Anal. Chem. 1981, 53, 1632-1636
1632
(2) Bruner, F.; Crescentini, G.; Mangani, F. Paper presented at the XIV International Symposium "Advances in Chromatography", Lausanne, Switzerland, Sept 24-28, 1979, unpublished. (3) Bruner, F.; Crescentini, G.; Mangani, F. Paper presented at the Third meeting of the "Serione Marchigiana" of the Itailan Chemical Society, Urbino, Italy, April 27, 1979; Chirn. Id.(Mllan) 1979, 9 , 695. (4) Bacaioni, A.; Goretti, G.; Lagan& A,; Petronio, B. M.; Rotatori, M. Anal. Chern. 1980, 52, 2033-2036. (5) Bruner, F.; Crescentini, G.; Mangani, F. Paper presented at the I1 National Congress of Analytical Chemistry of the Italian Chemical Socia ty, Padova, Oct 2-5, 1979. Proceedings of the Congress pp 146-148. Socletil Chimica Itallana, Rome, 1980. (6) Bertoni, G.; Brocco, D.;Di Palo, V.; Liberti, A.; Possanzini, M.; Bruner, F. Anal. Chem., 1978. 50, 732-735, and references therein.
(7) Dl Corcia, A.; Liberti, A. Adv. Chromalogr. 1976, 14, 305-307, and references therein. (8) Bruner, F.; Bertoni, G.; Crescentini, G. J . Chromatogr. 1978, 167,
- - .- . .
399-4[37 -
(9) Bruner, F.; Ciccioli, P.; Crescentini, G.; Plstoiesi, M. Anal. Chem. 1973, 45, 1851-1859.
RECEIVED for review February 25, 1981. Accepted May 12, 1981. This research has been partially supported by the Commission of the European Community under Contract No. 214-77-ENV 1-1.
Determination of Tetraalkyllead Compounds in Gasoline by Liquid Chromatography-Atomic Absorption Spectrometry J. D. Messman"' and T. C. Rains Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
A llquld chromatography-atomlc absorption spectrometry (LC-AAS) hybrid analytlcal technique Is presented for metal speclatlon measurements on complex llquld samples. The versatlllty and inherent metal selectlvlty of the technique are Illustrated by the rapld determlnatlon of flve tetraalkyllead compounds In comrnerclal gasoline. Separatlon of the Indlvldual tetraalkyllead species is achleved by reversed-phase llquld chromatography uslng an acetonltrlle/water moblle phase. The effluent from the liquid chromatograph is introduced dlrectly Into the aspiration uptake capillary of the nebulizer of an airlacetylene flame atomic absorptlon spectrometer. Spectral interferences due to coelutlng hydrocarbon matrix constltuents were not observed at the 283.3-nm resonance llne of lead used for analysls. Detectlon llmlts of this LC-AAS hydrld analytical technlque, based on a 20-bL injection, are approximately 10 ng Pb for each tetraalkyllead compound.
Antiknock fluids containing tetraalkyllead (TAL) compounds (see Table I) have been added to commercial gasoline in the United States since 1960 to improve the octane rating of the gasoline. Such additives containing variable TAL composition could be fitted to specific gasoline base stocks for maximum antiknock effectiveness in internal-combustion engines where the gasoline may not be evenly distributed among the cylinders (1,Z). However, analysts in the petroleum industry were confronted with the formidable task of developing rapid and reliable tetraalkyllead speciation techniques for control of refinery blending processes in the production of motor antiknock fluids containing variable TAL composition. The development of such techniques is also extremely important for hygienic concerns because of the toxicological behavior of tetraalkyllead compounds and their potential impact on the environment (2-5). Some type of chromatographic separation-detection scheme has generally been invoked for the determination of individual TAL compounds in gasoline. Parker et al. (6) initially sepal Present address: U.S. Geological Survey, 923 National Center, Reston, VA 22092.
