Anal. Chem. 1980, 52, 284-287
204
(9) Zandee, L.; Bernstein, R. B.; Lichtin, D. A . J . Chem. Phys. 1978, 69, 3427-3429; Zandee, L.; Bernstein, R. B. J , Chem. Phys. 1979, 70, 2574-2575. (IO) Bergman, A.; Jortner, J. Chem. Phys. Lett. 1972, 15, 309-315. (11) Bray, R. G.; Hochstrasser, R. M.; Wessel, J. E. Chem. Phys. Left. 2 , 1974, 27, 167-171; Hochstrasser, R. M.; Sung, H. M.; Wessel, J. E., J . Am. Chem. SOC.1973, 95, 8179. (12) . . Hurst. G. S.: Navfeh. M. H.: Youna. -. J. P. ADD/. . . Phvs. Lett. 1977. 30. 229-231. (13) Williams, A.; Sara, R. I . Intl. J . Appl. Radiat. Isot. 1962, 13, 229-238. (14) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Rei. Data, 1977, Suppl. No. 1 to V. 1.6. I
(15) Hughes, E. E.; Lias, S. G. "Vapor Pressures of Organic Compounds in the Range Below One Millimeter of Mercury". Natl. Bur. Stand. (U.S.), Tech. Note 1960, No. 70. (16) Brophy. J. H.; Rettner, C. T. Opt. Left. 1979, 4 , 337-339.
R~~~~~~~ for review ~~~~~t28, 1979. ~ ~~~~~~b~~ ~ 12, 1979. This work was supported in part by the U.S.Air Force Office of Scientific Research under Grant AFOSR-77in Part by the National Science under Grant CHE 77-16074, and in part by The Aerospace Corporation. 34383
Inductively Coupled Argon Plasma as an Excitation Source for Flame Atomic Fluorescence Spectrometry M. S. Epstein,' N. Omenetto,2 S. Nikdel, J. Bradshaw, and J. D. Winefordner" Department of Chemistry, University of Florida, Gainesville, Florida 326 11
Two unique applications of the inductively-coupled argon plasma (ICAP) to the excitation of atomic fluorescence in flames are demonstrated, which are difficult or impossible to perform using sources normally employed in atomic fluorescence spectrometry. A scatter correction procedure based on self-absorption in the ICAP is evaluated uslng an air-hydrogen flame for the determination of cadmium and zinc in a hlghly-scattering fly ash matrix (NBS-SRM 1633). The determination of high analyte concentrations by measuring the emisslon of the s?mple in the ICAP using a flame resonance spectrometer is demonstrated by the determination of iron in fly ash.
T h e inductively-coupled argon plasma (ICAP) has been shown to be an extremely useful source of resonance radiation for the excitation of atomic fluorescence in flames ( I , 2). Detection limits in a nitrogen-separated air/acetylene flame for elements such as Ca, Cd, Co, Cr, Fe, Mg, Mn, and Zn are equivalent to or better than atomic absorption detection limits for these elements. Furthermore, the ICAP has several unique characteristics which make it far more versatile than typical radiation sources employed in atomic fluorescence spectrometry iAFS), such as electrodeless discharge lamps. Scatter correction can not only be performed using the two-line technique ( Z ) , because of the significant number of intense ionic and nonresonance atomic lines excited in the ICAP, but also by a technique described here which takes advantage of differences in slope of the fluorescence excitation curve of growth and the emission curve of growth in the ICAP. High concentration analysis is also considerably simplified since the emission of the sample in the ICAP (rather than the sample fluorescence in the flame) can be monitored using the flame resonance spectrometer, providing excellent linear response typical of ICAP emission a t high analyte concentrations as well as the good spectral selectivity of AFS. This study demonstrates the versatility of the ICAP excitation source for both the measurement of high analyte On leave from the Center for Analytical Chemistry, Nationai Bureau of Standards, Washington, D.C. *On leave from the Institute of Inorganic and General Chemistry, University of Pavia, Pavia, Italy. 0003-2700/80/0352-0284$01 001U
concentrations and for scatter correction using flame atomic fluorescence spectrometry (AFS).
