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59-70. (8) . . Ferren, William P. Am. Lab. (Winston-Salem. N . C . ) 1978, 10,
52-58. (9) Valkovic, Viado “Trace Elements in Human Hair”; Garland STPM Press: New York, 1978;Chapter 4, 199 pp. (10) Yokel, Robert A. Clin. Chem. (Winston-Salem, N . C . ) 1982, 2 8 , 662-665. (11) Kliienlck, M. A,; Frederlckson, C. J.; Manton, W. I. Anal. Chem. 1983, 55,921-923. (12) Bagiiano, G.; Benischek, F.; Huber, I. Anal. Chim. Acta 1981, 123, 45-56. (13) Lau, H. K. Y; Ashmead, H. Anal. Lett. 1975, 8 (ll),815-824. (14) Assarlan, Gary S.;Oberleas, Donald Clln. Chem. (Winston-Salem, N . C . ) 1977, 2 3 , 1771-1772. (15) Clanet, P.; DeAntonlo, S. M.; Katz, S. A.; Scheiner, D. M. Clin. Chem. (Winston-Salem, N . C . ) 1982, 2 8 , 2450-2451. (16) Boumans, P. W. J. M.; de Boer, F. J. Spectrochim. Acta, Part 8 1973, 2 7 8 , 391. (17) Fassel, Velmer A.; Kniseley, Richard N. Anal. Chem. 1974, 46, lllOA-1120A. (18) Greenfield, S.;McGeachin, H. Mc D.; Smith, P. B. Talanta 1976, 2 3 , 1. (19) Ward, Arthur F.; Sobel, Harold R. Envlron. Anal. [Pap. Anal. Chem. Spectrosc. SOC,], 3rd 1976 1077, 245-252. (20) Munter, R. C.; Grande, R. A; Ahn, P. C. ICP I n f . News/. 1979, 5 , 368-383. (21) Kamakura, Mitsuhiro Nippon Elseigaky Zasshi 1983, 38. 823-838. (22) Zhu, Xlaofan; Chen, Shuqing; Xu, Guohua; Zeng, Yuiing; Li. Rongguang; Cheng, Guozhen; Yan, Hongzong; Han, Shishan Fenxl Huaxue 1983, 1 7 , 294-297. (23) Sakamoto, Takeshi; Kawaguchi, Hiroshi; Mizuike, Atsushi Bunko KenkYU 1978, 2 5 , 35-41. (24) Alder, J. F.; Samuel, A. J.; West, T. S. Anal. Chim. Acta 1976, 8 7 ,
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RECEIVED for review July 8,1985. Accepted October 16,1985. C.M. wishes to thank the Government of Venezuela for scholarship support. This research was funded throughout by grants from the Natural Sciences and Engineering Research Council of Canada (Grant A1126) and the Government of Quebec (Fonds F.C.A.C. EQ1642). In its latter stages considerable additional support was provided by the Ontario Ministry of the Environment (176 PL).
Determination of Trace Elements in Suspended Particulate Matter by Inductively Coupled Plasma Atomic Emission Spectrometry with Electrothermal Vaporization Akiyoshi Sugimae’ and Ramon M. Barnes* Department of Chemistry, GRC Towers, University of Massachusetts; Amherst, Massachusetts 01003-0035
A rapld, dlrect determination of trace elements In suspended particulate matter collected on glass fiber filter is performed by means of inductively coupled plasma atomic emission spectrometry with electrothermal vaporization. A sample placed in a mlcroboat was dissolved with 2 % hydrofluoric acid, and the evaporated residue subsequently was vaporized into the plasma for atomization and excitation in atomic emission spectrometry. The hydrofluoric acid treatment prevented glass formatlon, which limited attainable sensitivity when the sample was heated dlrectiy. Improvement in vaporization characteristics of analytes in the microboat allowed the determination of Cr, Mn, and Pb to proceed with high sensitlvlty. The relative standard deviations were usually below 13 %, which Is acceptable for large-scale alr pollution survey work.
The application of inductively coupled plasma atomic emission spectrometry (ICP-AES) to the determination of On leave f r o m E n v i r o n m e n t a l P o l l u t i o n C o n t r o l Center, Osaka Prefecture, 3-62, 1-Chome, Nakamichi, Higashinari-ku, Osaka 537, Japan.
trace elements in suspended particulate matter is well-established. Indeed, several investigators have examined the feasibility of ICP-AES for the rapid, precise, and accurate multielement analysis of suspended particulate matter (1-8). The U.S. Environmental Protection Agency (USEPA) established standard ICP-AES procedures for the routine analysis of suspended particulate matter collected by the National Air Surveillance Network (7).ICP-AES permits the determination of a large number of elements with high sensitivity and precision and with relative freedom from chemical interferences. However, reported methods have involved a two-step process comprised of an acid digestion of suspended particulate matter followed by ICP-AES determination. The acid digestion procedure is usually time-consuming and is liable to result in contamination and loss of sample constituents. A reduction of sample handling between the actual sample procurement and the ICP-AES determination would be advantageous from the standpoint of reduced errors and increased speed. Several investigators have attempted to minimize or eliminate the pretreatment steps with varying degrees of success. A simplified and single-vessel ultrasonic extraction has been designated equivalent to EPA’s lead reference procedures and enables a single analyst to prepare up to 50 samples/day on a sustained basis (9).