Table I. Tetzkyllead Compounds in Gasoline
lead dimethyldiethyllead methyltriethyllead tetraethyllead
TML TMEL DMDEL METL TEL
(CH,),Pb (CH,),(C,H,)Pb (CH3)2(C2H5)2Pb (CH,)( C,H, ),Pb (C,H,),Pb
rated all five TAL compounds by isothermal gas chromatography (GC), collected them individually in methanolic iodide scrubbers as they eluted from the column, and then measured the total lead content in each fraction by a dithizone spectrophotometric procedure. This lengthy and complex procedure was improved by gas chromatographic techniques which incorporated on-line electron capture (7-9), catalytic hydrogenation prederivatization flame ionization (10-12), and hot-wire thermal conductivity (12) detection systems. However, interferences due to coeluting gasoline matrix constituents frequently plaqued the unambiguous detection of all five TAL compounds using such coventional detectors. The severity of the interferences prompted analysts to investigate metal-selective detection systems for the gas chromatography procedure in which only lead-containing compounds would be detected. Atomic absorption spectrometry (AAS) (13-201, flame photometry (21), microwave plasma emission wavelength modulation (221, and hydrogen atmosphere flame ionization (23) techniques have been successfully applied as metal-selective GC detection systems for the determination of TAL compounds in gasoline. Although modern liquid chromatography (LC) is an attractive alternative to gas chromatography for many analytical separations, the LC technique has been used only sparingly for the separation of TAL compounds in gasoline. Liquid chromatography has been applied to the andysis of gasoline for a single TAL compound using flame AAS detection (24) and conventional molecular spectrophotometric detection at 254 nm (25). The LC technique has also been used in conjunction with Zeeman-effect AAS for the speciation of tetramethyllead (TML) and tetraethyllead (TEL) in a gasoline standard reference material (26). Although a standard mixture of five TAL compounds has been separated by LC using Zeeman-effect AAS detection (27),the LC technique has not been applied to the direct speciation of all five TAL cam-
This article not subject to US. Copyright. Published 1981 by the American Chemical Society
)bLJLN
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
pounds in a complex gasoline matrix. This paper reports on such an application in which the TAL compounds are determined directly by flame AAS as they elute from the LC. The versatility and inherent metal selectivity of LC-AAS as demonstrated in this paper permit simple optimization of the LC operating parameters. Only separation of the TAL compounds from each other must be accomplished because the AAS detector can be set to monitor only lead-containing compounds even in the presence of coeluting gasoline matrix constituents.
EXPERIMENTAL SECTION Apparatus. A Series 2 liquid chromatograph (The PerkinElmer Corp., Norwalk, CT) equipped with a Rheodyne Series 7105 sample injector was used in this study. A reversed-phase300 mm X 3.9 mm i.d. pBondapaE;CIS column (Waters Associates, Inc., Milford, MA) was used to separate the individual tetraalkyllead compounds. The mobile phase contained 70% acetonitrile and 30% water. The mobile phase flow rate was maintained at 3.0 mL/min through the anlaytical column which was operated at ambient temperature. The sample injector was modified slightly to accommodate a nominal 2O-pL sample loop to permit complete-loop filling. The sample loop was loaded with a 50-pL syringe. Atomic absorption measurements of the LC effluent were performed with a Model 5000 line-source atomic absorption spectrometer (The Perkini-ElmerCorp., Norwalk, CT) equipped with a conventional deuterium arc background corrector. A lead hollow cathode lamp (The Perkin-ElmerCorp., Norwalk, CT) waa used as the primary light fiource. Lead measurements were made at the 283.3-nm resonance line using a 0.7-nm spectral band-pass. An &/acetylene flame formed on a 10-cmsingle slot burner head of a permix burner assembly served as the atom reservoir. The flame gas flow rates were adjusted to give stoichiometric flame conditions when nebulizing the LC effluent. The stainless steel tubing from the analytical column and the aspiration uptake capillary of the atomic absorption nebulizer were connected by a short piece of small-diameter polyethylene tubing. An epoxy resin seal which was applied to both ends of the polyethylenetubing interface remained intact for several days before weakening due to continuous contact with the acetonitrile/water mobile phase. This necessitated a fresh layer of epoxy resin to be applied to re-form the seal. The AAS aspiration uptake rate was adjusted to be slightly less than the LC column flow rate so that compatibilitybetween the two could be achieved without creating a postcolumn reduced pressure region. The LC column flow rate and the AAS aspiration uptake rate used in this study provided optimum resolution of the five TAL compounds on the pBondapak CIScolumn. For