EXPERIMENTAL The apparatus and operating parameters for ICAP-excited AFS have been described previously ( I , 2 ) as well as the procedures used for standard and sample preparation (2). In the normal configuration, radiation generated by the introduction of 1to 10% solutions of pure analyte elements into a 1.5-kW ICAP is modulated by a mechanical chopper (600 Hz) and focused onto a nitrogen-separated air/acetylene flame. Standard and sample solutions are aspirated into the flame and the fluorescence is isolated and measured by a 0.1-m monochromator (16-nm spectral bandpass), photomultiplier, and lock-in amplifer. For high-concentration analysis, sample and standard solutions are aspirated into the ICAP, and the analyte emission is measured by monitoring the fluorescence from 100 pg/mL of analyte in the flame using the monochromator and associated electronics. RESULTS AND DISCUSSION Scatter Correction. The scatter of excitation source radiation by undissociated or unvaporized matrix constituents in a flame can be a major interference in AFS using resonance transitions (3). Any analytical instrument which is designed for AFS must therefore have a provision for distinguishing scatter from atomic fluorescence and making an appropriate correction. Scatter correction can be performed by several methods using the ICAP as an excitation source for AFS. Since the ICAP can be used to excite intense ionic and nonresonance atomic spectra for many elements, which are not observed in flames, the two-line correction technique works particularly well (2). Another method, which is evaluated here, is qimilar in principle to that described by Haarsma e t al. ( 4 ) for use with electrodeless discharge lamps, and is based on the shape of the fluorescence "excitation" curve of growth ( I ) relative to the ICAP-emission curve of growth. When concentrated solutions of the analyte are introduced into the ICAP to provide radiation to excite fluorescence in a flame into which the sample solution is introduced, the excitation curve of growth (relationship of fluorescence intensity from the flame to different concentrations of analyte in the ICAP) reaches a plateau caused by self-absorption in the ICAP. At this point, as illustrated in Figure 1 (case A), for cadmium a t 228.8 nm, there is relatively no change in fluorescence intensity from the flame for a several-fold change in analyte concentration ? 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2!, FEBRUARY 1980 iA,
“1
Cadmium
Concentratio?
in
CAP
( rng/ml
Flgure 1. Relationship of (A) fluorescence “excitation”curve of growth for 1 pg/mL cadmium in an air/hydrogen flame and (B) ICAPemission curve of growth reflected in the scatter signal from 1 % lanthanum solution in the air/hydrogen flame
in ICAP. The small decrease in flame fluorescence intensity at the higher concentration is due to the appearance of self-reversal in the ICAP. However, the ICAP-emission intensity, as reflected in the scatter signals from the flame in Figure 1 (case B), increases as approximately the square root of the increase in concentration of analyte in the ICAP, as expected for a case of complete self-absorption with minor self-reversal in the ICAP. Therefore, the scatter signal for a particular sample can be estimated by varying the analyte concentration in the ICAP and observing the effect on the signal from the flame. A simple relationship based on two different concentrations in the ICAP can then be set up: x+s=s1 (1) a x + ps = sz where x = sample fluorescence signal, s = sample scatter signal, S1 = total observed intensity from flame at lower analyte concentration in ICAP, S2 = total observed intensity from flame at higher analyte concentration in IC@, LY = ratio of analyte fluorescence intensity at higher and lower concentrations of analyte in the ICAP, and p = ratio of scatter intensity at higher and lower concentrations of analyte in the ICAP. When the plateau region of the fluorescence excitation curve is used, as in Figure 1 (case A), a = 1, since the change in fluorescence intensity is negligible. The value for p can be determined using a high-purity scattering solution, such as 1%lanthanum, and observing the change in scatter intensity as a function of a n a l e concentration in the ICAP, as in Figure 1 (case B). It is worth stressing here that this method is, in the authors’ opinion, much more reliable and simpler to use than the method proposed by Haarsma et al. ( 4 ) ,because of the capability to change the source parameters or to select a height in the ICAP which corresponds to total self-absorption, and because of the temporal stability of