940 (13) (14) (15) (16) (17) (18) (19) (20)
(21)
Anal. Chem. 1983, 55, 940-943 Eng, R. S.;Butler, J. F.; Linden, K. J. Opt. Eng. 1980, 19, 945. Maeda, Y.; Aoki, K.; Munemorl, M. Anal. Chem. 1980, 52,307. Albrecht. H. 0. 2.Phys. Chem. 1928, 136, 321. White, E. H. I n "Light and Life", 1st ed.; McElroy, W. D., Glass, B., Eds.; Johns Hopkins Press: Baltimore, 1962; P 183. Seitz, W. R. Methods Enzymol. 1978, 57,445. Schroeder, H. R.; Boguslaskl, R. C.; Carrico, R. J.; Bookler, R. T. Methods Enzymol. 1978, 57,425. Seitz, W. R.; Hercules, D. M. I n "Chemiluminescence and Bioluminescence"; Cormier, M. J., Hercules, D. M., Lee, J., Eds.; Plenum Press: New York, 1977; p 427. Kok, G. L.; Holler, T. P.; Lopez, M. B.; Nachtrieb, H. A,; Yuan, M. Envlron. Sei. Technol. 1978, 12, 1072. Kok, G. L.; Darnall, K. R.; Winer, A. M.; Pitts, J. N., Jr.; Gay, B. W. Environ. Sei. Technol. 1978, 12, 1077.
(22) Anderson, H. H.; Howard, H.; Moyer, R. H.; Slbbett, D. J.; Sutherland, D. C. US. Patent No. 3659 100, Aug 1970. (23) Anderson, H. H.; Howard, H.; Moyer, R. H.; Sibbett, D.J.; Sutherland, D. C. U.S. Patent No. 3700696, Oct 1972. (24) Warner, P. W., personal communlcation. (25) Levaggl, D.; Kothny, E. L.; Belsky, T.; de Vera, E.; Mueller, P. K. Environ. Scl. Technol. 1974, 6, 348. (26) Kelly, T. J.; Stedman, D. H.; Ritter, J. A.; Harvey, R. B. J . Geophys. Res. 1980, 85,7417.
RECEIVED for review December 9, 1982. Accepted February 16, 1983. We would like to thank General Motors Research Laboratories for supporting the field trip to Bermuda.
Bis(2,4-dinitrophenyI) Oxalate as a Chemiluminescence Reagent in Determination of Fluorescent Compounds by Flow Injection Analysis Kazumasa Honda, Jun Sekino, and Kazuhiro I m a l " Department of Analytical Chemistty, Faculty of Pharmaceutical Sciences, University of Tokyo, 7-3- 1, Hongo, Bunkyo-ku, Tokyo 1 13, Japan
The chemiluminescence (CL) reactlon of bis( 2,4-dlnltrophenyl) oxalate (DNPO) or bis( 2,4,6-trlchlorophenyI) oxalate (TCPO) with hydrogen peroxlde was used for the detectlon of the fluorescent compound dansylalanlne (DNS-Ala) In buffer solution. Factors affecting the CL intensities, such as water, halogen salts, and phenols, 2,4-dlnltrophenol (DNP) and 2,4,6-trlchlorophenoI (TCP), which are reactlon products, were studled. Halogen salts greatly quenched both CL Intensltles, and the water content In the reactlon medium also acted to reduce them. While the effect of TCP on the CL intenslty In the TCPO-CL reactlon was large, that of DNP in the DNPO-CL reactlon was small. The DNPO-CL reactlon was superlor to that of TCPO-CL In sensltlvlty In the flow injectlon analysls of DNS-Ala. The detectlon llmlt of DNS-Ala In the former reactlon was 5 fmol.
factors affecting the CL intensity in comparison with the TCPO reaction. The application of the DNPO-CL reaction to flow injection analysis will be reported.