0003-2700/66/0358-0785$01.50/00 1986 American Chemical Society
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A limitation of ICP-AES has been the almost exclusive use of nebulization to introduce samples into the plasma. This sample introduction method requires that samples either be liquid or be converted to a solution. The use of a solid sampling technique, which would obviate the acid digestion procedures, would be desirable. The analytical capability of ICP-AES would be significantly expanded if solid or powdered samples could be directly introduced into the plasma. Several approaches include laser ablation, spark vaporization, direct sample insertion, slurry atomization techniques, and electrothermal vaporization. The slurry atomization techniques has been applied already to the analysis of suspended particulate matter collected on polystyrene fiber filter (8). Although coupling of electrothermal vaporization (ETV) to an ICP system is fairly recent, utilization of electrothermal vaporization in ICP-AES analysis is continually increasing in popularity (10-29). In this type of analysis, the aerosol formed externally by electrothermal vaporization is transported to the plasma where atomization and excitation occur. A variety of ETV-ICP systems were employed for the direct determination of microliter sample solutions, including biological fluids such as urine and serum (29). Although information on the direct analysis for trace elements in solid or powdered samples is limited, this system shows considerable promise for the direct analysis of solid or powdered samples. In the method described here, a glass fiber filter on which suspended particulate matter has been collected is dissolved directly with hydrofluoric acid in a graphite microboat, and then the residue that remains in the microboat is vaporized into the ICP discharge. The method is useful for the rapid, routine determination of trace elements, including Cr, Mn, and Pb, in suspended particulate matter. EXPERIMENTAL SECTION Apparatus. Experimental facilities include a 40.68-MHz generator (Plasma-Therm Model HFL-2000G) and a 1-m Czerny-Turner monochromator described previously (2628). Emission signals observed in the plasma at the wavelength of interest were recorded with a chart recorder (Heath, Model EU-20-28). The electrothermal vaporization graphite furnace (Instrumentation Laboratory Model IL 655, controlled-temperature furnace atomizer) was modified for the ICP application. The cylindrical graphite cuvettes (IL part 29669) and the microboats (IL part 44119) were used. The cuvettes and the microboats were originally employed for the conventional graphite furnace-atomic absorption spectrometry (28). The dimension of the cuvette was 31.6 mm length and 4.65 i.d., with wall thickness of 0.8 mm. In the cuvette, a slot was cut for the insertion of the microboat, and a 2.4-mm-diameter hole was drilled for the injection of sample solution onto the microboat. During the course of establishing favorable operating conditions, the cuvette without a hole was also examined. A tungsten-based resistance thermometer measured the temperature, and the feedback mode in the temperature readout provided a reproducible temperature setting. However, the temperature value quoted here refers to the cuvette wall, and the actual temperature of the microboat is usually lower than the wall. Reagents. Analytical reagent grade chemicals (Fisher Scientific) and distilled, deionized water was used. Each stock solution of Cr, Mn, and Pb was prepared by dissolving a weighed portion of the high-purity metal or salt in a dilute acid. Standard solutions containing Cr, Mn, and Pb were prepared fresh by mixing requisite amounts of the stock solutions and diluting with distilled, deionized water. The amounts were based on the estimated concentrations of the elements in suspended particulate matter. Sample Collection. Suspended particulate matter was collected on a 8 in. X 10 in. glass fiber filter (Toyo GB-100 R) mounted on a standard Hi Vol sampler. The sampling time was 24 h at the average flow rate of 1.5 m3/min. The total exposed filter area was 7 in. X 9 in. of the 8 in. X 10 in. filter. Before and after the collection of suspended particulate matter, the filter was weighed so that the weight of the suspended particulate matter was known. The atmospheric concentratitm of suspended par-
Table I. Compromise Operating Conditions for the ETV-ICP System Plasma frequency forward power reflected power argon outer gas flow rate argon intermediate gas flow rate argon carrier gas flow rate observation height analytical wavelengths
40.680 MHz 0.75 kW 2500 "C. A satisfactory compromise vaporization temperature was thus found to be 2500 "C. However, a sudden excessive release of
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vapor from the microboat moved aside the microboat from the center of the cuvette and degraded the precision. For reproducible vaporization of analytical elements from the microboat, an ashing step was applied at 500 "C for 10 s followed by heating to 2500 "C. Very volatile compounds in the microboat would be removed largely during the ashing step. A portion of volatile silicon compounds formed in the microboat also was vaporized. However, no analyte emission signal appeared during the ashing step. At that temperature, samples including a volatile element such as Pb could be heated without any significant losses of analytical elements. Prolonged heating and ashing at higher temperatures were expected to make the vaporization of volatile silicon compounds more efficient. However, the possibility of losses of volatile analytical elements would increase. Since the solution in the microboat was completely evaporated on the hot plate prior to insertion into the cuvette, a drying step is not necessary. However, to ensure the complete drying of the sample, a drying step was applied at 110 "C for 10 8. Under the temperature program, aerosol generated from the microboat was successively transported into the plasma without extinguishing the discharge or changing the background. However, in this type of cuvette with a hole in the center of it, a portion of vapor from the microboat probably escaped through the hole. A cuvette without a hole should be more effective for the transport of aerosol (28). Indeed, higher emission signals were obtained up to the vaporization temperature of about 2200 "C. The plot of emission signal with increased vaporization temperature had about the same slope as was obtained for the cuvette with a hole. However, at temperatures above 2200 "C, emission signals decreased with increasing temperatures. Double peaks or even multiple peaks were obtained with poor reproducibility. Probably the extremely high vapor pressure built up within the cuvette moved the microboat from the center of the cuvette and introduced poor precision. Carrier Gas Flow Rate. A significant source of variation in response of the ETV-ICP system is a change in flow rate of argon carrier gas. As shown in Figure 4,in the case of the cuvette with a hole, emission signal increased at first and then gradually decreased with increasing carrier gas flow rate. In addition, the flow rate of carrier gas influenced the shape of the emission signal. Low flow rates,
