 ppm ammonium chloride, probably due to overlap between the appearance of cadmium and ammonium chloride. Finally, compared to the nitrate salts, the chloride salts do not show an increased level of interference. This agrees with the previous anion-effect results. It should be pointed out that significant improvement in the analysis precision and accuracy can be made by simply incorporating an ashing cycle into the atomization program. This is especially the case when the analyte vaporizes at a significantly higher temperature than does the matrix. Such is the case for copper, where we have found the interferences noted in Table I can be almost eliminated. Better understainding of these phenomena is achieved through a comparison of the atomization process in the MT'ES system and in furnace AAS. In both cases an atomic vapor is generated electrothermally utilizing a furnace atomizer in an inert environment. Thermal dissociation mechanisms play an important role in determining the atomization efficiency, especially in furnace AAS where it has been shown that vaporization of the anidyte from the atomizer surface into a high temperature environment ensures high atomization efficieincy (36,37). This is one area where a noticeable difference exists between these two s:ystems. In furnace U S , atomization takes place at atmospheric pressure in a semiclosed atomizer and diffusion and thermal convection are often the major factors in the removal of arnalyte vapor from the proximity of the hot atomizer surface. In the MTES system, the sample is atomized by an open atomizer under reduced pressure, typically a few torr,in a fast flow system. Once a particle is in the vapor phase, it will immediately encounter a low temperature environment and further thermal decomposition will not be possible. If thermal dissociation is an important atomization mechanism one would expect the atomization efficiency to be much lower in the MTES system than it would be in a furnace AAS system. The data obtained here show, however, that the atomization efficiencyof the MTES system remains near 100% until very high matrix concentrations are encountered. This, along with the results of the anion-effect study, suggests that the active nitrogen plasma, chemically and/or physically, plays a nignificant role in decomposing the undissociated particles. In addition, vapor phase association reactions, either chemical or physical in nature, are not likely to occur due to the low system pressure and rapid dilution of the sample vapor by
1244
ANALYTICAL CHEMISTRY, VOL. 55,NO. 8, JULY 1983
Time (sec)
Time (sec) Flgure 5. Signal vs. time profiles produced for the determination of copper in a NaCl matrix: (A) 0 ppm of NaCi; (B) 100 ppm of NaCI; (C) 5000 ppm of NaCI; (D) same solution as C but monitored at the 589 nm line of Na (sensitivity reduced by 100).
Flgure 3. Signal vs. time proflles produced for the determlnatlon of Cd In a MgCI, matrix: (A) 0 ppm of MgCi2; (B) 100 ppm of MgC12; (C) 1000 ppm of MgCI,.
p a,
e
K
I?
E
W
I
0
Time (sec) Figure 4. Slgnal vs. tlme proflles produced for the determination of copper in a MgCI, matrix: (A) 0 ppm of MgCI,; (B) 10 ppm of MgCI,; (C) 1000 ppm of MgCI,. the inert carrier gas. The interference that is seen is thus likely to occur on the atomizer surface. A great deal of information about interferences can be obtained by measurement of the emission-signalvs. time profiles for the analyte as a function of the amount of matrix material. The emission-signal vs. time profiles for cadmium in a magnesium chloride matrix are shown in Figure 3. The time axis can be thought of as a temperature axis, for the tantalum boat heating rate is constant with time until rather high temperatures, above 2000 "C, are reached. Analogous plots for cadmium in the other matrices all appear very similar. An examination of the peaks in these plots reveals that the appearance temperature shifts higher as the amount of matrix material is increased. This indicates that the analyte, cadmium, experiencesa reduced heating rate. This phenomenon might be due to loading of the tantalum boat, i.e., the fact that the boat is a relatively low-power low-termal mass heater. More likely, however, the increased appearance temperature is due to a reduced heating rate experienced by the analyte due to occlusion. The occluding matrix material acts as an insulator, since it must be vaporized before release of the analyte. The emission signal vs. time profiles for copper in a magnesium chloride matrix are shown in Figure 4. The shift in appearance temperature with an increasing amount of matrix material was not observed for any of the copper interferent studies. This is most likely because copper atomizes at a much higher temperature than does cadmium, thus allowing more of the matrix to be vaporized by the time the copper is released from the atomizer. In all chloride matrix studies with copper, the small peak at approximately 0.4 s was observed at high matrix concentrations. As explained earlier, this peak is due to cuprous chloride vaporizing and the resultant molecular emission.