In the conventional fluorescence detection system, a part of the stray radiation or Raman scattering derived from the light source, which raises the background level, often interferes with improvement of the system's sensitivity. To solve this problem, a chemiluminescence (CL) detection method, without light source, is a plausible candidate for the detection of fluorescent compounds. The reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) and hydrogen peroxide (H,O,) ( I ) (Scheme I) was applied to the detection of fluorescent compounds, such as dansyl amino acids, on thin-layer chromatograms ( 2 , 3 )and aromatic hydrocarbons ( 4 ) . In previous papers (5, 6), we applied the CL reaction using TCPO and H,02 to a fluorescence detection system for high-performance liquid chromatography (HPLC). With this method, dansyl amino acids and fluorescamine labeled catecholamines were separated and detected a t the 10 fmol and 25 fmol levels, respectively. For the quantification of a trace amount of biogenic substances, however, one or two orders of magnitude higher sensitivity is required. According to the results by Rauhut et al. (I, 7), among the oxalates, bis(2,4-dinitrophenyl) oxalate (DNPO) gave one of the highest CL quantum yields. Therefore, in this work we took DNPO and studied the various
EXPERIMENTAL SECTION Reagents. TCPO was prepared by following the method of Mohan and Turro (8) and recrystallizing from ethyl acetate. DNPO was prepared by following the method of Rauhut et al. (7) and recrystallizing from acetonitrile. Dansylalanine (DNS-Ala, cyclohexylamine salt) and H202were purchased from Sigma Chemical Co. (St. Louis, MO) and Mitsubishi Gas Kagaku Co. (Tokyo,Japan), respectively. All other chemicals were of reagent grade. Experiments in the Static System. Ethyl acetate and acetonitrile were selected as the solvent for oxalates (DNPO and TCPO) and H202, respectively. As a representative of fluorescent compounds, DNS-Ala was used. Two-hundred-fifty microliters of 0.05 M H202solution and 40 pL of 100 nM DNS-Ala solution (1.0 X M phosphate buffer, Na+) were premixed in a borosilicate glass tube (6 X 50 mm, Fisher Co., Boston, MA) for 10 s, and 100 pL of 1.0 mM oxalate solution was added instantaneously with a microsyringe and mixed vigorously with a mixer (type TM-100, Thermonics Co., Tokyo, Japan) for a few seconds. The tube was immediately set inside a CHEM-GLOW photometer (Aminco Co., Baltimore, MD) and the CL intensity was measured. The time course of the relative CL intensity was recorded after the addition of oxalate solution. Net CL intensity of DNS-Ala was obtained by subtracting the CL intensity of the reaction solution without DNS-Ala (blank) from that with DNS-Ala. Fluorescence (FL) intensity was also measured under almmt the same conditions where the CL reaction
0003-2700/83/0355-0940$01 SO10
Scheme I 0 0
i i R RO-C-C-OR
t H202+
f-7 ]
+2R,OH
10-0 D D + F l u 'Flu*(excited
FLU*
--+
state)+2C02
Flu + h u
Flu =Fluorophore
0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
-
Table I. Effect of Various Salts on DNPO-CL Intensitya salts added none NaCl NaBr
p--pi24d Solution
NaI Na,SO,
Figure 1. Flow di,agramfor chemiluminescence detection system for flow injection anallysis: D, damper; DC, delayed coil; Det, detector; I, injector; MD, rotating flow mixing device; P, pump; R, recorder.
644
\
Figure 2. Effect of pH of DNS-Ala solution on CL intensity. Various pH values of phosphate buffers (1.0 X lo-* M, Na') for DNS-Ala
solutions were used. was perfomed: 2.5 mL of acetonitrile, 0.4 mL of 1pM DNS-Ala solution and 1.0 mL of ethyl acetate were mixed, and the FL intensity was measured with a Hitachi 650-10s fluorescence spectrophotometric detector (Tokyo, Japan; excitation, 340 i 10 nm; emission, 5115 i 10 nm). Experiments in the Flow System. The flow diagram of the system is shown iin Figure 1. Minimicropumps (Kyowa Seimitsu Co., Tokyo, Japan), bellows type dampers (Developmentand Mfg. Co., Inc., Milford, CT), and a high-pressureuniversal injector (type KHP-UI-130, Kyowa Seimitsu) were used. At the point where two solutions were mixed, a newly developed rotating flow mixing device (9) was adopted. A detector, Schoeffel FS-970 LC fluoromonitor (SchoeffelInstrument Co., Westwood, NJ; PMT tube, SIC part No. 4800-0013) with a flow cell (3 mm o.d., 7 mm) was adopted with the light source off. PTFE tubings with 0.25 mm diameter were used in all the flow lines, a part of which, before and after the detector, were sealed with aluminum foil to prevent the light from coming through the tubing and into the detector. All the experimeints were performed at room temperature (ca. 25 "C).
