2008
Anal. Chem. 1983, 55,2006-2007
AIDS FOR ANALYTICAL CHEMISTS Determination of Lead in Gasoline by Zeeman Atomic Absorption Spectrometry Donald R. Scott* and 1;. E. Holboke’
U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory, Research Triangle Park, North Carolina 27711 Tetsuo Hadeishi Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 The determination of lead in gasoline is usually performed with flame atomic absorption spectrometry. This is the standard method adopted by the American Society for Testing Materials (1) and by the Environmental Protection Agency (2). The manual method of preparation involves the reaction of the alkyl lead components of gasoline with iodine, stabilization of the alkyl lead iodide complexes with tricaprylmethylammonium chloride (Aliquat 336), 10-fold dilution with methyl isobutyl ketone, and measurement of lead by atomic absorption spectrometry with an air-acetylene flame. The reaction with iodine reduces the variation in response for different alkyl lead compounds (3),and the dilution minimizes nonatomic absorption and matrix effects ( 4 ) . In actual use, this method is slow due to the sample preparation steps, which involve waiting periods, and is not very reproducible. An alternate method of analysis of lead in gasoline should involve fewer preparation steps and therefore be more rapid, particularly in the analysis of large number of samples. Zeeman atomic absorption spectrometry (ZAAS) is capable of correcting for large amounts of nonatomic absorption caused by smoke (5)and should be well suited for direct analysis of gasoline. The principal problem in the analysis is the volatility of the alkyl lead compounds in the gasoline which prevents their complete atomization before escape from a conventional electrothermal furnace. A “high gas temperature furnace” for Zeeman AAS analysis of volatile organolead compounds such as alkyl leads in gasoline has been described by Koizumi, McLaughlin, and Hadeishi (6). Another possible approach is the use of the dual chamber furnace for ZAAS lead analysis described by Hadeishi and McLaughlin (7). The use of a modified dual chamber cuvette in an electrothermal furnace combined with ZAAS analysis will be shown to provide a more rapid method of lead in gasoline analysis which yields results equivalent to the standard method.
EXPERIMENTAL SECTION Standard Flame AAS Method. Samples were received from an inspector in the field in glass containers and refrigerated until analyzed. The preparation steps described in the standard method (1,2)involving addition of iodine and Aliquat 336 and dilution with MIBK were followed. Calibration standards of 0.02, 0.10, and 1.00 g of Pb/gal were freshly prepared by using lead chloride in isooctane. Analyses were performed at 283.3 nm with a Perkin-Elmer 603 AAS equipped with a LKB sample changer. Each prepared sample was analyzed twice over a time period of ca. 1 h. A complete replication of sample preparation and analysis steps was always performed at least once per set of samples or at a frequency of one in ten samples. The NBS Standard Reference Material 1636 (lead in reference fuel) at a concentration of 0.0322 and/or 0.0725 g of Pb/gal was analyzed at a frequency of one in Present address: U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory,Las Vegas, NV 89114.
Table I. Comparison of Flame AAS and Zeeman AAS Lead Results amt of lead, g/gal flame AAS Zeeman AAS sample method a method
difference
0.0015 i 0.009 0.0015 t 0.0006 0.0000 0.004 t 0.002 0.0046 i 0.0004 -0.0006 0.004 t 0.002 0.0039 i 0.0005 0.0001 0.0105 t 0.001 0.011 i 0.007 -0.0005 0.082 t 0.002 0.075 * 0.009 0.007 bias 0.0012 detection 0.002 0.0006 limit Average of two analyses. Average of five analyses. Sum of differences divided by five. 1 2 3 4 5
ten samples or once per set of samples. Zeeman Furnace AAS Method, The Zeeman AAS which was used to analyze the samples utilized a magnetic field imposed upon the lamp. Details are given elsewhere (6,8). A commercially available ZAAS equivalent to the one used is available from Gruen Optik, Wetzlar, West Germany (Angstrom, Inc., Relleville, MI). It was used with a Massmann furnace equipped with a double chamber carbon cuvette, the construction of which has been described (7). The outer chamber was loosely packed with charcoal. Samples were introduced into the outer chamber where the volatile organic lead compounds were slowly transferred by gas flow to the inner chamber where complete atomization occurred. The samples were atomized by passing 400 A at 15 V through the furnace for 10 s. An magnetically confined lead lamp (6) was operated at 283.3 nm in a magnetic field of 10 kG. Peak signals were recorded on a strip chart recorder. The instrument was calibrated with NBS SRM-1636 samples containing 0.0322 and 0.0725 g of Pb/gal diluted 1:lO with MIBK. Samples were diluted 1 : l O with MIBK, and 10 p L was introduced into the furnace with a micropipet. Each sample was analyzed five times.
