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Anal. Chem. 1982,5 4 , 2162-2170
Spatial Profiles of Interelement Effects in the Inductively Coupled Plasma J. P. Rybarczyk,' Colleen P. Jester,* D. A. Yates, and S. R. Kolrtyohann" Department of Chemistry and Environmental Trace Substances Research Center, University of Missouri, Columbia, Missouri 6520 1
Detailed proflle studies were used to lnvestlgate the effect of aikall metal matrix elements on alkaline earth atomization, excltatlon, and loniratlon behavior. Three-dlmenslonai profiles which relate emlsslon Intensity or absorbance to height and radial dlstance In the plasma were generated. Atomlc and lonlc absorptlon as well as emlssion measurements were made. Thls allows a separatlon between processes affectlng atom or Ion formation from those affecting excitation. The region studied was low In the plasma where interactions wlth the sample begin. The aikall metal matrlces had no effect on atom formation and suppressed ionization. Excltatlon of both atoms and Ions was enhanced, as Indicated by Increased emlsslon. The effect on Ions was larger than on atoms. Nonthermal excltatlon processes appear to be Involved because a lithium matrlx had no effect on apparent excltatlon temperature as determined by the Iron two-line method. Generally slmllar profiles were found whether or not the aerosol entering the plasma was desolvated. The plasma region studled was not the optimum for analytical applications, but the results should lead to a better understandlng of atom/lon formation and excitation In Optimized plasmas.
One of the major factors which has led to the popular acceptance of the analytical inductively coupled plasma (ICP) is the great freedom from chemical interference in a properly optimized system (1-3). Numerous reports have appeared, however, which indicate that large matrix-dependent changes in emission intensities do occur in certain plasma zones (4-9). The difference in operating conditions between a plasma which shows severe interferences and one that does not can be as little as 100 W of power, a few millimeters in observation height, or less than 100 mL/min in central gas flow. Indeed a given matrix component can cause emission intensities to increase, decrease, or show no change depending on the exact experimental conditions. Blades and Horlick have documented this fact in two excellent recent papers ( 1 0 , I I ) . The interactions are obviously complex and most work directed toward understanding them has been done by using only emission intensities, although Blades and Horlick (11) did report results from one atomic absorption experiment that did not include spatial resolution. Spatially resolved emission-absorption comparisons were reported by Kornblum and de Galan (4) but only on a plasma with a central gas flow more than a factor of 3 above typical values. The conditions were so far from those in an analytically optimized system that extrapolation of their results to practical plasmas is quite hazardous. There is an obvious need for spatially resolved absorption as well as emission experiments because they will help separate effects due to atom or ion formation from those due to excitation. Our understanding of plasma matrix interactions is further complicated by the fact that most authors, including the very Present address: Department of Chemistry, Ball State Univer-
sit , Muncie, IN 47306.
TPresent address: St. Mary's Hall, P.O. Box 33430, San Antonio,
TX 78233.
recent ones (10, 11), have not clearly identified the plasma zones being observed. The relation between interelement effects and plasma structure has been described (9). In the current work we report detailed plasma profiles using the Abel inversion to convert lateral data to the more meaningful radial function. Absorption and emission profiles were made on a plasma operated under approximately optimized analytical conditions except for observation height. The observations were made low in the plasma (3-14 mm above the load coil) to study the initial phases of analyte-plasma interactions. All observations are related to the internal plasma structure using the nomenclature system described by Koirtyohann et al. (12). The precise zones sampled could be defined much better than with the usual practice of using the top of the load coil as a height reference.
EXPERIMENTAL SECTION Apparatus. The major portion of the instrument used for the current work has been described (9). Briefly, emission profiles were made by placing a mask with a 0.1 mm horizontal opening just outside the spectrometer entrance slit. The plasma was imaged at 2-fold size reduction at the mask position using a 150 mm focal length fused silica lens. The mask was driven, under computer control, to a series of positions along the vertical spectrometer slit, thereby admitting light from specific regions of the plasma image and generating a profile. The spectrometer wavelength setting was used to select the species for a given profile. The plasma system used was the Plasma-Therm ICP 2500. The cross flow nebulizer, spray chamber, and torches were used as supplied by the manufacturer, except as noted. Instrument modifications for this work included an additional lens to image light from a hollow cathode lamp at the plasma position. Images of the lamp and of the plasma were in turn formed at the slit mask position. The arrangement is similar to the usual two-lens system used for atomic absorption. When the light chopper was placed between the hollow cathode lamp and the plasma, the lock-in amplifier responded only to hollow cathode radiation and absorption data were obtained. Emission data were obtained by placing the chopper between the plasma and the spectrometer and turning the hollow cathode lamp off. PerkinElmer hollow cathode lamps were powered by a Kepco regulated dc supply. In order to obtain horizontal profiles, it was necessary to rotate the plasma image 90° from the vertical entrance slit. This was accomplished by turning the entire match box assembly of the Plasma-Therm Model 2500 ICP unit on its side. The normally vertical plasma axis was then horizontal and scans along the vertical spectrometer slit provided lateral profiles at a specific height (in normal orientation) in the plasma. This orientation was used for all of the data reported here. Throughout this paper we use the term height to represent distance along the normally vertical plasma axis. The plasma operated quite well in this orientation with no observable distortion in the regions of interest. A vertical, fanshaped stream of air was used to cut off the plasma tail plume about 75 mm from the load coil, and prevent overheating of the match box. The door of the plasma chamber was modified to provide about a 75 mm opening at the top for hot gases to escape. Short-term positional stability of the plasma zones was of the utmost importance for this work. Often plasmas which gave quite stable emission in the normal analytical zone (NAZ) were unacceptably noisy when the point of observation was lowered to the vicinity of the initial radiation zone (IRZ). Stability was greatly
