Anal. Chem. 1981, 53, 1627-1632
distribution coefficient relating to size exclusion mechanism (eq 1) overall distribution coefficient on molarity scale partition coefficient on molarity scale parameters given by eq 11 and 29, respectively parameters given by eq 13 and 31, respectively parameters given by eq 17 and 33, respectively gas constanlt ratio of the distance traveled by sample to that of Blue Dextran 2000 hydrated radius of species i. parameter defined by eq 14 absolute temperature activity coefficient of species i on molarity scale valency of ion i (negative for anion) valencies of cation and anion, respectively chemical potential of species i (eq 4) chemical potential of species i in standard state (eq 4) change in chemical potential
SlnlbaMi, M.; Corradini, D. J. Chromafogr. 1978, 148, 553-559. Ogata, T.; Yoza, N.; Ohashi, S. J. Chromatogr. 1971, 58, 267-276. Yoza, N.; Ogata, T.; Ueno, Y.; Ohashl, S. J. Chromatogr. 1971, 61, 295-305. Tarutani, T.; Watanabe, M. J. Chromatogr. 1973, 75, 169-172. Ackers, G. K. Biochemistry 1984, 3, 723-730. Aizawa, M.; Suzuki, S. Bull. Chem. SOC.Jpn. 1971, 44, 2967-2971. Aizawa, M.; Mizuguchi, J.; Suzuki, S.;Hayashi, S.; Suzuki, T.; Mitomo, N.; Toyama, H. Bull. Chem. SOC. Jpn. 1972, 45, 3031-3034. Alzawa, M.; Suzukl, S.; Suzuki, T.; Toyama, H. Bull. Chem. Soc. Jpn. 1973, 46, 116-119. Janado, M.; Yano, Y.; Kawamwi, H.; Nlshkla, T. J. Chromarogr. 1980, 193, 345-358. Heitz, W. Ber. Bunsenges. phys. Chem. 1973, 77, 210-217. Heltz, W. 2.Anal. Chem. 1975, 277, 323-333. Deguchi, T. J. Chromatogr. 1975, 108, 409-414. Deguchi, T.; Hisanaga, A.; Nagai, H. J. Chromatogr. 1977, 133, 173-179. Yamamoto, Y.; Tarumoto, T.; Iwamoto, E. Anal. Chim. Acta 1973, 64, 1-8. Shibukawa, M.; Ohta, N. Chromatogrephla 1980, 73, 531-537. Neddermeyer, P. A.; Rogers, L. 0. Anal. Chem. 1988, 40, 755-762. Shibukawa, M.; Oguma, K.; Kuroda, R. Talanfa 1977, 24, 699-700. Martin, A. J. P.; Synge, R. L. M. Biochem. J. 1941, 35, 1358-1368. Shibukawa, M.: Oauma. K.: Kuroda. R. J. Chromafwr. 1978.. 166.. 245-252. Porath, J. Pure Appl. Chem. 1983, 6 , 233-244. Davies, C.W. J. Chem. SOC. 1938, 2093-2098. Yamamoto, Y.; Sumimura, E.; Mlyoshi, K.; Tominaga, T. Anal. Chlm. Acta 1973, 64, 225-233.
-
LITERATURE CITED (1) Yoza, N. J . Chromafoq. 1973, 86, 325-349 (general review). (2) Yoza, N.; Ohashi, S. J . Chromatogr. 1989, 47, 429-437. (3) Ueno, Y.; Yoza, N.; Oshashl, S. J. Chromatogr. 1970, 52, 321-327.
1627
-
RECEIVED for review January 27,1981. Accepted May 18,1981.
Sample Enrichment for Determination of Chlorinated Pesticides in Water antd Soil by Chromatographic Extraction Fillppo Mangani, Giaincarlo Crescentlni, and Fabrizlo Bruner * Istituto di Scienze Chlmic)he, Unlversits dl Urblno, Plazza Rlnasclmento, 6-6 1029 Urbino, Ita&
The use of Carbopack B columns to recover chlorinated pesticides from water Is discussed In terms of recovery and elution curves. The same principles apply for the extraction of chlorinated pesticides from soli. The method presented uses small solvent volumes and saves time.