RESULTS AND DISCUSSION Five representative samples, covering the concentration range 0.0015-0.08 g of Pb/gal, were chosen from 100 samples collected in the field during routine inspection of vendors. The 100 samples had been routinely analyzed a t different times by using the standard flame AAS procedure. These five samples, which had been stored in a refrigerator, were later sent to Lawrence Berkeley Laboratory for analyses by the Zeeman AAS method. The analysis data are given in Table I. The differences between the two sets of results range from less than 0.0001 to a maximum of 0.007 g of Pb/gal with a bias of 0.0012 g of Pb/gal, the standard method results being higher. Both the maximum difference and the bias are below the EPA quality control guideline of 0.01 g of Pb/gal for replicate analysis of the same sample within one laboratory. The precision of analysis appears to be better a t low con-
This article not subject to U.S. Copyright, Published 1983 by the American Chemical Society
Anal. Chem. 1983, 55,2007-2009
centrations by Zeeman P A S and better at high concentrations by flame AAS. This is probably due to the lower detection capability of the furnace technique used in the Zeeman method and the requirement for diluting high concentration samples before analysis. Statistical analysis off the data yielded a correlation coefficient of 0.9998f 0.0009. A pairwise t test of the two sets of data gave a result of 0.82 which does not support, at any reasonable probability, the hypothesis that a difference exislts between the two methods. A linear regression analysis of the Zeeman AAS results vs. the standard flame AAS method yielded a Y intercept of 0.0007f 0.0003g of Pb/gal and a slope of 0.907 f 0.009with a overall standard error of fit equal to 0.0006 g/gal. The detection limit for the flame AAS procedure was 0.002g/gal and that of the Zeeman procedure was 0.0006 g of Pb/gal. The accuracy of the standard flame AA.S procedure was established during routine analysis with NBS standard reference samples (SRM 1636)at a frequency of one in ten samples. The concentration of lead in these samples was either 0.0322 or 0.0725 g/gal. The deviations from the certified values were less than 0.0035g of Pb/gal (5.9%)and averaged 0.0023 g of Pb/gal (3.9%) with an overall bias of -0.O009g of Pb/gal(1.5%). Since the Zeeman procedure was calibrated with the NBS SRM 1636,use of this SRM as an accuracy check would not provide much useful information. However, since the Zeeman AAS results agreed with the standard flame AAS data to within f9%, the accuracy of the Zeeman results certainly deviates less than f13% from the reference values. In actual practice the standard method of analysis of lead in gasoline using flame AAS is slow due to the various preparation steps. In the Zeeman AAS method the only prepa-
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ration step is dilution of the sample, making the overall analysis considerably faster. Although the Zeeman AAS procedure requires a Zeeman spectrometer and a double chamber furnace, it may well be worth the expense for a laboratory which processes large numbers of samples. From a safety point of view, the Zeeman procedure reduces the use of MIBK considerably. The Zeeman AAS technique used in this study could easily be extended to the analysis of lead in other fuels.