some studies the effluent from the LC was first directed through an 8-pL microcell of an ultraviolet/visible variablewavelength Model LC-551B absorbance spectrophotometer (The Perkin-Elmer Corp., Norwalk, CT) and then into the flame AA spectrometer. Separate electronic integrators (M-2 Computing Integrator, The Perkin-Elmer Corp., Norwalk, CT, and Autolab Minigrator, Spectra Physics, Inc., Bedford, MA) were connected to the phototube output of each detection system to compare data on retention time and peak area reproducibilities. Simultaneous tracings of chromatograms from both detectors were feasible by connecting the output of each individual electronic integrator to a separate strip-chart recorder (Honeywell,Inc., Fort Washington, PA). Reagents and Materials. Spectrograde acetonitrile (Eastman Kodak Co., Rochester, NY)l and distilled-demineralized water were used for the LC mobile phase. Tetramethyllead, tetraethyllead, and tetraalkyllead motor antiknock additive MLA500 Dilute (Ethyl Corp., Ferndale, MI) were used as received from the manufacturer for calibration standards. Appropriate dilutions of thase standard solutions were prepared in spectrograde acetonitrile. Commercial gasoline samples obtained from gasoline stations in the local area were stored in tightly capped glass bottles. Ampules of Lead in ReferenceFuel, Standard Reference Materid 1638 (NationalBureau of Standards, Washington,DC), were kept sealed until just prior to analysis. Appropriate dilutions of the
1633
DMDEL UV A B S MTEL ORBANCE
TML
1 t
INJECT
TEL
TML
TEL
lMEL
h
t,,,crh
MINUTES
Flgure 1. Separation of tetraalkyllead comounds in gasoline antiknock additive MLA500 Dilute by reversed-phase LC: 0.18 pg of TML (0.14 pg of Pb), 0.77 pg of TMEL (0.56 pg of Pb), 1.20 pg of DMDEL (0.84 Mg of Pb), 0.84 pg of MTEL (0.56 pg of Pb), and 0.22 pg of E L (0.14 pg of Pb). Analysis conditions are described in the text.
gasoline materials were prepared in “Distilledin Glass” methylene chloride (Burdickand Jackson Laboratories,Inc., Muskegon, MI). Methylene chloride was chosen as the diluent because of its miscibility with tetraalkyllead standard solutions, gasoline, reference fuel, and the mobile phase. All standard solutions and unknown samples were refrigerated in the dark when not in use to minimize evaporation and photodecomposition of the tetraalkyllead compounds.
RESULTS AND DISCUSSION The potential advantages of the LC-AAS hybrid analytical system over LC with conventional ultraviolet (UV) absorbance detection were investigated in the determination of tetraalkyllead compounds in a gasoline antiknock additive and in commercial gasoline. The effluent from the LC-UV system was introduced directly into the nebulizer of the flame AAS detection system to allow direct comparison of their selectivity and sensitivity. Retention time and peak area reproducibilities were also compared for both systems. Figure 1shows a comparison of chromatograms for the determination of TAL compounds in gasoline antiknock additive MLA500 Dilute which has a theoretical TAL composition of 16% by weight. The TAL distribution (in mole percent) as provided by the manufacturer is nominally 6% TML, 25% trimethylethyllead (TMEL), 38% dimethyldiethyllead (DMDEL), 25% methyltriethyllead (MTEL), and 6% TEL. Even in such a relatively simple matrix, the metal selectivity of the LC-AAS system is demonstrated. Whereas only TAL compounds are monitored at the 283.3-nm analysis line with the LC-AAS system, an additional component of MLA500 Dilute (undoubtedly xylene which is the predominant diluent) absorbs at the 254-nm wavelength with the L C - W system. With W detection the diluent peak is shown to elute just prior to tetramethyllead and could possibly preclude the accurate determination of this tetraalkyllead species. An attractive feature of the LC-AAS system compared to LC-UV for this particular analysis is the relatively greater convenience in calibration for quantitative purposes. A well-documented problem in the flame AAS determination of total lead in gasoline concerns appropriate calibration when more than one tetraalkyllead compound is present in the sample. This is a direct result of the dependence of lead sensitivity (that is, slope of the analytical curve) on the TAL compound. Numerous methods (28) to make the various tetraalkyllead responses more uniform have been reported. Careful control of aspiration variables, proper choice of organic diluent, and, more recently, addition of iodine in either methyl isobutyl ketone or methyl ethyl ketone to convert all tetraalkyllead forms to a single form have been studied. However, this dependence of the atomic absorption signal on the chemical form of lead is not observed with our LC-AAS
1634
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981 UV ABSORBANCE DETECTION ( 2 5 4 nrn)
FLAME AAS DETECTION ( 2 8 3 . 3 nm)
il
W
INJECT
DMQEL TMEL
INJECT MINUTES
& I 4
Figure 2. Separation of tetraalkyllead compounds In leaded gasoline by reversed-phase LC. Analysis conditions are described in the text.