The use of adsorbents such as Tenax GC and Carbopack B for the enrichment of water samples prior to the analysis of their chloropesticide content has been described in several reports and papers (1-5). Pesticide extraction from soil samples using elution with appropriate solvent mixtures has also been reported (6). Such methods show some advantages both on the usual solvent extraction using a separatory funnel for water and on Soxhlet extraction for soil. In this paper a detailed investigation on the possibilities of using chromatographic techniques for the extraction of pesticides from water and soil samples is reported. It has to be noted that, although the principles governing the adsorption and extraction process in water analysis and the extraction in soil analysis are the same as those that govern liquid-solid chr,omatography, the main feature of a chromatographic column, i.e., separation efficiency, is almost coimpletely absent. Thus, the “columns” used for the extraction should be regarded rather as an extraction apparatus than actual chromatographic column,. For water analysis, a short column is packed with Carbopa.ck B, a very well-known graphitized carbon black. This has its main application as a solid support for gas-liquid-solid chromatography (7). This material is also 0003-2700/81/0353-1627$01.25/0
used to make traps for sample enrichment in air pollution analysis (8). Water is allowed to pass through the Carbopack B column and the compounds of interest are adsorbed on the graphitized surface. Afterward, an appropriate solvent mixture is introduced into the column, to elute the pesticides from the graphitized carbon black. For soil analysis, a column is packed with soil according to the procedure described later. The soil behaves as an adsorbent that retains pesticides on its surface. These are eluted by a solvent mixture, that should be chosen for appropriate polarity characteristics. The two elution processes differ in the nature of the adsorbents and also in the story of the adsorption process. In fact, pesticides might have been spread over the soil several years before the analysis. The successive action of watering modifies the original adsorption energy distribution, making available deeper and stronger adsorption sites in the structure of the material. Several solvents or solvent mixtures have been tested in order to find the one with the best extracting power for the highest number of compounds. The pesticides investigated are structurally complex. EXPERIMENTAL SECTION Preparation of Extraction Columns. For water analysis Carbopack B (Supelco Inc. Bellefonte, PA) previously sieved to 60-80 mesh was extracted by Soxhlet for 6 h with a petroleum ether-toluene 2:l solution and then dried and packed gently in a glass column of 1cm i.d. X 15 cm. One gram of Carbopack B was used. Two glass wool plugs kept the adsorbent in place. The upper part of the column was connected via a ground glass joint to a separatory funnel containing the water to be analyzed. A 0 1981 American Chemlcal Society
1628
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
Table I. Pesticide Concentration in Artificiallv Polluted Soil (ng/g) and Water (ng/mL) soil water or-hexachlorocyclohexane (a-BHC) lindane p-hexachlorocyclohexane (8-BHC) heptachlor aldrin endosulfan I
15 20 17 20 40 15 p,p‘-dichlorodiphenyltrichloroethylene 41 (P,P’-DDW dieldrin 40 o,p’-dichlorodiphenyldichloroethane 20 (o,p’-DDD) 20
1.5 2.0 1.7 2.0 4.0 1.5 4.1
endrin ~...
80
p,p‘-dichlorodiphenyldichloroethane
57
4.0 2.0 2.0 8.0 5.7
(P.D’-DDD) p,p‘:dichlorodiphen yltrichloroethane @,p’-DDT) metoxichlor
130
13.0
150
15.0
~
slight nitrogen pressure ensured a water flow rate through the column of about 12 mL/min. In the case of soil analysis, a 2.0 cm i.d. X 30 cm column is used. Stones, roots, and other gross impurities were removed, and the soil was reduced to a size between 30 and 60 mesh, according to a procedure followed in the analysis of the Seveso soil for 2,3,7,8-tetrachlorodibenzo-pdioxin (TCDD) (6). Twenty grams of soil was packed into the column between two glass wool plugs. Artificial Pollution of Water and Soil. Bidistilled water was artificially polluted according to the following procedure: A reference solution of pesticides in acetone was added to 1 L of water in a separatory funnel. The concentration of pesticides in the solution was such as to obtain the final concentration in water reported in Table I. The master acetone solution was then added to the water with stirring. Stirring continued for 10 min to ensure complete solution of the pesticides. The choice of acetone as a solvent was important because it has the interesting property of being a solvent