546
Anal. Chem. 1981, 53, 546-549
ACKNOWLEDGMENT
(3) Beroza, Morton; Bowan, Malcolm C.; Bierl, Barbara A. Anal. Chem. 1972, 44, 2411-2413. (4) Solomon, John Anal. Chem. 1979, 51, 1861-1863. (5) Junk, A.; Richard, J. J.; Grieser, M. D.; Withk, D.; WRhk. J. L.; Arguello, M. D.; Vick, R.; Svec, H. J.; FrRz, J. S.; CaMer, G. V. J . Chromatogr. 1974, 99, 745-762. ( 6 ) Dunges, W. "Praechromatographische Mlkromethoden"; A. HClthlg Verhg: Heidelberg, Basel, New York, 1979; Chapter 3.
The expert technical assistance of G. Schmidinger and L. Kappinger is acknowledged.
LITERATURE CITED (1) "Pesticide Analytical Manual"; U.S. Department of HeaRh, Education and Welfare: Rockville, MD, 1972; Vol. 1, Chapter 1. (2) Zimmerli, B. M M . Gebiete LebensmMelunters. Hyg. 1973, 64, 528-532.
for review September
49
lg80. Accepted December
2, 1980.
Preparation of Multielement Solutions for X-ray Fluorescence Analysis with a Liquid-Aerosol Generator Robert B. Kellogg, Nancy F. Roache, and Barry Dellinger" Environmental Sciences, Northrop Services, Inc., P.O. Box 123 13, Research Triangle Park, North Carolina 27709
The number of techniques for analyzing solutions by X-ray fluorescence are far too numerous to describe individually ( I , 2 ) . Techniques such as preconcentration and precipitation are time consuming or not applicable if many elements are present in the sample. The most direct method of analysis is to use liquid-specimen holders (I). In this method the solvent as well as the Mylar film used to contain the liquid in the holders greatly absorbs the radiation from low atomic number elements. Such a method is best suited for relatively high concentrations of high atomic number elements. The capillary matrix method ( 2 )utilizes an array of capillaries which is dipped into the solution to be analyzed drawing an accurately known volume. One then touches an absorbent filter with the capillary array wetting the filter followed by freeze-drying. This leaves a pattern of dry spots containing the sample. One is unquestionably limited to analysis of very high atomic number elements because of the strong absorption by the filter material. After attempting the above techniques, we were convinced that rapid analysis of solutions against our t h i n - f i i calibration standards was most feasible through total evaporation of the solvent and subsequent analysis of the particulate matter. Such a method was readily achievable only through a liquid aerosol generation technique. This article describes in detail the results obtained in this laboratory by using this method.
EXPERIMENTAL SECTION Apparatus The liquid-aerosol generator, shown in Figure 1, consists of a Collison nebulizer (BGI, Inc., Waltham, MA), a nebulizer vessel, a dilution air coupling with a stainless she1 sleeve, a mixing chamber, and a filter holder (Millipore Corporation, Bedford, MA). All parts are made of Pyrex (Corning) except the nebulizer and stopper assembly, which are machined from Teflon (DuPont Co.), and the dilution air coupling sleeve and fdter holder, which are stainless steel. The construction materials reduce sample contamination and facilitate cleaning. The nebulizer vessel was constructed so that the bottom tip of the nebulizer clears the vessel by approximately 0.5 mm. This construction, which deviates from previous descriptions (3), is necessary because of the small volumes (5-10 mL) to be prepared by use of this apparatus. Also, a six-jet nebulizer is used to achieve higher aerosol generation rates with less preparation time. The dilution air coupling is fitted with a sleeve for introducing compressed air through the air inlet. The air flows between the sleeve and the coupling and then through a series of holes around the coupling's circumference, enters the appratus, and dries the aerosol. The dried particles then pass into the 10 cm by 46 cm cylindrical mixing chamber before being deposited on the filter. Operating conditions for the aerosol generator are given in Table I. Ease of cleaning reduces the possibility of cross-contamination during analysis and, thus, was an important design criterion during 0003-2700/81/0353-0546$01 .OO/O
Table I. Operating Conditions of Sample Aerosol Generator parameter nebulizer air flow dilution air flow sample volume sampling time
measurement 7.6 L/min at 25 psi 18-22 L/min 5-15 mL 1-30 min
the development of the generator. All parts of the generator, except the nebulizer and dilution air coupling, can be adequately cleaned with deionized water. The dilution air coupling is cleaned with a mild soap solution and a brush. The nebulizer is rinsed and submerged in deionized water, operated for a few seconds and then rinsed in methanol, and dried by a burst of air. Characterization Deposit Uniformity. Aerosol droplets generated for use in X-ray fluorescence analysis must be uniformly deposited across the measurement area because of nonuniform photon flux density of the excitation source. A series of 11 6.35mm diameter plugs were removed from the 35-mm diameter deposit from a sample containing S, K, and Cd to assess the uniformity of the deposit for samples from the aerosol generator. The plug pattern is shown in Figure 2. Each plug was fixed in the center of a sample holder with two-sided transparent tape. The count rate for S, K, and Cd was measured for each plug and normalized to the average for each element, respectively. These normalized count rates were then averaged (with 20 error bars) for each of the five distances from the axis of rotation. (The samples are rotated during analysis.) The results are shown in Figure 2. Evaluation of these data indicates that no one position on the filter received abnormally high or low aerosol deposits, indicating that the deposit is sufficiently uniform for analytical purposes. Generation of Calibration Standards. The liquid aerosol generator was used to prepare calibration standards for sulfur. Standard solutions were prepared from 0.1% KzSO4 and were collected on O.&pm pore size Nuclepore filters (Nuclepore Corporation, Pleasanton, CA) that had been exposed for 24 h to a constant temperature and relative humidity of 21 "C and 45%. After collection of the particulate, the samples were stored under the same conditions for 24 h, weighed to determine the areal concentration, and analyzed by X-ray fluorescence. A linear least-squares fit of the absorption corrected intensity data on Seven standards gave a sensitivity of 254.3 f 1.0counts cm2pg-* s-' which is in good agreement with 260.7 i~ 1.3 counts cm2pg-' s-' obtained with thin film standards from Micro-Matter Co., Seattle, WA. Similar good agreement was found for Na2S04and (NH&SO4. Analysis of Aerosol Samples. The applicability of the aerosol generator for the analysis of a broad range of acid-soluble elements was assessed. A solution was prepared by mixing 1mL of a lo00 ppm standard of each of the following cations: Al,Si, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Sn, Sb, Cs, and W (F & J Scientific, Monore, CT). Because of the nonavailability of sulfur standards from F & J Scientific and contamination of our 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
547
AIR INLET
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