RESULTS AND DISCUSSION The CL intensity at 5 s after addition of DNPO or TCPO M solution to the mixture of DNS-Ala solution (1.0 X sodium phosphate) and H z 0 2 solution was measured. As shown in Figure 2, the p H values which gave the maximum CL intensity were 3 and 6 in DNPO-CL reaction and TCP0-CL reaction, respectively. The ratio of sample to blank intensity was 12.5 in DNPO-CL reaction a t pH 3.3 and 4.3 in TCPO-CL reaction at pH 5.9, respectively. FL intensity did not change in the range from p H 3 to 9. The following experiments were done at the above best pH for each reaction. CL reaction with aryl oxalate such as DNPO or TCPO and H202in the presence of fluorescent compounds was extensively studied by Rauhut et al. ( I , 7, IO) and has been shown to occur via the following three major stages. (1) The high-energy intermediate (l,%dioxetanedione) is generated by the reaction
NaCIO, KNO,
concn,b M
re1 CL intens
1.03 x 10-3 5.19 x 10-3
100.0 i: 3 , l C 71.5 r 2.2 26.6 i 1.3
5.19 x 10-3 1.04 x 10-3 1.04 x 10-3 5.17 x 10-3 1.05 x 10-3 5.14 X l R 3 1.07 x 10-3 5.24 x 10-3
0.9 ? 0.1 N D ~ N D ~ 90.5 f 3.3 106.5 i 2.2 92.2 i 3.1 94.3 i: 1.1 101.8 ? 4.8 101.8 i 4.7
1.04 x 10-3
941
a Intensity ai, 4 s after addition of DNPO solution to the mixture of DNS-Ala solution (1.0 X lo-' M phosphate buffer (Na+,pH 3.3)) and H,O, solution; results are the average of five experiments; there was no difference between phosphate buffer (Na') and phosphate buffer Concentration in the reaction (K+) on CL intensity. mixture. Arbitrarily taken as 100. CL was not detected under the experimental conditions,
of aryl oxalate with HzOz. (2) The intermediate reacts with the fluorescent compound to excite it to the singlet state. (3) The excited fluorescent compound relaxes to the ground state with the emission of light. The last stage is said to be the same as the fluorescent emission ( I O ) . It is difficult to clarify how the following factors affect stage 1or 2 because it is not easy to elucidate the structure of the intermediate itself or to detect it because of its unstability ( I , 11). However, it is possible to discuss whether stage 3 is affected or not by measuring FL intensity in comparison to CL intensity. Effect of Various Kinds of Salts. In the DNPO-CL reaction, various kinds of salts were added to the DNS-Ala solution, and the relative CL intensity (at 4 s) was measured. As shown in Table I, quenching by halogen salts was observed though other salts did not affect the reaction. The degree of quenching increased with the increasing salt concentration. Quenching occurred according to the so-called Stern-Volmer equation ( I 2 ) ,which is generally applied to quenching phenomena such as fluorescence, liquid scintillation ( I 3 ) ,and chemiluminescence (14, I5), that is l o / l = 1 + r[Ql where Io is the CL intensity in the absence of halogen salts, I is the CL intensity when the concentration of halogen salt is [Q], and y is the quenching constant. The quenching constant calculated from the data in Table I1 increased with the increasing atomic weight of halogens (NaCl, 4.6 X lo2M-l; NaBr, 4.0 X lo4 M-l; NaI, 4.4 X lo4 M-l). In the TCPO-CL reaction, similar quenchings were observed (quenching constants were 2.2 X lo3 M-l by NaC1, 4.0 X lo4 M-l by NalBr, and 5.5 X lo6 M-l by NaI; CL intensity a t 4 s and pH 6.0). Fluorescence of DNS-Ala was not quenched by halogens, which means that quenching on CL reaction occurs in stage 1or 2. However, the role of halogens on quenching is not clear at present and