for pesticides and at the same time to mix with water in all proportions. Nonpolar solvents, such as benzene or aliphatic hydrocarbons,had to be avoided for this purpose because of their very low solubility in water. This might cause the formation of an emulsion rather than a solution, which could introduce a high degree of uncertainty or lead to erroneus results in the quantitative recovery. Furthermore, since acetone can be added to water without yielding a two-phase system, the volume of the master solution added can be large enough to obtain accurate results. The soil, chosen in an agricultural zone around Urbino, was of fluvial sandy-argillaceous type. Thus, the soil, previously washed with a solution of toluene-acetone 1:l by Soxhlet extraction for 5 h and then dried at 60 OC, was placed in a large glass container and the acetone solution of pesticides added. Additional solvent was added while the soil was gently stirred. The acetone was evaporated at room temperaturewith a procedure similar to that used to coat solid supports with low amounts of a liquid phase in chromatography (9). Elution and Recovery Curves. A master solution containing the same absolute amount, by weight, of pesticides as that dissolved in the volume of artificially polluted water to be extracted was prepared for every solvent mixture to be tested. The same procedure was followed for the solvent mixtures tested for soil. After the pesticides were separated from water on the Carbopack B column by the procedure described above, the solvent mixture was passed through the column at a flow rate of 2 mL/min. One milliliter fractions were collected separately. These fractions were diluted to 10 mL and analyzed by gas chromatography(GC). The resulting chromatogram is compared with the one obtained by using the master solution. From the ratio of the peak area of a certain compound in a fraction to the peak area of the same compound in the master solution, the percent of the total amount recovered in that particular milliliter of eluate was obtained. From the same data the integral curve for every compound was drawn, each value being the sum of the percentage recovered in all 1-mL fractions up to that point, divided by the total amount in the
standard solution. The same procedure was followed with the column containing the artificially polluted soil. In the case of actual soil samples the only way to evaluate complete extraction, Le., the total pesticide content, was to test an aliquot of the soil with prolonged Soxhlet extraction and consider the results obtained as 100%. Apparatus and Reagents. An established procedure, that turned out as extremelyuseful for pesticide analysis (6),was used in cleaning and handling the glassware and in maintaining the purity grade of the solvents. Pesticides were purchased from Supelco Inc. (Bellefonte,PA) and further tested for purity. A glass column, 3 m long, 2 mm i.d. containing Supelcoport coated with 1.5% SP2250 1.95% SP2401 was used for the analyses. The column was maintained at 200 “Cand flow rate was 40 mL/min. This ensured complete separation of most pesticides in the test mixture. High-purity nitrogen was the carrier gas. Measurements were obtained with a DANI (Milan, Italy) Model 3600 dual column gas chromatograph equipped with a frequency modulated @Nielectron capture detector (ECD). The ECD was operated in its linear range in all cases. Peak areas were measured with a Shimadzu Model CRlA integrator. Carbopack B was obtained from Supelco.
+
RESULTS AND DISCUSSION Sample Recovery with Different Solvents from Water. Six solvents or solvent mixture have been examined for the recovery of chlorinated pesticides from water samples previously adsorbed on Carbopack B according to the just described procedure. Benzene was avoided for occupational health reasons. On the other hand, from random tests, no practical difference in the extraction power between benzene and toluene has been found. Also, halogenated hydrocarbons such as methylene chloride have been avoided because of the high response of the ECD to these compounds. In Table I1 the recoveries obtained by using several solvent mixtures are reported together with those obtained by other researchers (column I, ref 4) and with the last results achieved with methylene chloride in the solvent extraction method (column