0003-2700/82/0354-2 162$01.25/0 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
Table I. General Conditions for Plasma Operation plasma power coolant gas flow plasma gas flow sample gas flow split mask opening optical magnification spatial resolution spectrometer slit width spectrometer band-parrs absorption source profile range, emission profile range. absorption
1.25 kW 15 L/min 0.0 L/min 1.1L/min (nondesolvated) 1.6 L/min (desolvated) 0.1 mm
-0.10.2mmmm at plasma center
-2x
0.08 nm hollow cathode lamp 12 mm 6-8 mm
improved by placing a 30 cm length of 6 mm i.d. Tygon tubing between the outlet of the spray chamber and the inlet of the torch. This is similar to the aerosol filter suggested lby Myers and Tracy (13) for controlling the droplet size distribution reaching the plasma. It is not lknown if the stability imiprovement was due primarily to the large droplet removal or to less turbulent gas delivery to center of the plasma. The result, however, was a plasma with no visible flicker in the IRZ position and one on which measurements could be reproduced within about 3 % relative standard deviation (RSD) even in this inherently unstable region. Some experiments were conducted with a desolvated aerosol using a desolvation system similar to those previously described (14,15). The heater was a quartz tube 7 mm i.d. and 30 cm long. It was wrapped with a heating tape which was operated at a power setting sufficient to heat the argon to about 175 "C. A condenser cooled with tap water (about 20 "C) was used to remove the water. We wished to make the mast direct comparisons possible between the behavior with and without desolvation. Therefore, the same nebulizer and spray chamber were used. The outlet from the spray chamber served as the input to the desolvation system. When the water was largely removed, the plasma zones shifted significantly. It was necessary to add more argon to the central gas flow to maintain the IRZ in a similar position relative to the load coil. The auxiliary argon was added at the base of the torch, thereby minimizing effects on the nebulizaition and transport processes. The general plasma operating conditions are given in Table I. The position of the tip of the IRZ, as determined by the transition from red to blue yttrium emission, was 13 mm above the top of the load coil. This IRZ tip, which could be visually determined within about 0.5 mm, was used as an internal plasma reference point for all observations reported here. The vertical plasma region profiled extended from -10 mm to +1 mm from this reference point. This or course, is not the optimum plasma region for analytical purposes. As stated earlier, the purpose here was to study initial processes within the plasma, not to produce analytical results. Reagents. Reagent grade salts and Fisher Atomic Absorption Standards were used to prepare analyte and matrix solutions. For most experiments the analyte concentration was 40 fig/mL and the matrix concentration either 0 or 1000 pg/mL. In some e x periments a constamt molar concentration of 0.026 M was used for the matrix solutions. Analytes included Mg, Ca, Sr, Ba, and Fe. Matrix solutions were prepared for Li, Na, K, and Cs. A11 solutions contained 0.16 M HN03 making nitrate the dominant anion. Procedure. For emission profiles the source region of interest was scanned by driving the mask with a 0.1 mm opening in 0.1 mm increments across the image. The spatial resolution at the point of optimum focus was -0.2 mm due to the 2-fold image size reduction. Relative emission intensity data were collected for each position, digitized, and stored in computer memory. A second data set could be collected and compared with the first using the same small computer which controlled the process. Output from this computer could be in the form of relative intensities for the two data eets or their difference, ratio, or log of the ratio (absorbance) all on a point-by-point basis. The data set thus obtained was for a lateral profie at a single vertical plasma position. The horizontal plasma was moved so that another vertical position was imaged on the slit and the process repeated
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until data for a complete three-dimensional profile of the region of interest were obtained. Absorption measurements were made by placing the light chopper in the appropriate position and f i t p r o f i g the intensity of the hollow cathode lamp transmitted by the plasma with only solvent being aspirated. These data points became Io. A similar profile with the test solution being aspirated gave an intensity, I, for each point. The output was absorbance (log I0/O calculated on a point-by-point basis corresponding to specific plasma positions. Again, data for a complete three-dimensional profile were obtained for a series of such lateral absorption data sets. The strongest resonance lines for the alkaline earth atoms and ions were used for both emission and absorption. For example, calcium atoms were measured at 422.7 nm and the ions, at 393.3 nm. Iron atomic emission was used to measure excitation temperature for the plasma region studied using the two-line method. Transition probabilities were taken from the work of Kalnicky et al. (16) and the wavelengths used were 382.588 and 382.444 nm. The upper states for these two lines are at 33 507 and 26 140 cm-', respectively. The lateral emission intensity data were collected as described above. The Abel inversion was used to convert to radial intensities (see next section) which, in turn, were used to calculate temperature profiles. The Abel Inversion. Details of the Abel inversion process have been published elsewhere (17) and only a very brief description will be given here. The lateral intensity or absorbance data were first fit to a polynomial of the form y = c2+ c3 - c1)2~-c4(x-c1)2
(x
