[ NOTES

Impact of Scattering on Axial Viewing Absorption Measurements within an Inductively Coupled Plasma

GARY D. RAYSON* and D A N I E L YANG S H E N Department of Chemistry, Box 30001, New Mexico State University, Las Cruces, New Mexico 88003

Index Heading: Atomic absorption spectroscopy.

INTRODUCTION

The detection of atomic species generated within an inductively coupled plasma (ICP) by atomic absorption (AA) has been described with the use of axial viewing of the discharge. 1,2 Briefly, those studies involved a "see- through" torch assembly which was oriented along the optical axis. That configuration was similar to that de- scribed earlier by Mermet and Trassy. 3

The present note addresses the potential impact of two types of matrix concomitants on the atomic absorp- tion measurement. The specific potential interferents in- vestigated were Na (an easily ionizable element) and NH4H2P04 (a vaporization interferent observed in flame atomic absorption). The impact of scattering of the sam- ple aerosol with the analyte in a pure water matrix on the measured absorption signal was also investigated.

EXPERIMENTAL

The experimental configuration used for these studies has been described previously. 1 The emission from a hol- low cathode lamp was directed through a see-through ICP torch with the use of a 24-cm-focal-length fused- silica lens with a magnification of 1.5. The transmitted light was then directed by the off-axis mirror pair optical assembly to the monochromator and detected by a pho- tomultiplier tube (PMT). The system was operated in a single-beam configuration. It was therefore necessary for three separate intensity measurements to be made for each absorbance calculation. The 100% transmittance

Received 1 December 1990. * Author to whom correspondence should be sent.

level was measured with the introduction of a blank so- lution to the torch. Subsequently, the attenuated light level was measured during the introduction of the sample solution. Because of the intense atomic emission from the plasma, and the limited dynamic reserve of the lock- in amplifier used for these measurements, a 0% trans- mittance measurement was made with the introduction of the sample solution and blocking of the incident ra- diation.

The see-through torch was a modification of a com- mercially available ICP torch (Plasmatherm, Inc., Kres- son, NJ). ' Briefly, the base of the torch was removed and an enlarged end was added which included an optical window. The sample inlet of this torch was then posi- tioned perpendicular to the axis of the torch, tangential to the torch, in a manner similar to that of the inter- mediate and outer gas flow inlets. The sample aerosol was introduced to the discharge along the same path as the incident radiation from the hollow cathode lamp passing through the base of the torch.

Plasma background line and continuum emission was discriminated against by a combination of two methods. The first involved the utilization of an off-axis over-and- under symmetrical mirror pair imaging system with field stops at the tangential and sagittal image planes. ~ As indicated above, the contribution of background emis- sion was minimized by the modulation of the light source and the use of a lock-in amplifier (Model 565, Stanford Research Systems, Palo Alto, CA). Again, this system was configured for single-beam measurements. No pro- vision was made for any type of continuous background correction.

Samples were introduced to the ICP AA system with the use of concentric-glass nebulizer (Plasma-Therm, Kresson, NJ) with a Scott-type, double-pass spray cham- ber. All solutions were delivered to the nebulizer with a peristaltic pump with a flow rate of 0.78 mL mL min -1.

The plasma was operated at an rf power input of 600 W with coolant gas flow at 16.3 L min -1 and a nebulizer gas flow of 0.85 L min -1.

Stock solutions of 1000 mg Ag, Cu, and Mg L -1 were each prepared by dissolution of the reagent-grade nitrate salts in doubly distilled deionized water. All sample so- lutions were prepared daily by serial dilution with doubly distilled deionized water. A stock solution of 10,000 mg Na L -1 was prepared with reagent-grade NaNO~ for all easily ionizable element (EIE) studies. The phosphate solution was prepared by dissolution of NH4H2P0 , for a stock solution concentration of 10,000 mg P043- L -1.

706 Volume 45, Number 4, 1991 ooo3-7o2s/91/~o4-o7o6~2.oo/o APPLIED SPECTROSCOPY © 1991 Society for Applied Spectroscopy

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C o n c e n t r a t i o n o f S o d i u m / p p m FIG. 1. The effect of added Na on axial absorbance with a water blank (O) and with a blank containing the corresponding concentrations of NaNO~ (1) for (A) Ag, (B) Mg, and (C) Cu.

RESULTS A N D D I S C U S S I O N

The effects of easily ionized e lements on the absor- bances of Ag, Cu, and Mg are shown in Fig. 1. Increasing amoun t s of Na (as the n i t ra te salt) were added to solu- t ions of 10 mg L -1 Ag, Cu, and Mg. Two different 100% t ransmi t t ances were measured by using (1) doubly dis- tilled deionized water, and (2) a series of solutions of different sodium concentrat ions. The absorbances were

then calculated according to those different 100 % trans- mi t t ance values. The addi t ion of sodium resul ted in an increase in the a p p a r e n t absorbances.