H). The worst results are obtained with completely nonpolar solvents, i.e., aliphatic hydrocarbons such as n-hexane and petroleum ether. Recovery with such solvents is extremely poor for heptachlor, aldrin, and endosulfan I. The loss of recovery is less dramatic in the case of other compounds. The presence of double bonds or polar groups such as S==O makes the extraction difficult with nonpolar solvents. Better results are obtained with toluene, as might be expected. Results obtained with toluene are comparable to those obtained by using a mixture of n-hexane and ethyl ether (1:l)or ethyl ether alone. However, the best results are obtained with a mixture of petroleum ether-toluene (21). Using only 10 mL of this mixture gives a recovery higher than 90% in all instances. In several cases values very close to a complete recovery are obtained. It is interesting to note that, in general, the recovery is higher with the mixture than when using the two solvents separately. By use of 25 mL of this solvent mixture a recovery very close to 100% is obtained in all cases. This also shows that when water passes through the column the entire content of pesticides is adsorbed on Carbopack B. Thus, the eventual recovery loss should be ascribed only to the elution phase. One should note that a slightly polar solvent, such as ethyl ether, is inadequate, not only for nonpolar or aromatic compounds but also for compounds containing double bonds or polar groups. In some cases the n-hexane ethyl ether mixture ( 4 ) ensures a higher recovery than with the two separate solvents, but in several other cases, such as with p,p’-DDE, dieldrin, endrin, and p,p’-DDD, the recovery is much lower than when using the two solvents separately. In fact, the extraction efficiency depends upon various factors, namely, the structure of the compounds to be recovered, the structure of the ad-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
Table 11. Percent Recovery of Pesticides from Water with the Present Methoda solvent A B C a-hexachlorocyclohexane (a-BHC) lindane p-hexachlorocyclohexane (p-BHC) heptachlor aldrin endosulfan I p,p'-dichlorodiphenyltrichloroethylene Cp,p'-DDE) dieldrin o,p'-dichlorodiphenyldichloroethane (o,p'-DDD) endrin p,p'-dichlorodiphenyldichloroethane(p,p'-DDD)
98 97 98 100 100 100 97 99 95 96 97 100 99
p,p'-dichlorodiphenyltrichloroethane(p,p'-DDT)
91 94 94.5 95 99 98 94 91
91 92 95 99 91
95 91 88 81 66 I8 95 96 13 95 96 81 78
1829
D
E
F
G
H
I
83 81 80 35 62 62 94 90 I8 90 93 90 86
82 80 81 33 61 68 81 85 13 85 89 86 81
95 92 91 90 IO 13 92 94 86 93 95 96 93
95 92 93 91 88 83 91 95 I9 96 91 I5 81
100 98 91 100 100 101 98 104
94 89
3
4
100 99 100 93
90 82 90 80 95 80 86 75
metoxichlor a A = 25 mL of petroleum ether (40-60"C)-toluene ( 2 : l ) ; B = 10 mL of petroleum ether (40-60"C)-toluene ( 2 : l ) ; c = 10 mL. of diethyl ether; D =: 10 mL of n-hexane; E = 10 mL of petroleum ether; F = 10 mL. of toluene; G = 10 mL of n-hexanediethyl ether (1:1);H = 180 mL of methylene chloride in three successive steps in separatory funnel; I = 25 mL of n-hexanediethyl ether (1:l)(from ref 4).
40
20
v1
4
1
2
3
4
5
1
2
5 t(rnin)
Flaure 1. Elution and recovsew curves for some Desticides wkh different solvents on Carbopack B traps: (0)petroleum ether-toluene (2:l): (0) toLene; (0)petroleum ethe; (40-60 O C ) . sorbent, and the structure of the solvent mixture. The best result is obtained with a compromise among all these factors, and this should be kept in mind when selecting a solvent mixture to extract pesticides from an adsorbent. Since both n-hexane rind ethyl ether have almost no similarities in structure with the adsorbent and with the molecules to be extracted, a very gold recovery cannot be expected. The results obtained with the mixture of column A, the one adopted, are fully comparable with those obtained by using a separatory funnel extraction (column H) but the amount of solvent required is about 10 times less. Elution and Recovery Curves in Extraction from Water. It is possible to determine the minimum amount of solvent necessary for an efficient recovery from the elution and recovery curves of different compounds obtained with various solvents. Such curves are also extremely useful in understanding the working mechanism of the various solvents. Figure 1shows some example of elution and recovery curves for three typical pesticides using different solvents. In the ordinate axis, qi is the amount recovered within the portion of solvent indicated; on the abscissa, qt is the total amount of pesticide that has been dissolved in water. In particular, the elution and recovery (curvesare reported for two solvents, toluene and petroleum ether, and for a mixture of the two. The behavior of heptachlor is particularly significant, showing that the extraction power of petroleum ether is very poor and that large amounts of this solvent should be used to obtain acceptable recoveries. By use of toluene a very fast recovery is obtained, with an elut,ion curve presenting its maximum at about 2 mL, and a recovery of 10% at 3 mL.