is now under investigation. It should be stressed that in the application of the CL reaction to the HPLC detection system, eluents containing halogen salts as a buffer or ion pairing reagents should be avoided. Effect of Ratio of Water Content. When H,Oz in acetonitrile solution was prepared, the total volume was kept constant with the various amounts of water. In DNPO-CL reaction (pH of DNS-Ala solution, 3.2), as the ratio of water in the final reaction solution increased (13-20%), the CL intensity at 5 s decreased by 55% and the FL intensity also decreased by 10%. The presence of water might thus affect
942
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
Table 11. Data for Calculating the Quenching Constant of DNPO-CL Reactiona salts
concn,b M
none NaCl
NaBr
1.03 2.07 3.10 4.14 5.17 1.04 2.07 3.11 4.15 5.18
NaI
x x x x x x x
1.00
10-3 10-3 10-3 10-3 10-3 10-4 10-4
X X X
2.07 x 4.14 x 6.21 x 8.28 x 10.35 x
I, II"
10-5
10-5 10-5 10-5
1.40 1.92 2.44 2.83 3.76 4.28 8.63 13.7 16.2 21.9 2.13 2.53 3.64 4.28 5.54
y,d
I
3D
M-'
4.6 x
4.0
X
4.4 x 10-4
pH of DNS-Ala solution, 3.3; temperature, 23 I1 "C. All other conditions are described in the text. Concentration in reaction mixture. I,, CL intensity in the absence of halogen salts; I, CL intensity in the presence of the halogen salt. CL intensity was measured at 4 s after the addition of DNPO solution to the mixture of DNS-Ala solution and H 0, solution; results are the average of five experiments. y, quenching constant.
Figure 3. Effect of phenols on CL intensity. DNP and TCP were respectlvely added to the DNPO and TCPO in ethyl acetate solutlons. Abscissa shows the final concentration of added phenols. CL intensity was measured at 4 s after the addition of the oxalate solution to the mixture of H 2 0 p solution and DNS-Ala solution. Relative CL intensity without addition of phenols is arbitrarily taken as 100. The pHs of DNS-Ala solution were 3.3 and 5.5 on DNPO- and TCPO-CL reaction, respectively.
not only the last stage but also stage 1 or 2. Therefore, it should be stressed that a reaction condition with less water content than that in the present experiment is much more favorable to give a higher CL yield in DNPO-CL reaction. In the TCPO-CL reaction (pH of DNS-Ala solution 5.9), CL intensity at 5 s and FL intensity decreased by the same degree (15%) when the water content increased (13 to 20%), meaning that the presence of water might lower the yield in the last stage. In our preliminary experiments (16),however, the effect of water on CL intensity could be dependent on the kind of fluorescent compound. For example, with a fluorescaminelabeled catecholamine as a fluorescent compound, the CL intensity increases with the increase of water in the TCPO-CL reaction mixture. Effect of Phenols Derived from the CL Reaction. 2,4-Dinitrophenol (DNP), a reaction product in DNPO-CL reaction, scarcely affected the CL intensity. However, when 2,4,6-trichlorophenol (TCP), a reaction product in the TCP0-CL reaction, was added to the TCPO-CL reaction solution,
Flgure 4. Response of DNS-Ala obtained by flow injection analysis: (A) DNPO-CL reactlon, 0.50 M H202 in acetonitrile (2.50 mL/min), 1.00 mM DNPO in ethyl acetate (1.00 mL/min), 0.01 M phosphate buffer (Na', pH 3.28) (0.40 mL/min), detected at 2.4 s after mixing. (6) TCPOCL reaction, 0.50 M H202 in acetonitrile (2.50 mL/min), 1.00 mM TCPO in ethyl acetate (1.00 mL/mln), 0.01 M phosphate buffer (Na', pH 5.90) (0.40 mL/min), detected 8.1 s after mixing; Ip, 1 pmol; 2p, 2 pmol; 3p, 3 pmol.