where Y is the emission intensity or absorbance at a specific lateral position, X is the lateral distance from the plasma axis, and C1 - C4 are coefficients which are adjusted for best fit. The use of only even powers of X assures the cylindrical symmetry required for the Abel inversion. The actual inversion was performed on the computed curve using a separate program. This method for doing the Abel inversion was chosen because it was convenient, it was not seriously affected by minor asymmetry in the plasma, it gave reliable intensity data in the critical region near the plasma center, and it gave apparently reliable inverted intensities even though the lateral data, in some cases, included significant noise. Another computer program was written to provide three-dimensional plots relating relative intensity or absorbance to height and radial distance in the plasma. RESULTS AND DISCUSSION Optical Considerations. The optical system has been shown (9) to give nearly constant response for the central 6 mm of the entrance slit or the corresponding 12 mm of the source. The data presented here are restricted to that range, and no calibration corrections were applied. The 0.1 mm mask opening gave a calculated resolution of 0.2 mm in the plasma center. Spatial resolution was similar for emission and absorption because light from only those protions of the sources which were imaged on the slit mask opening was admitted to the spectrometer. The reverse optics test was used for initial alignment and the slit image a t the plasma position allowed easy visualization of the source region actually sampled. Measurements with masks of known dimensions placed at the source position indicated that the actual resolution closely approached the calculated value. However, the depth of focus of the optical system must also be considered as discussed by Kalnicky et al. (16). The current optical system has a numerical aperature of about f / l O which would give insufficient depth of focus if the entire 10 mm plasma radius were of interest. However, we were primarily interested in studying the central channel low in the plasma where the radius of the emission or absorption zone is only about 2 mm for most analytes. The relatively large aperature did not cause serious deterioration in the effective spatial resolution of the system because of the small field depth studied. The serious resolution loss encountered at short wavelengths due to chromatic aberration was corrected by calculating the change in focal length using the simple lens formula (18) and
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 %
AI LATERAL A X I S C M M > . Flgure 2. Typical examples of the fP between a lateral profile (stepped curve) and the polynomial for a curve wRh a central minimum. Calcium ion absorption 3 mm below the tip of the IRZ.
D
%OO
UI.80
5'.60
6'UO
7'.20
S:OO
LATERAL A X I S CMM) Flgure 1. Typical example of the fit between a lateral profile (stepped curve) and the polynomial for a curve with no central minimum. Calcium atomic absorption 5 mm below the IRZ tip.
repositioning the lens and plasma accordingly. Absorption Measurements. Atomic absorption measurements posed few problems with the system. The hollow cathode lamps were operated at normally recommended currents and absorbances were reproducible within a few percent. The cathode image size allowed 6-8 mm of the plasma to be profiled from a single lamp position. In a few cases where this did not cover the plasma region of interest, it was a simple matter to adjust the lamp position and complete the profile with a second measurement series. Ion absorption measurements were somewhat more difficult because of low absorbances in some plasma regions and high ionic emission intensity in others. The hollow cathode lamps were operated at several times the recommended current (up to 40 mA) and shortened lamp life was an accepted price for obtaining the measurements. In spite of high current source operation and modulation, the signal-to-noise ratio for ion absorption measurements in the normal analytical zone (NAZ) was too poor to permit reliable measurement because of the noise produced by intense plasma emission. This is the primary reason that the profiles presented later extend to only 1mm above the IRZ tip. High current source operation could also lead to broadened lamp line profiles that would affect absorption measurements. No attempt was made to take line profiles for the source or the plasma int~account for this work. Precision i n Radial Intensities. In order to check the precision of the data gathering system, curve fitting, and Abel inversion, we prepared replicate profiles using magnesium ion emission from the vicinity of the IRZ tip. Ten profiles were run over a 3-day period with the major source variables readjusted between daily runs. The relative standard deviation (RSD) of the Abel inverted intensities were calculated at each 0.2 mm radial distance. The RSD was about 4.5% from 0-2 mm radii increasing to about 8% at 3.0 mm. A similar experiment when all profile data were collected in the shortest practical time gave RSD values of about 1% for the central 2 mm, increasing to 2% at greater radial distances. These experiments established only the reproducibility of the processes. Additional error sources affecting accuracies in the Abel inversion process are discussed by Choi and Kim (17).
Typical examples of radial profiles after Abel inversion. All were taken at -5 mm relative to the IRZ tip. The alkali metal concentrations were either 0 or 0.026 M and the calcium 40 pg/mL: (A) Ca atomic emission, (B) Ca atomic absorption, (C) Ca ionic emission, (D) Ca ionic absorption; (1) Ca, (2) Ca + Li, (3) Ca -I- Na, (4) Ca + K, (5)Ca 4- Cs. Flgure 3.
Curve Fitting and Abel Inversion. Typical data for the curve fitting are shown in Figures 1 and 2. Figure 1 is for calcium atomic absorption at 422.7 nm and is a lateral profile with no central minimum. The stepped curve shows the raw data and the smooth line is the polynomial fit. The lateral data were initially taken with an arbitrary zero. The computer program calculates a center of symmetry, sets this value to X = 0, and uses distances measured relative to it for the curve fitting calculations. The center of symmetry was constant within the resolution of the measurements. Figure 2 shows similar data for calcium ion absorption at 393.3 nm. In this case the curve shows off axis maxima with a central minimum. The fit is not as good because of the higher noise levels typical of ion absorption measurements. Typical inverted data are shown in Figure 3. In this case the radial profiles of calcium atomic and ionic emission and
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
n
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A
..