system. Presumably the acetonitrile/ water solvent mixture, nebulization rate, burner design, and flame type, stoichiometry, and observation area used in this study all combine to successfully alleviate this problem and permit a single TAL compound to be used for calibration for all the TAL compounds in the sample. However, a significant variation of the molar absorptivities of the individual TAL compounds, especially for TML, at the 254-nm wavelength complicates quantitation in LC-UV. Calibration with a single TAL compound would require application of appropriate predetermined response factors for each TAL compound present in the sample. The other alternative would be to calibrate with standards which contain each TAL compound that is present in the unknown sample. Chromatograms obtained with the UV detector set at alternate wavelengths in the 210-270-nm wavelength region showed no significant improvement in relative sensitivity for the TAL compounds. In fact, based on integrated peak areas of a mixture containing 0.98 pug of TML (0.76 pg of Pb) and 1.59 pg of TEL (1.02 pg of Pb), the AAS responses (pg of Pb/unit area) at the 283.3-nm analysis line for TML and TEL were within 5% of each other; however, the UV response (pg of TAL/unit area) at 254 nm for TEL was about 6.3 times greater than that for TML. The analytical utility of LC-AAS is more clearly demonstrated in Figure 2 which illustrates the separation and detection of the five TAL species in commercial leaded 89-octane-number gasoline diluted 5-fold with methylene chloride. The LC-UV chromatogram is of little practical value because of the strong absorption at 254 nm of coeluting unsaturated and aromatic hydrocarbons in the gasoline matrix which completely obscures the TML and TMEL peaks. Accurate quantitation of the remaining TAL components (DMDEL, MTEL, and TEL) is considerably limited even though the five TAL compounds themselves are completely resolved. The LC-UV chromatogram in Figure 2 illustrates optimum analytical performance because a new column was used for the separation. When older and less efficient analytical columns were used severe tailing of olefinic and aromatic gasoline hydrocarbons obscured completely all five TAL species. Because resolution of the five TAL species themselves was much less affected when older columns were used, performance of the LC-AAS system was less dependent on the condition of the reversed-phase column.
The LC-AAS chromatogram shows complete base line resolution of the five TAL species which permits accurate peak height or peak area measurements for quantitative purposes. The entire analytical determinationwas accomplished in about 4 min. Nonspecific broad band absorption or scatter signals were not observed using the LC-AAS system as a result of the excellent combustion properties of the gasoline matrix and suitable flame conditionsused to maximize dissociation of the hydrocarbon matrix. Even had some broad band absorption occurred, simultaneous compensation could be automatically accomplished with the deuterium arc technique commonly employed in conventional atomic absorption analyses. Leaded gasoline samples from various refineries were analyzed by LC-AAS. In all cases each of the five TAL species was detected although the relative concentrations varied from sample to sample. The concentrations of TAL compounds in “unleaded” gasoline samples were below the detection limits using the flame AAS detection system in all but one commercial brand. For this particular “unleaded” gasoline sample, peaks for four TAL compounds were just detectable above the base line noise; no peak was observed for TML in this sample. Lead in Reference Fuel (NBS-SRM 1638)was also analyzed by LC-AAS to ascertain the relative proportions of TML and TEL present. This 91-octane-number reference fuel is a mixture of 91% by volume 2,2,4-trimethylpentane and 9% by volume n-heptane. Lead is added in the form of a TMLTEL mix of unspecified proportions. However, at 20 “C,the total lead content is certified a t 1.94 g of Pb/gallon or 513 pg of Pb/mL. Twenty microliters of this Standard Reference Material (SRM) was injected into the liquid chromatograph for analysis. A small peak eluted at a retention time characteristic of TML and a very large peak eluted at a retention time characteristic of TEL. Addition of quantities of TML and TEL individually to the SRM confirmed these peaks as TML and TEL. Integrated areas of the two peaks indicated that at least 95% of the lead present in the reference fuel at the time of analysis was in the form of TEL. This is in complete agreement with Koizumi et al. (26) who analyzed the SRM by LC-Zeeman AAS. However, this is not conclusive evidence that the lead was originally added in these same proportions of TML and TEL because some photodecomposition, volatilization, or conversion of TML to TEL could possibly have occurred even though the SRM had been stored in the dark until use. The possible interchange of methyl and ethyl radicals on the c18 reversed-phase column was investigated since this alkyl conversion phenomenon had been reported with gas chromatography