self-absorption conditions in the source. In order to adequately test this method for scatter correction, we employed an air/hydrogen flame, rather than the nitrogen-separated air/acetylene flame which we typically use for analytical measurements, since scatter signals from the latter flame are significant only at very low analyte concentrations in “high-solids’’ matrices (2). Cadmium and zinc were selected as the test elements. The initial problem encountered was that the emission from the region of the ICAP which provided the greatest fluorescence sensitivity in the flame was unsatisfactory for this method of scatter correction, since the absence of self-absorption in that ICAP region caused the values of a and f7 to be almost identical. It was necessary to
285
focus on the flame a region of the plasma (4 cm above the coil) considerably higher than the optimum region (1.5 cm above the coil) before self-absorption was significantly great enough to result in a useful difference in cy and P. For cadmium, however, this resulted in a sensitivity loss slightly greater than an order of magnitude for introduction of annlyte concentrations from 2.5 to 10 mg/mL Cd into the ICAP. The first test of the scatter correction technique was the determination of cadmium in NBS SRM-1633 (Fly Ash) a t a certified concentration of 1.45 f 0.06 pg/g Cd. While this element in t h e f l y ash matrix can bc determined using t h e nitrogen-separated airlacetylene f l u m e (limit of detection = 0.8 n g / m L without difficulty, our purpose was to evaluate the scatter correction technique using the air/hydrogen flame. Because of the sensitivity loss resulting from focusing the less intense area higher in the ICAP (4 cni above the coil) on the flame in order to maximize self-absorption, the limit of detection using the 228.8-nm line (20 ng/mL) was larger than the actual concentration (15 ng/mL) of cadmium in the fly ash solutions. However, the scatter signal generated by the fly ash matrix, which was 2OX greater than the fluorescence signal from the analyte, gave an apparent cadmium concentration in the fly ash of 30 pg/g. By varying the cadmium concentration in the ICAP from 2.5 to 10 mg/mL, and using Equations 1 and 2, the scatter-corrected cadmium concentration was found to be less than 2 pg/g (Le., less than the limit of detection), illustrating adequate scatter correction. The sensitivity problem encountered because self-absorption is not significant in the optimum region of the ICAP can be improved somewhat by the aspiration of much higher concentrations of analyte than we typically employ (100 mg/ mL rather than 10 to 20 mg/mL). Although prolonged aspiration of solutions at such elevated concentrations may clog the nebulizer and ICAP torch orifice, we did not experience any problems for the short time periods (5 niin) that we employed. Furthermore, nebulizers which are designed specifically for high solids, such as those based on the Babington principle could be used to avoid clogging problems ( 5 ) . We repeated the determination of cadmium in fly ash using cadmium concentrations of 25 and 100 mg/mL in the ICAP. which improved the sensitivity of the analysis by a factor of 5 , since self-absorption was now significant closer (3 cm above the coil) to the optimum intensity region of the ICAP. A value of 1.6 f 0.5 pg/g Cd was obtained for the fly ash, which is in reasonable agreement with the certified value. The large standard deviation is, of course, a result of measurements at concentrations near the limit of detection and also a result of the propagation of errors from the several measurements required to correct for scatter signals which were an order of magnitude larger than the analyte fluorescence signal. The scatter correction procedure was also used to determine zinc at 213.9 nm in the fly ash using the airlhydrogen flame. Since the solution concentration of zinc was of the order of 1 to 2 pg/mL, sensitivity was not a problem. Using zinc concentrations of 10 and 100 mg/mL in the ICAP, self-absorption was complete ( a = 1) at 2.7 cm above the coil. The scatter signal was found to be only about 2% of the analyte fluorescence signal. A value of 211 pg/g Zn was obtained, which was in excellent agreement with the certified value of 210 f 20 pg/g Zn. Not only can this method correct for scatter, but it should also be effective for the correction of spectral interferences originating f r o m contamination in t h e solutions used for excitation in t h e ICAP. Such interferences occur when a nonanalyte element, which is present both in the sample matrix, and as a contaminant at appreciable levels in the ICAP excitation solution, emits radiation which can excite fluorescing lines within the spectral bandpass of the monochro-