Figure 4. 40 pg of
Three-dimensional profile of calcium atomic emission: (0) Ca/mL, (X) 40 pg of Ca 4- 1000 pg of Li/mL.
absorption are shown when calcium is present alone or with 0.026 M concentrations of the various alkali metals. All of the alkali metals show similar effects. Atomic absorption is unaffected (Figure 3B) and atomic emission is enhanced (Figure 3A). Ionic absorption is suppressed (Figure 3D) and ionic emission is enhanced (Figure 3C). Also, all of the alkali matrices enhance the tendency toward a central minimum in the atomic emission and the ionic absorption profiles. Blades and Horlick (11) report effects on emission for equimolar concentrations of alkali metal matrices which rank in order of ease of ionization of the metal. Some of the same tendency is seen here though nalt consistently. ]Lithium shows the smallest effect on atomic emission (Figure 3A) with the other matrices the same within experimental error. Lithium, SOdium, and potassium rank in order of ionization potential for the effect on ion emission (Figure 3C) with cesium giving the same curve as pokwsiumi. The order is reversed, however, for ionic absorption (Figure 3D) with lithium giving the greatest suppression and cesium the least. In other similar experiments, the alkali metal concentration was kept constant at 1000 pg/mL. Lithium then showed the greatest matrix effect and cesium the least, as one might expect from the relative molar concentrations. Three-Dimension Profiles. The most advanced form of data output from our system is three-dimensional profiles as shown in Figure 4. The three axes are relative intensity or absorbance, height, and radial distance in the plasma. Heights are measured relative to the IRZ tip as previously described. A rather complete descrilption of analyte behavior is presented quite efficiently though such plots. Figure 4 shows the effect of lithium on calcium atomic emission. One sees a central minimum in the intensity profile indicating that the IRZ is “hollow” when defined by calcium atomic emission. The maximum intensity is seen in the center at heights above -3 mm. The intensity reaclhes a maximum at about -1 mm and then declines due to depletion of the atomic population by ionization. The lithium matrix enhances the emission at all locations below the IRZ tip and shows little effect above the tip. A similar profile of calcium atomic absorption is shown in Figure 5. The most striking differences from emission are the absence of a central minimum at any plasma location observed and the lack of enhancement due to lithium. This latter point was shown for one vertical location in Figure 3. At plasma heights below -3 mm the lithium matrix suppresses calcium absorption slightly, probably due to a reduced atomization rate for calcium atoms imbedded in salt particles. With or without lithium, however, calcium atomization is surprisingly complete in this relatively cool central channel low in the plasma. At -10 mm the absorbance for calcium alone was already about 70% of its maximum value. At heights above
Flgure 5. 40 pg of
Three-dimensional profile of calcium atomic absorption: (0) Ca/mL, (X) 40 pg of Ca 4- 1000 pg of Li/mL.
R-RXIS
IMMI
Flgure 6. Threedimensional profile of calcium pg of Ca/mL, (X) 40 pg of Ca -t 1000 pg of
R-RXIS Figure 7. 40 pg of
ionic emission:
40
Li/mL.
[MMI
Three-dimensional profile of calcium ionic absorption: (0) Ca/mL, (X) 40 pg of Ca -t 1000 pg of Li/mL.
-3 mm calcium atom absorption drops off much like the emission but more rapidly. The radial widths of the calcium atomic profiles are quite constant with height. There is a tendency for profile widths to be greater in the presence of the matrix. Calcium ionic emission and absorption profiles are shown in Figures 6 and 7. Both profiles show central minima low in the plasma. The off axis maximum response takes place at greater radial distances for ionic emission than for absorption . Lithium enhances emission intensities 2 mm from the central axis at all plasma heights. However, a suppression is seen in the center of the plasma at all heights where central emission was measurable. Similar observations were reported by Blades and Horlick ( 1 1 ) . Ionic absorption is suppressed
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
R - R X I S IMMJ
R-AXIS lMMl
Flgure 9. Three-dimensional profile or iron 382.4 nm atomic emission: (0)40 p g of Fe/mL, (X) 40 p g of Fe 1000 p g of Li/mL.
+
R-RXIS (MMI
R - R X I S LMMI
R - R X I b LMMI
Flgure 8. Three-dimensional profiles for magnesium: (A) atomic absorption, (B) atomic emission, (C) lonlc absorptlon, (D) ionic emission; (0)40 p g of Mg/mL, (X) 40 p g of Mg 1000 p g of Ll/mL.
