15) . . W. H. Henry, E.P.A. Contract No. 68-02-0447, Battelle Memorial Institute, Columbus, OH, 1972. (6) R. E. Lee, Jr., S.S.Goranson, R. E. Ehrione, and G. B. Morgan, fnviron. Sci. Techno/., 6, 1025 (1972). (7) N. L. Morrow and R. S. Brief, Environ. Sci. Technol., 5, 786 (1971). (8) U S . Public Health Service, "Air Quality Data from the National Air Sampling Networks and Contribution State and Local Networks", 1966 ed., Durham, NC, 1988. (9) D. F. S. Natusch. J. R. Wallace, and C. A. Evans, Jr., Science, 183, 202 (1974). (10) J. R. Rhodes, A. H. Prodzynski, and R. D. Sieberg, lnstrum. Soc. Am. Trans., 11, 337 (1972) (11) P. Grennfelt, A. Akerstrom, and C. Brosse, Atmos. fnviron., 5, 1 (1971). (12) J. W. Cares, Am. lnd. Hygiene Assoc. J, 29, 463 (1968). 113\ -, J. - R. Rhodes. Amer. Lab.. 5. 57 (19731. (14) J. R. Rhodes, A . ~ H .Prodzynski; C. B. Hunter, J. S. Payne, and J. L. Lundgren, fnviron. Sci. Technol., 6, 922 (1972). (15) D. Gray, D. M. McKown, M. Kay, M. Eichor, and J. R. Vogt. /.€.€.E. Trans. Nucl. Sci., 19, 194 (1972). (16) W. H. Zoller and G. E. Gordon, Anal. Chem., 42, 256 (1970). (17) R . Dams, J. A. Robbins, K. A. Rahn, and J. W. Winchester, Anal. Chem. 42, 861 (1970). (18) S. S. Brar and D. M. Nelson, "Modern Trends in Activation Analysis", NBS Spec. Pub/., 312, 43 (1969). (19) D. H. Peirson, P. A. Cawse, L. Salmon, and R. S. Cambray, Nature. 241, 252, (1973). (20) G. L. Hoffman, R. A. Duce, and W. H. Zoller, fnviron. Sci. Techno/., 3 , . 1207 (1969). (21) M. E. Hoschler, E. L. Kanabrocki, C. E. Moore, and D. M. Hattori, Appl. Spectrosc., 27, 185 (1973). (22) C. D. Burnham, C. E. Moore, E. Kanabrocki, and D. M. Hattori, fnviron. Sci. Technol., 3, 472 (1969).
(23) P. W. West, "Chemical Analysis of Inorganic Pollutants", in "Air Pollution", Vol. II, A. C. Stern, Ed., Academic Press, New York, 1969, p 172. (24) T. Y. Komitani, J. L. Bove, B. Nathanson, S.Siebenberg. and M. Magyar, Environ. Sci. Techno/.. 6, 617 (1972). (25) J. Y. Hwang and F. L. Feldman, Appl. Spectrosc., 24, 371 (1970). (26) R. J. Thompson, G. B. Morgan, and L. J. Purdue, At. Absorption News/., 9, 53 (1970). (27) S.H. Omang, Anal. Chim. Acta, 55, 439 (1971). (28) S.P. Matousak and K. G. Brodie, Anal. Chem., 45, 1606 (1973). (29) H. P. Loffin, C. M. Christian, and J. W. Robinson, Spectrosc. Lett., 3, 161 (1970). (30) R. Woodriff and J. F. Lech, AnalCbem., 44, 1323 (1972). (31) S.A. Clyburn. T. Kantor, and C. Veillon, Anal. Chem., 46, 2213 (1974). (32) W. C. McCrone and J. G. Delly, "The Particle Atlas, VoI. 11: The Light Microscopy Atlas", 2nd ed.. Ann Arbor Science Publishers, Ann Arbor, MI, 1973, p 472. (33) 9. C. Begnoche, M.S. Thesis, The Pennsylvania State University, 1974.
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RECEIVEDfor review December 13, 1974. Accepted February 21, 1975. The authors are grateful to The Center for Air Environment Studies of The Pennsylvania State University for financial support during the period of this research. This irivestigation'was supported by Research Grant No. R 800397, Grants Administration Division, Environmental Protection Agency, administered through the Center for Air Environment Studies of The Pennsylvania State University.