286
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
mator. An example of such an interference is the small enhancement observed when the copper resonance lines at 216.5, 217.8, and 218.2 nm are included in the spectral bandpass (16 nm) of a monochromator used for the analysis of zinc (213.9 nm) in high purity copper ( 2 ) . Since the contamination in the ICAP excitation solution will be at a low enough concentration so that it will not be self-absorbed, it should respond linearly (to a reasonable extent, depending on the effect of high solids on nebulization efficiency) to the increase in concentration of the ICAP excitation solution (/3 > l),while the analyte fluorescence will not change appreciably (a= 1). Correction is then performed in the same manner as for scatter. Measurement of High Analyte Concentrations. One major advantage claimed for atomic absorption spectrometry (AAS) compared to AFS is the simplicity of measuring high concentrations as analyte. The options available to the AAS analyst include: (a) dilution of the sample to the linear concentration range; (b) using a less sensitive absorption line; (c) reduction of the aspiration rate; and (d) reduction of path length by turning the burner head. While methods a and c are also available to the AFS analyst, method d is not and method b, the use of a less sensitive absorption line, is subject to several difficulties. The availability of less sensitive lines which result in useful fluorescence is poor for some elements. Less sensitive fluorescing lines are more subject to scatter and other spectral interferences, since the signal-to-background ratio is decreased relative to the most sensitive line. Furthermore, the difficulty in choosing alternate, less-sensitive fluorescing lines is more complicated than in choosing a less sensitive line for AAS, since the ICAP is not a monochromatic light source and the flame will be irradiated by all the radiation characteristic of the analyte. At the wide spectral bandpasses used for AFS, the fluorescence radiation from the flame will often be the sum of fluorescence from several lines, which may include resonance transitions as well as direct-line and stepwise line transitions. Effects due to absorption of excitation or fluorescence radiation (prefilter and postfilter effects, respectively) result in analytical curve nonlinearity in AFS and will differ depending on the particular type of transition. Since several transitions may be included in the spectral bandpass, prediction of analytical curve linearity at high analyte concentrations is difficult and must be evaluated experimentally. The versatility of the ICAP as an excitation source for AFS simplifies high concentration analysis. The normal procedure for ICAP-excited AFS (2),is to aspirate a high concentration of pure analyte (10 to 20 mg/mL) into the ICAP, focus the radiation though a mechanical chopper onto a flame into which the analytical solution is introduced, and observe the fluorescence using a monochromator. The flame itself acts as a “resonance spectrometer” (6),with a spectral bandpass on the order of 0.002-0.004 nm, the half-width of the absorption line in the flame (7). The 0.1-m monochromator acts only as a rough optical filter to reduce flame emission noise. In this configuration, however, as shown in Figure 2 (case A) for the iron 248.3-nm line, the analytical growth curve is linear only to slightly above 10 pg/mL, which is typical of the fluorescence technique. If the excitation of analyte in the sample is performed by the ICAP, rather than the flame, t h e analytical growth curue c a n be extended t o high concentrations, as also illustrated in Figure 2 (case B). A solution containing 100 pg/mL of the analyte is constantly introduced into the nitrogen-separated air/acetylene flame and the analytical solutions (i.e., the standards and samples) are aspirated into the ICAP. This configuration combines the high-resolution advantage of the flame “resonance spectrometer” with the excellent linearity
/
4
,A
,Llrnlt
3
of
Cetec’IoP
-2
-’
Log
IRON
0
I
2
3
CONCENTRATION ( p g / m l )
Flgure 2. Analytical growth c w e s for iron atomic fluorescenceat 248.3 nm using (A) the ICAP as the excitation source and the flame as the
analyte atom reservoir and (B) the ICAP as analyte atom reservoir and the flame as a resonance monochromator of the ICAP-emission a t high concentrations, a result of the negligible self-absorption and/or self-reversal of that source (1, 8).