+
by the presence of lithium at all plasma locations tested. Lithium also causes the central minimum to persist to greater heights. The effect of lithium on ionic absorption appears to reach a minimum at about -1 mm relative to the IRZ tip. In both plots of ionic behavior the tendency for the lithium matrix to broaden the profiles is seen. Unlike the behavior for atoms, ionic emissions and absorption increase rapidly at
and above the IRZ tip. In fact, the intense emission prevented ion absorption measurements higher in the plasma. Similar data showing magnesium profiles with and without lithium are given in Figure 8: Again, one sees relatively little effect on atomic absorption (Figure 8A). The profiles are broadened somewhat by the matrix and atom formation may be delayed as for calcium, but it is nearly complete very low in the plasma in both cases. The reduction in magnesium atomic absorption is evident at and above the IRZ tip but is much less pronounced than it was for calcium. The magnesium atomic emission (Figure 8B) shows an off axis maximum at much greater radial distance than either Mg absorption or Ca emission. The lithium enhancement is more pronounced than with Ca though the same tendency to suppress in the center and enhance at the wings is evident. Above the IRZ tip, no reduction in magnesium atomic emission with height is seen, indicating that the decrease in atom population through ionization is more than compensated for by the gain in excitation. Magnesium ionic absorption (Figure 8C) is severely suppressed by lithium at all locations. Surprisingly, the ion absorption profiles in the absence of the matrix show no central minimum except at the lowest observation height. The Mg ionic emission profiles (Figure 8D) have a very pronounced central minimum. Again, the off axis maximum for Mg ionic emission takes place at greater radial distances than calcium ionic emission or magnesium ionic absorption. A large lithium enhancement is observed. The “hole” in the center of the plasma is larger for magnesium ionic emission than for the other species observed and it is further expanded by the lithium matrix. Evidently, the few magnesium ions which are present at plasma radii greater than 2 mm are very efficiently excited. Excitation Temperature. The behavior described in Figures 4-8 indicates that the presence of lithium enhances excitation efficiency for both atoms and ions. The emission/absorption temperature as defined by Kornblum and de Galan (19) increases substantially. In an equilibrium plasma, this would indicate an increase in excitation temperature as measured by other established methods. Figure 9 shows the effect of lithium on iron 382.4 nm atomic emission. The behavior is similar in many respects to that of Ca and Mg atomic emission. Lithium causes enhanced emission, expansion of the central channel, and a general broadening of the profiles. However, when these data were used with similar radial profiles of the iron 382.6 nm line to calculate excitation temperature profiles, the data in Figure 10 were obtained. The radial limit of these profiles was defined by diffusion of the
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13,NOVEMBER 1982
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Three-dimensional profile of excitation temperature calculated from iron 382.6 and 382.4 nm emission in the desolvated system: 40 pg/mL Fe + 1000 bg/mL Li.
Flgure 11.
Flgure 10. Three-dimensional profile of excltsltion temperature calculated from iron 382.6 and 382.4nm emission: (upper plot) 40 pg of Fe/mL; (lower plot) 40 pg of Fe 1000 pg of Li/mL.
+
analyte species out of the central channel. Intensities were too low for reliable calculations at greater than about 2 mm radius low in the plasma and about 3 mm near the IRZ tip. These data are intended to show changes in excitation temperature rather than absolute values even though there is reasonable agreement in measured temperature above the IRZ tip with previous reports (16). After examination of the data in Figure 10, one notes modest excitation temperatures low in the central channel which increase with both radius and height reaching a plateau a t approximately 5000 K for the central 1.5 mm radius in the vicinity of the IRZ tip. The temperature profiles with and without lithium are virtually identical. Similar results were reported for a sodium matrix by Mermet and Robin (5). The profile at -7 mm appears to be anomalous both with and without lithium in Figure 10. We chose not to invest the considerable effort required to resolve these apparent anomalies because the fiial conclusions were unlikely to be affected. The conclusion is that the apparent excitation temperature of the plasma region studied as measured by the two-line method with iron is unaffected by the matrix. Therefore, some nonthermal mechanism must be responsible for the enhanced excitation evident in Figures 4-9. It may be noted that lithium apparently affects the two excited iron levels in the same way but in a way that differs from the Boltzmann distribution relative to the ground state, Kornblum and de Galan (19) have suggested the possibility of equilibrium among excited levels but nonequilibrium relative to the ground s t a b Desolvation Studies. The amount of water reaching the plasma was measured by passing the carrier gas through two U tubes (9.6 cm i.d. X 20 cm long) placed in series where the transfer line would norm,ally connect to the base of the torch.
The tubes were loosely packed with 5-16 mesh silica gel, dried at 150 O C for 2 h and weighed. The carrier gas, with or without desolvation, was passed through these tubes for a measured time, and the amount of water reaching the plasma was calculated from the weight gain of the first tube. The second tube was a back-up to assure complete collection although virtually no water escaped collection in the first tube. Sample solution was pumped to the nebulizer at 2.90 mL/min. The amount of water transported to the plasma without desolvation was 0.0305 f 0.004 mL/min. If the aerosol was passed through the entire desolvation apparatus with neither the heater nor the condenser turned on, the water introduction rate was 0.020 mL/min. With the desolvation heater turned on and water flowing through the condenser, 0.0036 0.0003 mL/min of water reached the plasma, or about 12% of the amount without the desolvation apparatus. The location of plasma zones was significantly different with the desolvated aerosol because plasma energy was no longer reuired to vaporize and decompose the water. The IRZ tip was lowered from 13 mm above the load coil to about 6 mm. Auxiliary argon (0.5 L/min) was added at the base of the torch as previously described to restore the position of the IRZ tip. The effect of the desolvation system on the amount of sample transported to the plasma was tested by observing calcium ionic emission in the normal analytical zone of the plasma, about 8 mm above the IRZ tip. This is a plasma region where ion emission is both intense and relatively unaffected by most external influences. A 20% reduction in emission intensity was observed between the desolvated and nondesolvated aerosols, and this was taken as a reasonable approximation of the analyte loss in the desolvation system. Temperature profiles with desolvation were constructed by using the same method discussed previously. Results were similar to those for the nondesolvated aerosol in that the intensity of the iron lines was increased by the presence of lithium but the final calculated temperature profiles were unaffected by the matrix. The temperature profile for iron plus lithium is shown in Figure 11. Trends similar to those in Figure 10 may be noted. The main difference is that the central channel temperature approached 5000 K at -5 mm with desolvation whereas this central temperature was attained at -1 mm without desolvation. Profiles of calcium atomic and ionic emission and absorption are given in Figure 12. Many similarities with the nondesolvated system may be noted but there are also significant differences. Atomic absorption (Figure E A ) is suppressed by the matrix low in the plasma as before but the absorption reaches a maximum at -7 mm with calcium alone and at -5
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
A
B
A
R - R X I S IMMl
R-8x15 ( M M I
R - A X I S IHMI
Figure 12. Three-dimensional profiles for calcium from the desolvated system: (A) atomic absorption, (B) atomic emission, (C) ionic absorption, (D) ionic emission; (0)40 pg of Ca/mL, (X) 40 pg of Ca 1000 pg of Li/mL.