Rapid Determination of Lead in Gasoline by Atomic Absorption Spectrometry in the Nitrous Oxide-Hydrogen Flame R. J. Lukasiewicz, P. H. Berens, and B. E. Buell Research Laboratories, Union Oil Company of California, Brea, CA
9262 1
The nitrous oxide-hydrogen flame provides efficient combustion for a wide range of organic solvents. Effects of gasoline composition on the determination of lead by atomic absorption are minimized in this flame. Direct aspiration of gasoline is practical and is thus the basis for a rapid determination of low levels of lead. An in situ reaction of alkyl lead with iodine and complexation with a liquid ion exchanger levels response for all alkyl types. The effects of iodine and liquid ion exchanger concentration upon lead measurement are studied. Lead can be determined accurately from trace levels up to 0.10 gram/US gallon by simple addition of an excess of iodine and ion exchanger followed by measurement vs. standards which contain a complex of lead chloride in gasoline. Higher concentrations are measured with greater accuracy if a dilution is used. The limit of detection for lead in gasoline is 0.0001 g/gal. using this procedure.
Determination of lead in gasoline by atomic absorption spectrometry was first reported by Robinson ( I ) . A limitation of the method was variation in atomic absorption response with the chemical form of lead in solution. Inorganic lead standards could not be used ( I , 2 ) to determine tetraalkyl lead in gasoline. In addition, response was found to be a function of lead alkyl type (3, 4 ) . The sensitivity for tetramethyl lead was 2.5-fold greater than tetraethyl lead. Effects of the composition of gasoline ( 5 ) ,the diluent sol-
vent used (2, 6), and position of the flame viewed by the detector ( 4 , 5 ) were complex and the subject of additional studies. The difficulties associated with the determination of alkyl lead in gasoline were in part overcome by careful choice of solvent and control of sample aspiration rate ( 3 ) . A procedure based upon reaction of alkyl lead in petroleum products with aqueous iodine monochloride and subsequent back extraction into methyl isobutyl ketone has been reported (7). Lead was measured in the MIBK phase by atomic absorption. This procedure, however, is quite lengthy and requires careful control of solution parameters. Kashiki et al. (8) proposed an in situ reaction of alkyl lead in gasoline with iodine, which included a 50-fold dilution with methyl isobutyl ketone and measurement in the airacetylene flame. This procedure eliminated the problem of variations in response due to different alkyl types by leveling the response of all alkyl lead compounds. The method proposed by Kashiki was modified (9) by the addition of the liquid ion exchanger, tricapryl methyl ammonium chloride. Use of the ion exchange material is reported to improve response and increase stability of the alkyl lead iodide complex (9). With minor modification, the latter method was adopted by the American Society for Testing Materials as the standard method for the determination of lead in gasoline (IO). All of the methods outlined above require a dilution of gasoline with an appropriate solvent prior to measurement. Dilution compensates for severe non-atomic absorption and scatter from unburned carbonANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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containing species. Matrix effects caused in part by the burning characteristics of gasoline ( 4 ) were also minimized by dilution. We wish to report the use of the nitrous oxide-hydrogen flame for determination of lead in gasoline. Use of this flame allows direct aspiration of gasoline with virtually no non-atomic absorption a t the 2833-A lead resonance line. Effects of gasoline composition are practically eliminated by virtue of the efficient combustion process. Lead can be determined by simple treatment of gasoline with an excess of iodine and liquid ion exchanger, prior to measurement vs. standards containing lead chloride in unleaded gasoline. This procedure provides a rapid direct measurement of lead in gasoline from trace levels to 0.10 g/US gal. At lead levels above 0.10 glgal., a dilution with unleaded gasoline is recommended due to nonlinearity in the analytical working curve a t high concentrations and possible complications in the iodine conversion reaction. EXPERIMENTAL Apparatus. Measurements were made using a Perkin-Elmer Model 306 atomic absorption spectrophotometer. Light source was a Westinghouse Model WL-36039 lead hollow cathode lamp operated a t 8 mA. The lead resonance line a t 2833 A was used and slit width was set so that the spectral band pass was approximately 2 A. A 5-cm slot type titanium burner was positioned about 5 mm below the center of the hollow cathode beam image. A fuel lean nitrous oxide-hydrogen flame was used for all measurements. Nitrous oxide flow rate was set a t 9.7 l./min, and hydrogen flow rate was a t 5.3 l./min. Uptake rate of gasoline was set a t 5.2 mlimin by adjusting the nebulizer. Reagents. Tricapryl methyl ammonium chloride (Aliquat 336) was obtained from General Mills Chemicals, Inc., Minneapolis, MN, as a 90% aqueous-alcoholic solution. Unleaded gasoline (Union Oil Co. of California, Los Angeles, CA) was used as the solvent for preparation of standard solutions. Procedure. A standard stock solution which contained 5.0 g lead per gallon (1.32 g/l.) was prepared by dissolving the appropriate amount of dried reagent grade lead chloride in a solvent composed of 10% Aliquat 336 solution, described above, and 90% methyl isobutyl ketone (IO).Working standards were prepared by diluting the stock solution with unleaded gasoline. Trace lead content of the unleaded gasoline was determined by measuring the absorbance of the gasoline a t two wavelengths: the lead resonance line a t 2833 A, and the close non-absorbing lead line a t 2802 A, under expanded scale conditions. Using the absorbance measured a t 2833 A for a 5 mg/gal. lead solution, and applying a small correction for non-atomic absorption as measured above, it was determined that lead content of the unleaded gasoline was less than 0.0001 g/gal. Prior to final measurement, an estimate of lead content in gasoline was made by direct comparison of samples to standards containing tetraethyl lead in unleaded gasoline. Samples above 0.10 g/gal. lead were diluted so that final measurements could be made in the linear working range. Lead response was very nearly linear to 0.10 g/gal. in gasoline, which corresponds to 26 mg/l. Approximately 25 ml of gasoline sample was poured directly into a calibrated 50-ml glass stoppered Erlenmeyer flask. By means of a micropipet, 0.5 ml of 3% (w/v) iodine in toluene was added to the gasoline. The sample was allowed to react for about 5 minutes and then 0.5 ml of 50% (v/v) Aliquat 336 in MIBK was added. Samples were measured by direct comparison to standards containing lead chloride in unleaded gasoline.