This procedure is satisfactory for high concentrations only, since the detection limit for iron a t the 248.3-nm line as 2 pg/mL compared to 6 ng/mL for analysis using the ICAP (with aspiration of 20 mg/mL iron) as the AFS excitation source ( 2 ) . However, the useful linear ranges of both configurations show considerable region of overlap, so that all concentration ranges are effectively covered by one configuration or the other. The analytical usefulness of the high concentration configuration was tested by the determination of iron in National Bureau of Standards (NBS) Standard Reference Material (SRM) 1633-Fly Ash at a concentration reported by Ondov et al. (9) to be 6.2%. The sample preparation has been described previously (IO). An iris diaphragm was placed a t the focal point of the focused radiation from the ICAP near the mechanical chopper to exclude radiation from the tail plume where matrix effects might occur. A shield was also placed between the flame and the chopper to reduce flame flicker caused by air draughts. The analyzed concentration was found to be 6.0 0.2% iron, in good agreement with the reported concentration. Analytical precision was found not to be significantly different from that obtained by viewing the emission directly with a spectrometer, as expected, since the major noise source at high analyte concentrations is analyte flicker noise and the flame resonance spectrometer operates on the plateau of the fluorescence curve of growth where fluorescence flicker noise measured in intensity units is very small.
*
CONCLUSION The versatility of the ICAP as an excitation source for flame atomic fluorescence spectrometry is clearly evident since with simple changes in measurement procedures, high concentration analysis and scatter correction can be performed. The choice of scatter correction method, between the two-line technique and the self-absorption technique, will depend on the analytical situation. When sensitivity is the major concern and the scatter signal is not too large, the two-line technique is preferable. When sensitivity is adequate and accuracy is of prime concern, the self-absorption method should be used, since it corrects for scatter a t the same wavelength as the fluorescence excitation and also compensates for spectral interferences due to contamination in ICAP excitation solutions.
Anal. Chem. 1980,
The determination of high concentrations of analytes using the emission of the sample in the ICAP with the help of a flame resonance spectrometer eliminates the requirement for dilution or the search for and the disadvantages of a less sensitive fluorescing line. As stated before ( 2 ) ,the great value of the ICAP-excited AFS technique lies in the excellent spectral resolution of the atomic fluorescence measurement. Therefore, specific analytical problems can be solved without the need for an expensive, high-resolution monochromator.
LITERATURE CITED (1) N. Ornenetto, S, Nikdel, J. D. Bradshaw, M. S.Epstein, R . D. Reeves, and J. D. Winefordner, Anal. Chem., 51, 1521 (1979). (2) M. S. Epstein. S. Nikdel, N. Ornenetto, R. Reeves, J. D. Bradshaw, and J. D. Winefordner, Anal. Chem., 51,2071 (1979). (3) N. Omenetto and J. D. Winefordner, Prog. Anal. Atom. Spectrosc., 2, 1 (1979).
52, 287-290
207
(4) J. P. S. Haarsma, J. Vlogtman, and J. Agterdenbos, Spectrochim. Acta.. Part E, 31, 129 (1976). (5) R. C. Fry and M. B. Denton, Anal. Chem., 49, 1413 (1977). (6) A. Walsh, Analyst(London), 100, 764 (1975). (7) T. Hollander, B. J. Jansen, J. J. Plaat, and C. Th. J. Alkemade, J . Quant. Spectrosc. Radiat. Transfer, IO, 1301 (1970). (8) H. G. C. Human and R. H. Scott, Spectrochim. Acta, Part E,31,459 (1976). (9) J. M. Ondov, W. H. Zoller, I. Olmez, N. K. Aras, G. E. Gordon, L. A. Rancitelli, K. H. Abel, R. H. Filby, K. R. Shah, and R. C. Ragaini, Anal. Chem., 47, 1102 (1975). (10) M. S. Epstein, T. C. Rains, and 0. Menis, Can. J . Spectrosc., 20, 22 (1 975).