However, the elution curve is tailing in the right part, The overall recovery is thus incomplete. Observing the elution curve obtained using the toluene-petroleum ether mixture, one can note that its shape is more symmetrical than that obtained with other solvents, although the retention time is longer. This results in a larger overall recovery using only 10 mL of the mixture. It seems that the importance of petroleum ether is related to its ability of removing heptachlor from the relatively few active sites for which toluene is inactive. Further, the elution curve of heptachlor using petroleum ether as the mobile phase shows that the ehromatographic process is, in this case, extremely poor, yielding a very broad curve that cannot be considered as a chromatographic peak. This can be regarded as an extreme case of the wrong choice of a solvent when used alone. In spite of this, petroleum ether, used together with toluene, contributes to a higher recovery, as shown by the integral curve. The elution and recovery curves for lindane represent an intermediate case, with the elution curve from petroleum ether shaping as a chromatographic peak. The overall recovery at 10 mL is the same obtained in the case of heptachlor. This shows that in some cases, when using a solvent mixture, it is of little importance whether the elution curves of the single components represent a correct chromatographicbehavior, the important parameter being the removal operated by one of the two solvents over the pesticide molecules adsorbed. Finally, the behavior of p,p'-DDT, in which the curves of the two solvebts are almost symmetrical, represents the very best solution to the problem. As a result, 100% recovery is obtained and the elution curve becomes higher, narrower, and
1630
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
tv
20
1.O
Figure 2. Elution and recovery curves for an actually polluted soil (0)and an artlflclally polluted soil (0)using acetone-toluene (1:l).
symmetrical, approaching the shape of a chromatographic peak. It is appropriate to point out that the full success of the toluene-petroleum ether mixture for p,p'-DDT is due to the structure of this compound, that has both an aromatic and aliphatic character. A further consideration to be made is that it is always possible to find a solvent mixture that is able to ensure a complete recovery for a certain compound with a minimum amount of eluate. However, the solvent mixture chosen should be the one that ensures the best compromise for all compounds, as was pointed out before. With 10 mL of the petroleum ether-toluene mixture, the recovery is within the margin of error, the same obtained with the separatory funnel extraction method in most cases. However, with the latter method, three consecutive extractions of 1L of water with 60 mL of methylene chloride are required. Thus, at the end of the extraction procedure, the sample is dissolved in a solvent volume almost 20 times greater. Thus evaporation of the solvent is needed prior to analysis or purification. This is an undesirable step in many instances. Recovery from Soil. Several solvents have been tested for pesticide recovery from soil. The recovery tests were carried out on soil artificially polluted. However, as outlined in the introduction of this paper, artificially polluted soil differs to some extent from the actual soil in the way pesticides are adsorbed. However, since the comparison among the various solvents is on a relative basis, this situation does not affect the importance of the recovery tests. Table I11 reports the recoveries obtained. The values in column D refer to those obtained from the current literature using the accepted Soxhlet extraction method. It is important to keep in mind that these values refer to the overall method, which includes the cleanup procedures. This may account for a loss of recovery somewhat higher than in our method and can be estimated as ranging around 2 % The ethyl ether-petroleum ether mixture (column C) yields results that are comparable to the n-hexane-acetone mixture (column H), used in Soxhlet extraction. The latter behaves in a very similar way to the petroleum ether-acetone mixture, as might be expected, due to the analogous structure on nhexane and the hydrocarbons present in petroleum ether. The best results are given by the toluene-acetone mixture (1:l) which has been adopted in the present method. Furthermore, the recovery power of this mixture is higher than that obtained with the single components, values very close to 100% recovery being obtained in all cases. In order to check different adsorption of pesticides on artificially and actually polluted soil, elution and recovery curves have been drawn for the pesticides present in an actual soil sample, using a toluene-acetone mixture (1:l). These are
.
I II
Figure 3. Comparison between direct elution and Soxhlet extractlon: (a) direct extract, (b) Soxhlet extract, (c) Soxhlet extraction of the soil after dlrect elutlon; (1) pp'-DDE, (2) pp'-DDT, (3) metoxichlor.