RESULTS AND DISCUSSION Use of t h e Nitrous Oxide-Hydrogen Flame with Organic Solvents. The analytical utility of the nitrous oxidehydrogen flame for atomic absorption and emission spectrometry has been studied rather extensively in aqueous systems ( 1 1 , 12). Relatively poor atomization of refractory elements has prevented widespread use of this flame. However, the degree of atomization for some of the more easily atomized elements such as lead, copper, iron, and magnesium equals or exceeds the degree of atomization for such elements in other commonly used flames ( 1 1 ) . For the current application, the nitrous oxide-hydrogen flame is suit1048
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
able and has been found to provide advantages when used with organic solvents. Very few reports appear in the literature (23) describing the use of this flame with organic solvent systems. We have determined, however, that the nitrous oxide-hydrogen flame provides distinct advantages over other flames for the direct atomization of selected elements in organic solvents, In particular, substantial improvements in ease of use and in the flame background absorption over other flames are realized. Spraying gasoline into this flame causes little expansion of the primary reaction zone and no luminosity. No discernible secondary cone is formed in the nitrous oxide-hydrogen flame regardless of the hydrogen flow rate or the gasoline uptake rate. Thus, the optimum fuel-to-oxidant flow ratio and solvent uptake rates are easily obtained. Formation of Cz, CH, and CN species above the primary reaction zone is minimal with gasoline being sprayed into the flame. Nitrous oxide flames generally have a higher burning velocity than the air-acetylene flame (12, 14). However, the nitrous oxide-hydrogen flame is much safer from flashbacks than the nitrous oxide-acetylene flame and does not require the use of air in lighting and extinguishing (12, 13). The air-acetylene flame is less desirable for this application because introduction of gasoline and other organic solvents into the flame results in bright luminosity and strong absorption due to large concentrations of incandescent carbon. This effect can be greatly reduced by minimizing the fuel-to-oxidant ratio and by reducing the solvent uptake rate; however, sensitivity is also significantly reduced. Similar, but greatly reduced effects, are also observed when gasoline h aspirated into the nitrous oxide-acetylene flame. Luminosity can be completely eliminated by operating this flame under extremely fuel lean conditions. The use of either the air- or nitrous oxide-acetylene flames requires critical attention to flame parameters and interferences caused by variation in the composition of gasolines are more likely to occur than with the nitrous oxide-hydrogen flame. It is perhaps more appropriate to describe the combustion system existing while gasoline is being aspirated into the nitrous oxide-hydrogen flame as a nitrous oxide-gasoline-hydrogen flame. Gasoline contributes sufficient fuel to sustain combustion even in the absence of hydrogen. Indeed, absorbance for lead in gasoline increases slightly if the hydrogen fuel is shut off while gasoline is being aspirated into the flame. This suggests that hydrogen plays a minimal role in the combustion process and, in fact, may act solely as a diluent. For the sake of simplicity, the flame described above will be referred to as a nitrous oxide-hydrogen flame below. A simple spectrometric examination of some of the prominent species produced in the flame while gasoline was being aspirated was undertaken by observing emitted specTable I . Effect of Solvent on Absorbance of Lead in the Nitrous Oxide-Hydrogen Flame Absorbance of Solvent
solvent x io3 Sensitivitya
X Difference*
GasolineC Benzene Special naphtholited Methyl isobutyl ketone Nonene a
...