when certain stationary phases were employed. When a 1:l mixture of TML and TEL was injected onto the C18 reversed-phase column, no intermediate TAL peaks were detected. Moreover, the sensitivities of TML and TEL in a TML-TEL mixture were the same as those of TML and TEI, individually. This suggests that interchange of methyl and ethyl radicals on the reversed-phase column was negligible for the chromatographic parameters used. Retention time reproducibility (relative standard deviation of the mean of six determinations) of the five tetraalkyllead compounds in MLA5OO Dilute as shown in Table I1 was in the 0.3-1.0% range. The total TAL mass injected was 3.21 pg of TAL (2.24 pg of Pb). The lead content present in the different TAL forms ranged from 0.1 to 0.9 pg of Pb. The retention time reproducibility for each of the TAL species remained the same even when the total lead mass was reduced to 0.9 pg of Pb. Table I1 also illustrates the relative reproducibility of peak area measurements with both detection systems. Peak area integrations were generally more precise with the AAS de-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
1835
Table 11. Comparison of AAS and UV Absorbance Detectors for the LC Separation of Tetraalkyllead Compounds in Gasoline Antiknock Additive MLASOO Dilute
TAL TML TMEL DMDEL MTEL TEL total a
Ng of Pb
injected 0.14 0.56 0.84 0.56 0.14
retention times RSD, %(“ AAS~ UVC 0.6 1.0 0.8 0.6 0.6
0.8 0.5 0.4
0.3 0.6
2.24
Six replicate measurements.
283.3 nm.
peak heights RSD, %‘ AAS~ 4.9 4.4 4.2 4.3 8.2
uvc
peak areas
RSD, %‘ AAS
uvc
11.1 4.6 4.5 5.3 6.1
41.9 7.0 5.6 6.4 12.7
4.8
6.0
254 nm.
tection system. Except for TML, the relative standard deviation of the mean peak area of lead in each tetraalkyl form, based on six measurements, was on the order of 5%. The poorer precision for TML may be partially due to its rapid elution from the column which limits accurate peak area quantitation because of the more narrow widths of TML peaks. Using the UV detection system, peak area reproducibility of the three intermediate TAL compounds (TMEL, DMDEL, and MTEL) was about 6%; however, precision of the TEL areas was considerably worse and reproducibleTML peak areas were impossible to achieve. The poor precision of TEL may be partially attributed to peak broadening but, more importantly, to its greater sensitivity to base line drift since it is the last TAL compound to elute. Even with isocratic elution, significant basia line drift occurred with the UV detection system due to background absorption. Base line drift was not observed with the AAS detection system even when gradient elution was attempted. The difficulty encountered in making reproducible UV measurements of TML can be largely attributed to the low mass being measured. Taking into account the relativlely low molar absorptivity of TML at 254 nm, 0.2 pg of TML is simply too low to be reliably determined with the UV detector. However, its corresponding mass of lead (0.14 pg of Pb) can be determined much more reliably with the AAS (detection system, Detection limits for lead in the various tetraalkyl forms using the LC-AAS system described were calculated as that concentration of lead which produced a peak whose height was approximately three-fifths of the peak-to-peak baseline noise. On the basis of a 20-bL injection of gasoline antiknock additive MLAW Dilute, the detection limit for lead was about 10 ng for each tetraalkyl species. Such detection limits were determined under LC-PAS conditions that were required for optimum resolution of the five TAL compounds. The signal-to-noise ratio (SNR)of the LC-AAS sy&m may be slightly improved by operating the pneumatic nebulizer of the AAS burner in a nonaspiration orientation so that a slight backpressure is formed on the LC column. Such an improved SNR is reportedly due to improved droplet characteristics and transport efficiencies during nebulization of the LC effluent (29)
I
Calibration curves for the five tetraalkyllead compounds using the LC-AAS system were constructed from MLA500 Dilute in amounts which contained 1.1-11 pug of total lead. Linear calibration was achieved over this total lead concentration range for each tetraalkyllead species. Such a linear dynamic range makes the LC-AAS system analytically useful for the determination of TAL compounds in commercial leaded gasoline. Moreover, the unity response factors for each TAL compound permit a single TAL compound to be used for calibration for all of the TAL compounds in the sample. A final study was conducted to evaluate the relative merits of peak height and peak measurements for means of quantitation in LC-AAS. A 20-pL portion of gasoline antiknock