+
mm for calcium plus lithium. This contrasts with the -3 mm for maximum atomic absorption without desolvation. For calcium atomic emission (Figure 12B) the reduction in intensity at heights above -3 mm is more rapid than with the nondesolvated system. Ion emission and absorption are influenced more by desolvation. Figure 7 shows the ground-state ion population in the absence of lithium and without desolvation increasing rapidly with height at and above the IRZ tip, while in Figure 12C the absorbance reaches a maximum slightly below the tip and decreases a t greater heights. In contrast, the ion emission profiles show a tremendous increase in intensity at and above the -1 mm height. The same increase, though to a much less degree, is seen in Figure 6. The same intensity scale is used in these two figures to emphasize the difference. Lithium enhances ion emission low in the plasma (Figure 12D)while at and above the IRZ tip the primary effect of the matrix is to broaden the profiles both with and without desolvation. Similar profiles of the effect of lithium on strontium and barium emission and absorption were also obtained. The trends were quite similar to those for calcium and they will not be discussed in detail here. Comparison with Previous Work. Since the most intensive reported emission/absorption comparisons in an ICP
were by Kornblum and de Galan (4),comparison of our results with theirs seems appropriate. Neither data set was gathered on an analytically optimized ICP. However, our system was very close, the primary difference being observation height. The effects reported are taking place low in analytical systems. In some cases they may extend into the observation zone and in others only a short extrapolation is needed. Both data sets indicate complex behavior with effects due to vaporization, ionization, and nonequilibrium excitation. In both cases the presence of an alkali metal enhanced excitation of both atoms and ions. The high flow plasma (4) showed steadily decreasing ion emission with height in contrast to the large increase we see at and above the IRZ tip. Atom number densities were reduced by cesium in the high flow plasma and unaffected by lithium in the current work. Atom densities also showed a central minimum at all observation heights indicat!ng a large "hole" through the high flow plasma. Ionization was completely suppressed by cesium in their system and modestly reduced in most plasma locations in ours. Phosphate suppressed calcium emission with high central flow in contrast to our previous report (9) of an enhancement. We see a consistent tendency for the alkali metal to broaden the profiles and enhance any central minimum which may be present,
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
while they often saw a less pronounced minimum with the matrix. These differences are not surprising given the very large central flow through the previous plasma. Comparison of our magnesium emission data with those from Kornblum and de Galan’s (4)low flow plasma indicates quite different behavior. This iu became their lowest observation point (7.5 mm above the load coil) was probably above the IRZ tip and therefore effectively higher in the plasma than our highest point. Interpretation of Results. We cannot yet provide a complete interpretation ffor all of the observed profile changes. For some, the data will support reasonably firm conclusions and for others some possible explanationsi can be eliminated. The fact that ground-state atom populations are almost unaffected by the matrix: rules out nebulization and transport as important contributors to the observed matrix changes. Thus, the aerosol ion redistribution reported by Browner’s group (20) or transport problems of the type described by Skogerboe and Olson (21)are not major (contributors to the observed effects. Similar conclusions based on different experimental evidence were reached by Bladies and Horlick (11). We believe that the general broadening of the profiles in both emission and absorption in the presence of the lithium matrix is a lateral diffusion effect similar to those reported for flames (22-24, even though the effect is in the opposite direction. In flames, the analyte tends to be more concentrated in the center because of slow lateral diffusion of heavy salt particles formed upon desolvation of droplets from concentrated solution. In the case of the plasma, the sample particles are confined to the central channel initially by thermal and flow gradients. Thus, there is an active force maintaining a narrow analyte channel. Matrix salt particles have greater inertia and resist the central acceleration more effectvely than particles containing only the analyte, resulting in broadened profiles falr the analyte plus matrix. Similar broadened profiles in the presence of a sodium matrix are reported by Blades and Horlick (11) who predict the profile changes from diffusion effects but do not, explain their reasoning. Several fadors contribute to the observation that the atomic absorption and emission profiles for calcium are nearly the same width, while for magnesium the emission profile is much broader. The light magnesium atoms, once formed, would diffuse more rapidly than calcium and somle broadening could be expected. However, if this were a major factor, broadened magnesium absorption profiles would also be observed. A more probable explanation is that the few magnesium atoms which do diffuse to greater radii resist ionization more effectively than calcium and are quite efficiently excited by their proximity to the induction region, perhaps by a different mechanism. Nonthermal processes must make major contributions to analyte behavior in the plasma regions studied as previously reported (4,10, 11, 19). Indeed, the lithium matrix appears to reduce the ionization tiemperature ( 4 ) while enhancing the emission/absorptioln temperature (19)for both atoms and ions. Additional evidence for nonthermal processes comes from comparing ion absorptioin with atomic emission. The magnesium profiles at -5 and -7 mm in Figure 8 show ionic absorption in the center of the plasma when magnesium is present alone. The atom emission intensity approaches zero for these same locations. Therefore, we see magnesium ionization, a process requiring 7.6 eV and not atomic excitation which requires only 4.3 eV. There are plasma locations where calcium shows a similar effect but they are smaller and less obvious than for magnesium. Obviously, if thermal processes predominated, one would see atomic emission at lower heights and smaller radii than ionic absorption. Given the above data,