the CL intensity decreased markedly (Figure 3). T C P was reported to quench some of the excited states formed (17),but in our experiments no quenching effect on FL intensity was observed. On the other hand, DNP reduced markedly the FL intensity. The less quenching by DNP on the CL in DNPO-CL reaction might be a color quenching of DNP to absorb the emitted light from DNS-Ala. Experiments in the Flow System. Detection of DNS-Ala in the flow injection analysis (FIA) system using the DNP0-CL reaction (Figure 1) was attained. The pH of the phosphate buffer was selected as 3 for the line for DNS-Ala considering the results obtained in the static system. The length of the PTFE tubing between the mixing device and the detector was varied in order to change the CL reaction time, and the relative CL intensity (peak height) of DNS-Ala was measured. The CL intensity decreased according to a pseudo-first-order equation as the reaction proceeded. The reaction time was fixed a t 2.4 s (length of tubing, 1.4 m) in the following experiments. Figure 4 shows the response of DNS-Ala using the DNP0-CL reaction in comparison with the TCPO-CL reaction. In the TCPO-CL reaction, the best condition for the CL intensity (pH of DNS-Ala solution, 5.9; reaction time, 8.1 s) was employed. The detection system using the DNPO-CL reaction afforded a DNS-Ala determination range from 20 fmol to 1 nmol with a relative standard deviation of 3.6% ( n = 5) at the 20 fmol level. The detection limit was 5 fmol at the signal to noise ratio of 3. As shown in Figure 4, the CL intensity of DNS-Ala with DNPO-CL reaction was 10 times higher than that of the TCPO-CL reaction under the selected condition of optimum pH and reaction time for each reaction. This might be ascribed to the fact that, firstly, DNPO is superior to TCPO in giving higher CL quantum yield (1). Secondly, TCP, a reaction product in the TCPO-CL reaction, is a stronger quencher than DNPO for the reaction. The DNPO-CL reaction might be applicable to the detection system for HPLC (5,6) to obtain higher sensitivity and also to another analytical system using the TCPO-CL reaction (2-4, 18-21).
ACKNOWLEDGMENT The authors thank Zenzo 'i'smura of the University of Tokyo and W. Rudolf Seitz of the University of New Hampshire for their valuable suggestions. The authors are also grateful to Kyowa Seimitsu Co. (Tokyo, Japan) for the use of pumps and to Atto Corp. (Tokyo) for the use of the detector and dampers.
Anal. Chem. 1983, 55, 943-946
Registry No. DNPO, 16536-30-4;TCPO, 1165-91-9;IDNS-Ala, 35021-10-4;DNP, 51-28-5;TCP, 88-06-2;HzO, 7732-18-5; NaC1, 7647-14-5;NaBr, 7647-15-6;NaI, 7681-82-5. LITERATURE CITED Rauhut, M. M. Acc. Chem. Res. 1989, 2 , 80-87. Curtis, T. G.;Seitz, W. R. J. Chromatogr. 1977, 734, 343-350. Curtis, T. G.:Seitz, W.R. J. Chromatogr. 1977, 734, 513-516. Sherman, P. A.; Holtrbecher, J.; Ryan, D. E. Anal. Chim. Acta 1978, 9 7 , 21-27. Kobayashi, S.; Imai, K. Anal. Chem. 1980, 5 2 , 424-427. Kobayashi, S.; Sekino, J.; Honda, K.; Imai, K. Anal. Biochem. 1981, 712, 99-10,4. Rauhut, M. ht.; Bollyky, L. J.; Roberts, B. G.; Loy, M.; Whitman, R. H.; Iannota, A. V.; Semsel, A. M.; Clarke, R. A. J. Am. Chem. SOC. 1987, 89,6515-6522. Mohan, A. GI.; Turro. N. J. J. Chem. Educ. 1974, 51, 528-529. Kobayashi, S.;Imai, K. Anal. Chem. 1980, 52, 1548-1549. Rauhut, M. M.; Roberts, B. G.; Maulding, D. R.; Bergmark, W.; Coleman, R. J. Org. Chem. 1975, 4 0 , 330-335. Richardson, W. H.; Q'Neal, H. E. J. Am. Chem. SOC. 1972. 9 4 . 8665-8668. Stern, 0.; Volmer, M. Phys. Z . 1919, 2 0 , 183-188.