These appa ren t absorbance increases could result f rom the addi t ion of sodium chloride in the sample, increasing the n u m b e r of foreign part icles in the optical pa th before the p l a sma discharge. Al though those foreign part icles do not absorb any light of the wavelength of interests, they would sca t ter the light f rom the hollow cathode

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C o n c e n t r a t i o n o f P h o s p h a t e / p p m FIG. 2. The effect of added PO4 3- o n axial absorbance with a water blank (O) and with a blank containing the corresponding concentrations of NH4H2P04 (I) for (A) Ag, (B) Mg, and (C) Cu

APPLIED SPECTROSCOPY 707

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FIG. 3. Measured axial absorbance of Ag at 328.1 nm with a water blank (O) and with a blank containing the corresponding concentrations of Cu (IlL

lamp and result in a loss of light intensity. This type of wavelength-nonspecific scattering would result in the in- creases with the apparent absorbances that were ob- served. The impact of these proposed scattering pro- cesses on the measured absorbance signal can be evaluated by the use of blank solutions containing the same con- centration of added NaCl as each sample solution. The results of these experiments are also indicated in Fig. 1. For these metals studied, the apparent enhancement ob- served with added Na is effectively compensated for with the use of an accurate blank solution.

The presence of phosphate anions in the sample matrix can result in the formation of relatively nonvolatile metal salts. This could result in a change in the atomic con- centration of the analyte. Because of the lower applied rf powers used when the ICP is used as an absorption source, interferences of this type were also studied with the present system. This study was undertaken by adding increasing amounts of phosphate (as NH4H2PO4) to 10 mg L -I solutions of silver, copper, and magnesium. In a procedure similar to the procedures described above, two different 100% transmittance light intensities were ob- tained by using different blanks: (1) doubly deionized water, and (2) a series of solutions containing different concentrations of phosphate equal to those added to the corresponding analyte solutions. Two sets of absorbances were then calculated for each metal. The results of this study are displayed in Fig. 2 for each analyte. Instead of a depression in the atomic absorption signal as a result of a vaporization interference, an apparent enhancement was observed with added phosphate. Although the use of a phosphate-containing blank significantly decreased this enhancement, the correction was not complete for Ag and Cu atomic absorption measurements with the addition of 1000 mg PO43- L -1.

The observed apparent absorption enhancement re- sulting from the addition of NaNO3 and NH4H2PO4 might cause one to doubt that the reduction in transmitted light intensity could come from the scattering of light from desolvated analyte salt particles in the plasma. The introduction of samples will, of course, bring more par- ticles into the pathway of the incident light. To explore this possibility, we designed an experiment which in-

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volved a series of solutions with increasing amounts of copper as the respective blank solutions for silver ab- sorption measurements. The corresponding Ag and Cu solutions were equal in molarity. Therefore, the intro- duction of silver solutions and its copper blank solution into the system brought a nearly equal number of metal- containing particles into the optical pathway. The 100 % transmitted light intensity was measured while the cop- per blank solution was introduced into the system; the 0 % transmitted light intensity and the absorption mea- surement were then read while the silver solution was introduced into the system. The results are displayed in Fig. 3. This plot of atomic absorbance demonstrates, as a function of Ag concentration, that the apparent de- crease in transmitted light intensity induced by the in- troduction of appropriate samples is due to the absorp- tion of the Ag and is not a result of the scattering (except at higher metal salt concentrations).

CONCLUSIONS

The utilization of an ICP as an atom reservoir in atomic absorption spectrometry must involve correction for nonspecific scattering of the incident light by desolvated aerosol particulates. The impact of this scattering is most pronounced when the concentration of concomitant salts is greater than 100 mg L -1. The use of an accurate blank enables the correction of this effect. Similarly, back- ground correction would be expected to accurately com- pensate for this effect.

1. G. D. Rayson and D. Y. Shen, Anal. Chem., 62, 1239 (1990). 2. G. D. Rayson and D. Y. Shen, J. Anal. Atom. Spectrom., paper in

press (1990). 3. J. M. Mermet and C. Trassy, Appl. Spectrosc., 31, 237 (1977). 4. S. G. Salmon and J. A. Holcombe, Anal. Chem., 50, 1714 (1978). 5. J. P. Walters, Anal. Chem., 40, 1540 (1968). 6. R. J. Klueppel, D. M. Coleman, W. S. Eaton, S. A. Goldstein, R. D.

Sacks, and J. P. Walters, Spectrochim. Acta, Part B, 33, 1 (1978).

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