10
20
Time (sec) Flgure 6. Slgnal vs. time profiles produced for the determination of lead in a NaCl matrix: (A) 0 ppm of NaCI; (B) 10 ppm of NaCI; (C) 100 ppm of NaCI; (D) 500 ppm of NaCI; (E) 5000 ppm of NaCI. The emission signal vs. time profiles for copper in a sodium chloride matrix are shown in Figure 5. A small increase in appearance temperature is seen for 100 ppm NaCl concentration, but when the matrix concentration reaches 5000 ppm, the appearance temperature has shifted to a lower value. This shift to earlier time (lower temperature) is due to the copper salt being carried into the vapor phase by the more volatile sodium chloride (bp = 900 "C at 2 torr). The signal labeled D in Figure 5 is the signal due to sodium emission when the 1ppm copper solution containing 5000 ppm NaCl is atomized. It can be seen that this signal peaks concurrently with the early peak of the copper signal. Lead determinations have been the subject of a great number of studies, and lead is the most common analyte in reports concerning matrix interference in furnace AAS. Many of these studies that have been concerned with lead in chloride matrices for these matrices are troublesome and the results produced are somewhat confusing. In the MTES system the shift of the appearance temperature of lead to higher values with increasing matrix mass corresponds almost exactly to the cadmium data when ammonium chloride or nitrate salts constitute the matrix. Again, this interference can be explained by the large matrix mass simply occluding the analyte. The results for lead in a sodium chloride matrix or a magnesium chloride matrix are as perplexing as the analogous results in furnace AAS. Figure 6 shows the emission signal vs. time profiles for lead in a sodium chloride matrix. In the lead-sodium chloride system the appearance temperature shifts to earlier time as the amount of sodium chloride present increases to a certain level, about 50 ppm in this case. Beyond this level the peak stops shifting but the magnitude of the signal drops with further increase in matrix mass. This is most likely due to a carrier distillation effect similar to that seen for copper, but in the case of lead it appears that after a certain level of matrix mass is reached all of the lead is carried into the vapor phase via this mechanism. The shift in appearance temperature for
ANALYTICAL CHEMISTRY, VOL.
lead in the sodium chloride matrix is almost identical with the shifts seen by Czobik and Matousek for the lead-sodium chloride system in furnace AAS (38). Results obtained for the determination of lead in a magnesium chloride matrix appear nearly identical with those shown in Figure 6 for thle lead-sodium chloride system. The boiling point data for magnesium chloride at reduced pressure is not available, but the boiling points of sodium chloride and magnesium chloride at atmospheric ]pressure are nearly identical. Thus, analogous arguments as to the mechanism of the time shift of the lead peak can he made. At very high matrix mass levels a signrd reduction was seen for any case where the matrix vaporization could not be separated in time from the analyte vaporization. In these cases the amount of matrix material present in the observation region of the flow tube at the same time as the analyte is sufficient to lower the metastable concentration. Wlhen this occurs eq 3 no longer applies and a negative interference is noted. As stated earlier, some of these problems might be alleviated by simply incorporating an auhing cycle into the atomizationprogram. Further, matrix modification steps, such as used in furnace atomic absorption, designed to ciiuse the matrix to volatilize a t 11 low temperature, or steps taken to stabilize the anal@ so that it does not leave the surface until after the matrix has been removed might be useful. If, however, the situation arises where the vaporization of a heavy matrix cannot be separated in time from the analyte, a fundamental limitation exists. Thus, the conclusion can be made that the source producing the highest metastable concentration possible would be the least susceptible to this type of interference. Registry No. Cd, 7440-43-9; Cu, 7440-50-8; Pb, 7439-92-1.
LITERATURE CITIED (1) Ne, H. C.; Niernczyk, T. M. Anal. Chem. 1982, 54, 1839-1843. (2) Capelle, G. A.; Sutton, D. G. Appl. Phys. Left. 1077, 30, 407-409. (3) Capelle, 0. A.; Sutton, D. G. Rev. Scl. Insfrum. 1978, 49, 1124-1 129. (4) Dodge, W. EL 111; Allen. R. 0. Anal. Chem. 1081, 53, 1:!79-1286.
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RECEIVED for review February 4,1983. Accepted April 7,1983.