RECEIVED for review July 19,1979. Accepted October 22,1979. N. Omenetto would like to thank the Committee of Internal Exchange of Scholars for the granting of a Fulbright travel fellowship*Research by AF-AF0SR-44620-76-C005 and by WPAFB Contract number F33615-78-C-2036.
Aerosol Monitoring System for the Size Characterization of Droplet Sprays Produced by Pneumatic Nebulizers John W. Novak, Jr. and Richard F. Browner" School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
An aerosol monitoring system has been developed which gives size distributions of spray droplets produced by common nebulizers used for analytical atomic spectrometry. The system provides a means for systematic characterization of droplet sizes of low volatility liquid sprays in the range 0.1-10 bm. Droplet distributions have been obtained using a cascade impactor operated In series with an electrical aerosol analyzer. These measurements are necessary to provide a sound practical and theoretical bask for the development of more effective means of solution transfer to flames and plasmas.
The nebulization of liquid samples is a critically important process in analytical atomic spectrometry, in that sample solutions are normally introduced into flames and plasmas as finely dispersed mists generated by pneumatic nebulizers. Willis ( I ) has made a detailed study of the operation of certain pneumatic nebulizers and has related a number of atomization problems experienced in analytical atomic spectrometry directly to nebulizer characteristics. Willis has suggested that for flame atomic absorption spectrometry, both atomization efficiencies and chemical interferences are a function of the droplet size distribution produced by the nebulizer. The droplet size distribution of the analyte solution spray determines: (i) the transport efficiency of the sample from solution to flame or plasma (2); (ii) the rate of desolvation of the wet droplets (3); (iii) the rate of vaporization of the dried salt particles remaining after desolvation (3) and finally, (iv) the magnitude of many interference effects (4,most commonly solute vaporization interferences. Consequently, both the magnitude of the analytical signal and its degree of freedom from some common interference effects are critically related to the nature of the spray entering the flame or plasma. The spray characteristics themselves are determined by the design of the nebulizer/spray chamber combination. O003-2700/80/O352-0287$0 1 O O / O
In order to understand better the operation of atomic absorption spectrometry ( U S ) and inductively coupled plasma (ICP) nebulizers in common use, it is essential first to accurately determine the droplet size distributions of the sprays they produce. Only with this information can design improvements be made which are based upon the actual data of relevance, rather than on a combination of indirect data such as signal magnitude, signal stability, interference effects, etc. To date, the information available on droplet size distributions produced by various pneumatic nebulizers is of little direct relevance to any of the parameters of importance noted above. This is a direct consequence of the techniques used for collection of the data ( I , 3, 5 , 6 ) which are incapable of reliably measuring droplets smaller than approximately 10 pm in diameter. As will be shown in this and later studies: (1)a significant fraction of the mass of a typical droplet spray is contained in particles below 5-pm diameter, and (2) the great majority of droplets greater than approximately 20 pm do not reach the flame or plasma in any case and so are largely irrelevant to analytical spectrometry. There are very real difficulties associated with the determination of droplet size distributions from pneumatic nebulizers operating under realistic conditions. The high particle number densities of the sprays (approximately lo6 particles ~ m - and ~ ) wide particle size distributions can lead to significant measurement errors, even when sophisticated instrumentation is used for their determination (7-9). Recent reviews of available particle sizing techniques ( I O , 1I ) aided in the selection for this study of a cascade impactor for large droplets (12-14) (20.4 pm) and an electrical aerosol analyzer for droplets in the range 0.032-1.0 pm diameter ( 1 5 - l a . However, these systems are generally used at much lower particle number densities than those found in pneumatic nebulizer sprays (12, 16, 18, 29). Instrument requirements led to the need for extremely high dilution of the aerosol 1580 American Chemical Society