ACKNOWLEDGMENT We thank Howard Kelley of the US. EPA, Environmental Monitoring Systems Laboratory, Las Vegas NV, for suggesting the modification of the furnace used in the analysis. Registry No. Pb, 7439-92-1. LITERATURE CITED "Annual Book of ASTM Standards"; Amerlcan Society for Testing Materials: Philadelphia, PA, 1973; Part 17, D-3237. Fed. Regisf. 1974. 39 (July 28), 15449. Kashiki, M.; Yamazoe, S.; Ohima, S.Anal. Chim. Acta 1971, 53, 95. Lukasiewiez, R. J.; Berens, P. H.; Buell, B. E. Anal. Chem. 1975, 47, 1045. Koizumi, H.; Yasuda, K. Spectrochim. Acta, Part B 1978, 318, 237. Kolzumi, H.; McLaughlin, R. D.; Hadeishl, T. Anal. Chem. 1979, 51, 387. Hadeishl, T.; McLaughlin, R. D. Anal. Chem. 1976, 48, 1009. Hadelshl, T.; Church, D. A.; McLaughiln, R. D.; Zak, B. D.; Nakamura, M.; Chang, B. Science 1975, 187.
RECEIVED for review April 29,1983. Accepted June 24,1983. This article has not been subjected to Environmental Protection Agency review and does not necessarily reflect the views of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Gas Chromatographic Determination of Residual Solvents in Lubricating Oils and Waxes I. M. R. De Andrade Bruening Petro'leo Brasileiro SIA, Centro de Pesquisa e Desenvolvimento, Leopoldo A. Miguez De Mello (PetrobrbsICenpes), Rio de Janeiro. Brazil The use of methyl ethyl ketone (MEK)-toluene mixtures as a solvent for dewaxing lubricating oils is a common refining practice. After extraction, the solvents are recovered and the lubricating oils and waxes are analyzed in order to control t,he efficiency of the solvent recovery unit and to determine if the residual solvent contents remain within the required product specifications. A gas-liquid chromatographic procedure ( I ) has been proposed, but it included a previous treatment of the samples, which involved heating, stripping, and trapping steps; in addition the last eluted peak, toluene, had a retention time of approximately 20 min. Recently, another method (2) reported the use of the azeotropic distillation of toluene, ME:K, and methanol as sample preparation prior to the GLC analysis. This procedure demanded large amounts of sample and a moderately polar solvent which could dissolve both methanol and wax or lubricating oil. This paper describes a direct gas-liquid chromatographic analysis of the residual solvents, using tert-butylbenzene as internal standard. In order to prevent the lube oils and waxes from contaminating the chromatographic analytical column, the samples were injected directly in a precolumri containing a silicone stationary phase. The samples of lube oils and waxes were similarly treated, .the latter being dissolved in an equal amount of light neutral oil, with eventual heating. With another stationary phase, 1,2,3-tris(2-cyanoethoxy)propaneand proper operating con-
ditions, it was possible to reduce the analysis time to 7 min.
EXPERIMENTAL SECTION Apparatus. The analyses were performed in a Varian Aerograph Model 2440 gas chromatograph equipped with a dual flame ionization detector using nitrogen as carrier gas. The instrument was connected to a 1-mV Hewlet-PackardModel 7127A recorder. The carrier and detector gases were dried in molecular sieve and silica gel filters. Chromatographic Columns. The analytical column consisted of a 4.0-m length of 0.32 cm 0.d. stainless steel tubing packed with 1,2,3-tris(2-cyanoethoxy)propane(TCEP) on Aeropak 30,80/100 mesh (0.5 g of TCEP per 9.0 g of support). The precolumn contained about 0.1 g of 10 wt % SE-30 on 60/80 mesh Chromosorb W. The precolumn dimensions were 10 cm length X 0.32 cm o.d., and the precolumn was installed in the column oven, between the injection outlet and the analytical column. Chemicals. Toluene and tert-butylbenzene were standards from Phillips Petroleum Co., Bartlesville, OK. MEK was available as pure grade chemical from Merck S.A. Indfistrias Quimicas, Rio de Janeiro, R.J., Brazil. Operating Conditions. The following operating conditions were used: oven temperature, 70 "C; injector and detector temperatures, 170 " C ; nitrogen flow rate, 35 mL/min. Preparation of Standard Solutions. In order to prepare standard solutions, it was necessary to use a diluent, which did not interfere with the solvent peaks. A light neutral oil, available
0003-2700/83/0355-2007$01.50/00 1983 American Chemical Society