additive MLA5OO Dilute was injected so that 2.2 pg of total lead was introduced onto the column. The quantity of lead in the different tetraalkyl forms was in the 0.1-0.9 pg range. It is shown in Table I1 that peak height measurements generally are as precise as the corresponding electronic integration of peak areas. However, broadening of the TEL peak degrades its reliable quantitation by the peak height method because of the added uncertainty in fiiding the peak height maximum. This is much less a problem in peak integration. Conversely, peak height measurement has a decided advantage over peak area measurement for the determination of TML. Because of its rapid elution from the column, and extremely narrow TML peak is reproducibly obtained. However, the extremely narrow peak shape of TML makes it difficult to precisely integrate the area beneath it. Quantitation by either method is greatly simplified by the inherently “clean” LC-AAS chromatogram because a stable base line and base line resolution of all TAL species greatly minimize measurement errors. Although specific organolead compounds were the only lead species studied, the versatility of AAS detection and an appropriate LC separation scheme would also permit ionic and particulate lead species to be conventiently measured in the samples. However, the LC-UV technique is limited to the determination of only those lead species which contain a suitable chromophore. The enhanced metal selectivity of the LC-AAS system minimizes the effort to optimize chromatographic parameters because the only concern is to resolve the analytical compounds containing the metal of interest. Nonanalyte compounds will go undetected even if they coelute with the metallo compounds of interest. Moreover, the need for high-purity solvents for the chromatography is much less critical and alleviates the need of “blanking out” solvents which contain organic impurities that strongly absorb. With a metal-selective detection system such as AAS, the LC mobile phase must only be free of the metal of interest in the organometallicanalysis. In addition to better metal selectivity over conventional UV detection, atomic absorption spectrometry provides better sensitivity. The only major disadvantage of a metal-selective detector such as AAS is that it is destructive in nature. However, this is seldom of practical concern because the effluent can be split, if desired, before it reaches the atomizer, The LC-AAS system offers analytical advantages over other hybrid techniques for metal speciation measurements such as GC-AAS because it allows the direct determination of individual organometallic compounds which are nonvolatile and thermally labile. Speed, resolution, and the amenability to diverse liquid sample solutes including ionic species make LC-AAS a logical and versatile metal speciation technique. Several instrumental improvements can be made in the existing LC-AAS system which should extend the analytical capability of the hybrid technique. A more permanent and durable interface is needed. More fundamentally, however, a need exists to improve the sensitivity of the system by
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
designing an atom reservoir in the AAS detector which is more efficient than the combustion flame in the present system. The use of electrothermal atomization may improve the detection limits of the LC-AAS technique by more than an order of magnitude. Studies involving discrete sampling (microliter quantities) of LC effluents with graphite furnace atomic absorption detection have recently been reported (26,27,30,31) which demonstrate the viability of this approach. The LC-AAS hybrid technique may be easily extended to metal speciation analyses other than the test application presented here. The technique is currently being employed in this laboratory for metal speciation measurements on waste lubricating oils and on leachates of potentially hazardous wastes. Unfortunately, the technique is presently limited to the analysis of liquid samples. Metal speciation measurements on solid samples using LC-AAS would require utilization of selective extraction or ion-exchange schemes which would reliably isolate the different chemical forms of the metal without destroying the integrity of the sample. Clearly, the formidable chemical and instrumental challenges of metal speciation measurementson solid samples will be the emphasis of future research.
ACKNOWLEDGMENT The authors express their gratitude to Michael H. Thomas and the Perkin-Elmer Corp. for loan of the LC instrumentation. We also thank the Ethyl Corp. and D. C. Reamer of the Department of Agriculture for supplying the tetraalkyllead compounds and W. E. May and S. A. Wise of the National Bureau of Standards for their suggestions and helpful discussions. LITERATURE CITED (1) Shaplro, H.; Frey, F. W. ”The Organic Compounds of Lead”; Intersclence: New York, 1968 Chapter 17. (2) Frey, F. W.; Shapiro, H. Top. Curr. Chem. 1971, 76, 243-297. (3) Mahaffey. K. R. EHP, Envlron. Heallh Perspect. 1977, 19, 265-295. (4) Committee on Biological Effects of Atmospherlc Pollutants “Lead: Alrborne Lead In Perspecthre”; National Academy of Sciences: Wash lngton, DC, 1972 Chapter 8. (5) Posner, H. S.; Damstra, T. “The Biogeochemistry of Lead In the EnvC ronment, Part B. Blologlcal Effects”; Nrlagu, J. O., Ed., Elsevier/NotthHolland Blomedlcal Press: Amsterdam, 1976; Chapter 15. (8) Parker, W. W.; Smlth, G. 2.; Hudson, R. L. Anal. Chem. 1981, 33, 1170-1 17 1. (7) Dawson, H. J. Anal. Chem. 1983, 35, 542-545.