reported in Figure 2. Although the overall recovery using 25 mL of solvent for the extraction is the same in both instances, a very significant difference is observed in the elution curves. These tail to a greater degree in the naturally polluted soil than in the artificially polluted soil, This shows definitely that pesticides can be adsorbed on higher energy sites due to the porous structure of the material. Longer time is required by the organic molecules to be occluded with the pores of the soil. It is interesting to note that petroleum ether alone gives poor results with soil and most pesticides are recovered only to a small degree using 25 mL. This behavior is due to the nature of the soil, that acts mainly as a specific porous adsorbent. Petroleum ether, a nonpolar solvent, does not show the extraction capability toward the pesticides adsorbed on specific active sites. However, it has been shown that the same solvent is able to extract the compounds adsorbed on Carbopack B to a greater extent. This is to be ascribed to the different nature of this material which behaves as a nonspecific, nonporous adsorbent. The problem of the specific nature of adsorption on soil has been overcome by using acetone, which is polar enough to remove the pesticides from the specific active sites of soil. Toluene acts in intermediate way. For the sake of comparison, in Figure 3 three chromatograms are reported. A sample of soil was divided into two portions. Chromatogram a was obtained by injecting the solution resulting from the present method, chromatogram
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
1631
Table 111. Recovery of Pesticides from Soil with This Method Using Different Solvents Compared with the Soxhlet Extraction Method' (Column D ) A B C D E F G H a-hexachlorocyclohexane ( a-BHC) lindane p-hexachlorocyclohexane (p-BHC) heptachlor aldrin endosulfan I p,p'-dichlorodiphenyltrichloroethylene (p,p'-DDE)
dieldrin
o,p'-dichlorodiphexyldichloroetharie (o,p'-DDD)
endrin p,p'-dichlorodiphenyldichloroethane(p,p'-DDD) p,p'-dichlorodiphenyltrichloroethaae(p,p'-DDT)
metoxichlor
84 85 85 86 81 91 82
86 85 87 90 76 88 87
84 86 86 82 83 87
24 23 54 10 32
88
83.1
96 90 89 88 92 87
81 93 91 84 89 90
93 86 95 87 88 85
88.2
85.2
0
91.2 94.2
3.5
87 89 93 85 97 96 94
99 99 100 101 100
98 102
92 90 92 91 94 92
0 0 0 0 10
98 99
102 99 102 100
85 86 85 92 77 89 85 88 88 89 90 91 89
a A = 25 mL of toluene; B = 25 mL of petroleum ether-acetone (1:l); C = 25 mL of diethyl ether-petroleum ether (4060 "C) (1:1); D = 65 mL of acetone-hexane (1:l)in Soxhlet; E = 1 0 mL of petroleum ether (40-60 "C); F = 1 0 mL of acetone; G = 25 mL of toluene-acetone (1:l);H = 25 mL of hexane-acetone (1:l). -
100
z z$oo 80
60
40
20
v (mi) ...-I
I
1
2
I
3
t
I
I
4
5
1
2
3
4
Flaure 4. Elution and recovery curves for two1 uesticides wlth different solvents on artificially polluted sol: (0) acetone;
(171);
(m) toluene.
b by injecting the one from Soxhlet extraction, and chromatogram c by injecting the solution obtained after Soxhlet extraction of the soil which has been previously treated with the present method. This shows that our method ensures an extraction is as complete as the one obtained by using Soxhlet. The peak in chromatogramb just before peak no. 3 has not been identified. However, being absent in chromatogram c, it cannot be ascribed to a compound that is not extracted with our method. Since the area of peak no. 3 in chromatogram a is equal to the sum of the areas of' peak no. 3 and the unknown, it is possible that some side effects happen in the Soxhlet extraction. Elution and Recovery Curves for Soil. The elution curves, reported in Figure 4,clearly indicate the effects of the two solvents. Acetone shows a peak of shorter retention time than toluene and this means a high extraction power for most of the active sites. However, after 6 mL, the recovery reaches a steady state. Toluene gives a poor recovery when used first, and the average retentilon is higher. However, it turns out to be very useful in recovering the last traces. By comparing the two adsorbents, it turns out that the nonpolar character of petroleum ether is exploited in the extraction from the strong nonspecific active sites of carbon black. On the other hand, the polar character of acetone makes the extraction from the highly strong specific active sites of the siliceous material of the soil possible. The retention time obtained with the mixture is intermediate between those
(mi")
5
(e)acetone-toluene
of the two separate solvents, and the recovery, in turn, is higher. The results of this paper indicate that the proper choice of the solvent mixture plays a very important role on the size of the final volume of solution in which the pesticides are collected. For water analysis, a recovery similar to the best obtained by Lagan6 et al. (4) in their extensive work on this topic is obtained by using only one-fifth of the solvent volume. Using 25 mL of solvent mixture, Lagan&et al. obtained a much lower recovery (compare columns A and I in Table 11). Where soil extraction is concerned, 25 mL of the solvent mixture ensures better recoveries than those obtained with the current methods. Another advantage of the present method which should be pointed out is that the time required for sample concentration is short, 5 min in the case of soil extraction, without the necessity of solvent evaporation. It should also be pointed out that in routine work, when many samples must be treated within the same day, the advantage of a substantial volume reduction is extremely important for both safety and economic reasons. ACKNOWLEDGMENT The authors thank A. R. Mastrogidcomo for outstanding technical assistance. LITERATURE CITED (1) Leoni, V.; Puccetti, G.; Colombo, R.; D'Ovidio, A. M. J. Chromtogr. 1978, 125, 390-405.
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