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even the partial local thermal equilibrium concept proposed by Jarosz et al. (25) is difficult to support for the plasma regions currently studied. A suprising conclusion from this work is that atom formation is well advanced in the center of the plasma even at the lowest heights observed. The plasma channel does not show a central minimum for calcium or magnesium atom populations. Excitation of atoms is seen only at larger heights and radii. Again, if thermal processes predominated as they probably do in flames, one would expect to see appreciable excitation before atomization is complete, especially for calcium. The data also show the hazard of interpreting emission plasma data based on a model which considers only atom formation processes (26). The calcium ion profiles in the desolvated system at and above the IRZ tip are quite interesting. Ion absorption decreases with height while ion emission shows a tremendous increase. Increased line width due to higher translational temperature in this region could be part of the explanation. The measured absorbance could then decrease with a constant ion population. However, ion populations could also be depleted by formation of multiply ionized calcium or by unusually high population of excited ionic levels. The large increase in emission at the same location suggests that greatly over populated excited ionic levels may exist and that depletion of ground-state populations by this mechanism should not be excluded. In other respects there are many similarities between the profiies with and without solvent removal. This was expected because of central gas flow was increased to restore the IRZ position, as determined by yttrium emission to 13 mm above the load coil. Differences which do exist indicate processes taking place at lower heights in the desolvated system. However, the value of using the IRZ or an internal plasma reference point is established by the general similarity of the two sets of profiles. Removal of about 90% of the water accompanying the analyte caused a major shift in the plasma zones relative to the load coil. Yet, when the IRZ position was restored by auxiliary argon, the general analyte behavior in a given plasma region was also restored to a large degree. The lithium matrix had only a minor effect on calcium and magnesium atom formation. It suppressed ionization in the plasma regions studied which contrasts with the conclusions of Blades and Horlick (11). However, in agreement with these authors, we found that the presence of lithium matrix greatly enhanced excitation of both atoms and ions. The excitation enhancement was much greater for ions than for atoms and appears to be of a nonthermal nature for both species. Blades and Horlick (11) suggest that collisions with electrons is the primary mechanism which is affected. However, Mermet and Robin (5)found no change in electron number densities when sodium was added to the plasma. We prefer to leave the mechanism by which the increased excitation takes place as an open question at this time. The observations reported here are not a challenge to the established freedom for interelement effects in a properly optimized analytical ICP. Our observations were restricted to the vicinity of the IRZ which should be, and normally is, avoided for analytical purposes. We present these data for their intrinsic interest, as an aid in understanding the processes occurring in the early stages of interaction between the ICP and the sample which is introduced, and as a first step toward learning the processes which occur throughout the plasma.
ACKNOWLEDGMENT We are grateful to the Columbia National Fisheries Research Laboratory for the use of a liquid argon storage tank and a gas flow controller. We also thank H. Kim for help with
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Anal. Chem. 1982, 5 4 , 2170-2174
the curve fitting and Abel inversion and for many valuable discussions.
LITERATURE CITED Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, I l l O A and 1155A. Boumans, P. W. J. M.; de Boer, F. J. Spectrochlm. Acta, Part 8 1975, 308 - - - , 309 ---. Greenfleld, S.; Jones, I. 11.; McGreachln, H. McD.; Smith, P. B. Anal. Chlm. Acta 1975, 74, 225. Kornblum, G. R.; de Gaian, L. Spectrochim. Acta, Part 8 1977, 328, 445. Mermet, J. M.; Robin, J. Anal. Chim. Acta, 1975, 70, 271. Savage, T. N.; Hieftje, G. M. Anal. Chem. 1980, 52, 1267. Blades, M. W.: Horlick, G. A m / . Soectrosc. 1980. 34. 696. Kawaguchl, J.; Ito, T.; Ota, K.: Mizutke, A. Spectrochli. Acta, Part B 1980, 358, 193. Koirtyohann, S . R.; Jones, J. S.; Jester, C. P.; Yates, 0. A. Spectrochim. Acta, Part B 1981, 358, 49. Blades, M. W.; Horllck, G. Spectrochim. Acta, Part B 1981, 368,861. Blades, M. w.; Horllck, G. SPeCtrOChi” ACa, p a r t 8 1981, 368,881. Koirtyohann, S. R.; Jones, J. S.; Yates, D. A. Anal. Chem. 1980, 52, 1965. Myers, S. A.; Tracey, D. H. 1981 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Abstract 10. Veillon, C.; Margoshes, M. Specfrochim , Acta, Part 8 1968, 238, 553.