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Barrail, E. M.; Balllnger, P. R. J. Gas Chromatagr. 1983, 7, 7-13. Bonelli, E. J.; Hartmann, H. Anal. Chem. 1983, 35, 1980-1961. Soulages, N. L. Anal. Chem. 1988, 38, 28-33. Souiages, N. L. Anal. Chem. 1987, 39, 1340-1341. Soulages, N. L. J. Gas Chromatogr. 1988, 6 , 356-360. Koib, B.; Kemmner, G.; Schleser, F. H.; Wiedeking, E. Fresenlus Z. Anal. Chem. 1988, 227, 166-175. Segar, D. A. Anal. Lett. 1974, 7 , 69-95. Coker, D. T. Anal. Chem. 1975, 47, 386-369. Robinson, J. W.; Vldaurreta, L. E.; Wolcott, D. K.; Goodbread, J. P.; Kiesel, E. Specfrosc. Lett. 1975, 8 , 491-507. Chau, Y. K.; Wong, P. T. S.; Cbulden, P. D. Anal. Chlm. Acta 1978, 85, 421-424. Robinson, J. W.; Klesel, E. L.; Godbread, J. P.; Bliss, R.; Marshall, R. Anal. Chlm. Acta 1977, 92, 321-328. Bye, R.; Paus, P. E.; Solberg, R.; Thomassen, Y. At. Abs. Newslett. 1978. 77. 131-134. Radziuk, B.; Th&ssen, Y.; Van Loon, J. c.; C ~ U Y. , K. Anal. Chlm. Acta 1979. 105. 255-262. Mutsaars, P. M.;Van Steen, J. E. J . Inst. Pet. 1972, 56, 102-107. Reamer, D. C.; Zoller, W. H.; O’Haver, T. C. Anal. Chem. 1978, 50, 1449-1453. DuPuis, M. D.; Hill, H. H. Anal. Chem. 1979, 51, 292-295. Botre, C.; Cacace, F.; Cozzanl, R. Anal. Lett. 1978, 9 , 825-830. Ruo, T. C. S.; Seiucky, M. L.; Strausz, 0. P. Anal. Chem. 1977, 49, 1781-1 765. Koizumi, H.; McLaughlln, R. D.; Hadelshi, T. Anal. Chem. 1979, 51, 367-392. Vickrey. T. M.; Howell, H. E.; Harrison, G. V.; Ramelow, G. J. Anal. Chem. 1980, 52, 1743-1746. Keyworth, D. A. Flame Emlsslon and Atomic Absorption Spectrometry”; Dean, J. A., Ralns,T. C., Eds., Marcel Dekker: New York, 1975; Vol. 111 Chapter 18. Koropchak, J. A.; Coleman, G. N. Anal. Chem. 1980, 5 2 , 1252-1255. .-. - .-. -. Brlnckman, F. E.; Blair, W. R; Jewett, K. L.; Iverson, W. P. J. C h m a fWr. SCl. 1977. 15. 493-503. Brhckman, F. E:; Jewett, K. L.; Iverson, W. P.; Irgoiic, K. J.; Ehrhardt, K. C.; Stockton, R. A. J. Chromatagr. 1980, 197, 31-46.
RECEIVED for review February 17,1981. Accepted May 4,1981. To adequately describe materials and experimental procedures, it is occasionally necessary to identify commercial products by manufacturer’s name or label. In no instance does such identification imply endorsement by the National Bureau of Standards nor does it imply that the particular products or equipment is necessarily the best available for that purpose. This work was presented in part at the 179th National Meeting of the American Chemical Society, Houston, TX, March 23-28, 1980, and at the 7th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, Sept 28-0ct 3,1980.