943
(13) Kaczmarczyk, N. "Organic Sclntillators and Liquid Scintillation Counting"; Horrocks, D. L., Peng, C. T., Eds.; Academic Press: New York, 1971; pp 977-990. (14) Fletcher, A. N.; Heller, C. A. Photochem. Photobioi. 1985, 4 , 105 1-1058. (15) Belyakov, V. A,; Vassil'ev, R. F. Photochem. Photoblol. 1987, 6 , 35-40. (16) . . Sekino, J.: Honda. K.: Imai, K. R o c . Symo. . . Chem. Physlol. Pathol. 1981, 27, 17-19. (17) Lechtken, P.; Turro, N. J. Mol. Photochem. 1974, 6 , 95-99. (18) Wllliams, D. C., 111; Huff, G. F.; Seitz, W. R. Anal. Chem. 1978, 4 8 , 1003. .. 1006. . .... (19) Williams, D. C., 111; Seitz, W. R. Anal. Chem. 1978, 4 8 , 1478-1481. (20) Arisue, K.; Marul, Y.; Yoshida, T.; Ogawa, 2 . ; Kohda, K.; Hayashi, C.; Ishida, Y. Rlnsho Byori 1981, 2 9 , 459-462. (21) Tsuji, A.; Maeda, M.; Arakawa, H. R o c . Symp. Chem. Physioi. Patho/. 1981, 21, 5-8.
RECEIVED for review August 28, 1982. Resubmitted and accepted January 27, 1983. Presented in part a t the 102st Annual Meeting of the Pharmaceutical Society of Japan, Osaka, April 3-5, 11982.
Determination of Free Silica and Manganese in Airborne Particles by Flameless Atomic Absorption Spectrometry G. H. Chen' alnd T. H. Risby" Division of Environmental Chemistry, D e p a t f r " Baltimore, Maryland 2 1205
of Environmental Health Sciences, The Johns Hopkins School of Hygiene and Public Health,
A method has been developed for the determinatlon of free silica and manganese in evaluating occupational exposure during the mixing, transporting, crushing, and sieving of manganese ore. This procedure uses phosphoric acid to dissolve silicates and manganese from the slllca residue. Flameless atomic absorptlon spectrometry Is used to determine the manganese in the flitrate and to determlne the dlicon in the form of the siilcomolybdate complex. Various experimental condltlons and Interference by different cations are reported.
Manganese is not the only occupational hazard during the mining, transporting, crushing, and sieving of manganese ore. Free crystalline silica usually coexists with manganese in the dusts; workers in this industry are susceptible to both manganese poisoning and silicosis. In order to evaluate the workers' exposure to both manganese and free silica, separate methods of sampling and analysis are usually employed. For the determination of manganese, the colorimetric methods and modified spectrographic procedures of the early 1960's have now been superseded by flame atomic absorption spectrometry (1). For the determinatilon of free silica, three principal methods are currently used (2): the colorimetric procedure, infrared spectrophotometry, and X-ray diffraction. The latter method is recommended by NXOSH for the quantitative and qualitative analysis of dust containing free crystalline silica although Visiting scientist from Institute of Industrial Health, Anshan Iron & Steel Co., Anshan, Liaoning Province, The People's Republic of China.
the other two are also acceptable. The sensitivity, speed, sh.ort sample preparation time, ability to identify the polymorphs of free silica, and possibilities for automation make X-ray diffraction the method of choice. However, it is limited by its requirement for expensive equipment and a high degree of technical skill. At present, the colorimetric procedure is still widely used. In this investigation, an attempt was made to develop a method to quantify both manganese and free silica in the same sample, with minimum sample preparation, using flameless atomic absorption spectrometry. This technique has a high sensitivity and is appropriate for measuring both manganese and silicon. This work of Talvitie (3-5) and Sweet (6) has proved a source of valuable information for colorimetric determinations as well as for this investigation. The proposed procedure is a modification of the colorimetric method.
EIXPERIMENTAL SECTION Apparatus. A flameless atomic absorption spectrometer (Jarrell Ash Model 810 and FLA-100) was used for the analysis. Aliquots of the anal@ solutions were introduced into the atomizer cell with an automatic pipet (5 pL) with precleaned disposable polyethylene tips. Chemicals. Deionized distilled water, high-purity acids (Ultrex, J. T. Baker Co.), atomic absorption standards (Fisher Scientific Co.), and high-purity reagents (Analyticalgrade, J. T. Baker Co.) were used throughout this study. The method was standardized by using the NIOSH recommended standard for quartz (Mm-U.Sil 5) which has a particle size of less than 5 pM. Procedure. All glassware was cleaned with aqua regia and polyethylene with EDTA solutions, prior to use, and were stored filled with distilled water to prevent contamination of the standards or samples (7). Polycarbonate membrane filters (25 mm or 37 mm, 0.2 pm, Nuclepore Corp.) were selected for the collection of airborne
0003-2700/83/0355-0943$01.50/00 1983 American Chemical Soclety