(15) Olson, K. W.; Haas, W. J., Jr.,; Fassel, V. A. Anal. Chem. 1977, 49, 833
(16) Kalnlcky, D. J.; Kniseley, R. N.; Fassel, V. A. Spectrochim. Acta, Part E 1975. 308. 511. (17) Choi, B: S.rKim, H. Appl. Spectrosc. 1982, 36, 71. (18) Resnlck, R.; Halliday, D. “Physics, Part 2”; Wiley: New York, 1967. (19) Kornblum, G. R.; de Galan, L. Spectrochlm. Acta, Part 8 1977, 328, 71. (20) Borowiec, J. A.; Boom, A. W.; Dliiard, J. H.: Cresser, M. S.; Browner, R. F.; Matteson, M. J. Anal. Chem. 1980, 52, 1054. (21) Skogerboe, R. K.; Olson, K. W. Appl. Spectrosc. 1978. 32, 181. (22) Koirtyohann, S . R.; Plckett, E. E. Anal. Chem. 1968, 40, 2068. (23) West, A. C.; Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1973, 45, 2420. (24) Boss, C. B.; Hieftje, G. M. Anal. Chem. 1979, 51, 1897. (25) Jarosz, J.; Mermet, J. M.; Robin, J. Spectrochim. Acta, Part 8 1976, 3 1 8 , 377. (26) Li, K. P.; Ll, Y. Y. Anal. Chem. 1981, 5 3 , 2217.
RECEIVED for review December 23, 1981. Accepted July 15, 1982. This material is based on work supported by the N ~ tional Science Foundation under Grant CHE 77-22915. It was presented at the 9th International Conference on Atomic Spectroscopicum InterSpectroscopy and xxll nationale in Tokyo, Sept 1981.
Direct Determination of Arsenic in Shale Oil and Its Products by Furnace Atomic Absorption Spectrometry with a Tetrahydrofuran Solvent System Joseph L. Fabec Gulf Science and Technology Company, P.0. Drawer 2038, Pittsburgh, Pennsylvania 15230
A furnace atomic absorption spectrometry method was developed to directly determlne arsenic In shale oil and its products by uslng aqueous and organlc standards In a tetrahydrofuran solvent system. Repllcate analyses of organlc arsenical standards and shale oll samples revealed very good preclslon and accuracy, coefflclents of variatlon being, with one exception, 5 % or less. Results on shale and hydrotreated shale 011s are compared to Instrumental neutron activation and energy dispersive X-ray fluorescence data. Problems probably related to graphite tube porosity that can cause low recovery of organlc arsenlcals and a callbratlon curve shift are dlscussed. This procedure may have appllcations In determining other elements In a variety of organlc or aqueous matrices.
Because arsenic will poison catalysts during process operations and also is an enviromental contaminant, the determination of total arsenic in shale oil and its products is of primary concern. Many methods available for the determination of arsenic ( I ) , i.e., gravimetric and chemical methods such as colorimetric measurements based on the arsenicmolybdenum blue complex and arsine generation in combination with silver diethydithiocarbamate, differential pulse polarography, heated vaporization atomic absorption, and arsine generation in combination with atomic absorption spectrometry (AAS) require sample pretreatment and subsequent analysis on an aqueous matrix. Arsenic has been determined by furnace atomic absorption in catalytic re-former feedstocks (2) after extraction into an
aqueous media and in coal (3) after extraction from a sulfuric acid-iodide solution into toluene. Few analytical studies have appeared dealing with arsenic determinations in shale oil and its product using furnace AAS. The methods of choice have been energy dispersive X-ray fluorescence (4-6) and instrumental neutron activation ( 4 , 5 , 7, 8), since little or no sample preparation prior to analysis is required. This paper describes a procedure developed to directly determine total arsenic in shale oil, hydrotreated shale oil (HTSO), and its products by furnace AAS. Furnace tube problems when determining inorganic and organic arsenicals in the solvent system used are discussed.
EXPERIMENTAL SECTION Apparatus. Analyses were performed on a Perkin-Elmer Model 5000 equipped with a HGA-500 and a PRS-10 printersequencer. Except for the conditions given below, instrument parameters were set according to specificationsin the manufacturer’s literature. Background correction was used, and the absorption signals were evaluated by reading peaks. Nonpyrolyticdy coated tubes were used. The HGA 500 program conditions were as follows: dry cycle, 110 OC-5-2 (Temp-Ramp, S-Hold, S); char cycle, 900 OC-l@-lO;atomize cycle, 2700 oC-l-lO; and 0 internal argon flow. Prepared solution quantities of 10,20, or 40 p L were manually injected into the furnace using a 25 pL 702 or 50 pL 705 Hamilton syringe. Reagents. Analytical reagent grade sulfuric acid, Ni(N03)2.9H,0, and tetrahydrofuran (THF) containing butylated hydroxytoluene as a preservative, deionized water, and medium neutral oil were used. Medium neutral oil is a noninhibited finished lubricating oil base stock having a viscosity range of 175-325 cSt with a minimum VI of 95.
0003-2700/82/0354-2170$01.25/00 1982 American Chemical Society
