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Anal. Chem. 1980, 52, 1157-1158
Table 111. (Continued)
-
silica-R(NH, ) * (6% methylene chloride ref. no. t o Figure 2
20.
21.
22. 23. 24. 25. -
hydrocarbon benzo[ b Ifluoranthene benzo[k Ifluoranthene perylene difluorenyl benzo[ b Ichrysene anthanthrene rubicene benzo [ghilperylene di benz [ a,h ]anthracene indeno[ 1,2,3-c,d]pyrene dibenz [ a , c ]anthracene picene coronene dibenzo [ e , h ] pyrene dibenzo[a,h]pyrene
aromatic hydrocarbons on this column were calculated from experimentally-determined retention volumes (Table 111) as previously described by Popl et al. (7). T o evaluate the effect of alkyl and phenyl substitution on the retention of fused-ring aromatics, representative derivatives have been included in the silica-R(NH,), column testing. Values of the appropriate retention indices (Table 111) corresponding to decreasing or increasing retentions due to alkyl and phenyl substitution of ring structures are included. Methyl substituents contributed insignificantly to changes in the retention of two-, three-, and four-ring types studied. Only n-decyl substitution of pyrene decreased the retention to any great extent. I n all cases phenyl-substituted hydrocarbons were retained more than parent ring structures. Table I11 also presents the retention indices for alkyl and phenyl substitutions calculated from earlier published results ( 3 ) and additional measurements on the silica-CI8 column. Increments in the retention indices are considerably higher than those corresponding to differences in selective behaviors on the silica-diamine column. I n the reversed-phase system, alkyl derivatives of PAHs overlap PAHs of higher ring number. The normal-phase diamine-based system studied here appears to offer better ring-size selectivity because alkyl PAHs elute near the parent PAHs. Furthermore, separation based on the number of condensed rings was achieved up to 4-ring aromatics. Higher condensed ring structures did not follow the increasing ring-number sequence. In contrast to the reversed-phase system (silica-CI8 and acetonitrile-water (80:20) as eluent). in this normal phase system phenyl-substituted structures tend not t o increase the parent aromatic retention to such a
in n-heptane) log I 4.435 4.435 4.559 4.852 5.000 >5
silica-C,, ( 3 ) (230% acetonitrile in water) ___ log I 4.376 4.461 4.404 3.954 5.000
-
>5 >5 >5
4.955 4.766
>5 >5 >5
-
>5
-
-
>5
-
>5
degree. Retentions of phenyl derivatives in the normal-phase diamine-bonded system seem to be more dependent on molecule structural factors than on the number of x electrons or the number of molecular carbons.
CONCLUSIONS PAH separations achieved on the diamine sorhent are based on the number of fused aromatic rings up to 4. It differentiates this sorbent from classical liquid chromatography sorbents as silicas and aluminas offering selectivities based on general chemical affinities. It also differentiates the diamine-bonded phase from the octadecyl-bonded phases. The silica-R(NH2)2 sorbent, because of its retentive properties, seems to be superior to other polar bonded phases studied for PAH chromatography. I t is very promising for the separation and characterization of PAH mixtures of various origins. Studies on developing separations schemes for PAH fractions from fossil fuel products are presently being performed. LITERATURE CITED Wise, S. A.; Chesler, S. N.; Hertz, H. S . ; Hilpert, L. R.; May, W. E. Anal. Chem. 1977, 4 9 , 2306-2310. Thomas, R.; Zander, M. Erdol Kohle Erdgas Petrochem. 1977, 3 0 , 403-405. Chmielowlec, J.; Sawatzky, H. J . Chromatogr. Sci. 1979, 17, 245-252. Thomas, R. S.: Lao, R. C.; Wang, D. T.; Robinson, D.; Sakuma, T. "Carcinogenesis, Volume 3: Polynuclear Aromatic Hydrocarbons", Jones, P. W., Freudenthal, R. I . , Eds.; Raven Press: New York, 1978. Ogan, K.; Katz, E.; Salvin, W. Anal. Chem. 1979, 51, 1315-1320. Dark, W. A.; McFagden, W. H. J . chromatogr. Sci. 1978, 16, 289-293. Popl, M.; Dolansky, V.; Mostecky, J. J . Chromatogr. 1976, 177. 117-127.
RECEIVED for review September 14, 1979. Accepted January 29, 1980.
Determination of Ethyl Sulfate by Reversed-Phase Ion Pair Chromatography Grace Chiu' Monsanto Chemical Intermediates Company, P.O. Box 12830, Pensacola, Florida 32575
Almost all of the existing methods for the determination of ethyl sulfate ions are indirect in nature. They involve the conversion of ethyl sulfate to sulfate ( I ) ,the determination of the hydrolysis products ( I ) ,the determination of the cation
' Permanent address: Department of C h e m i s t q , rniversit? of West Florida, Pensacola, Fla. 32504. 0003-2700/80/0352-1157$01 O O / O
of the salt via ion-exchange ( 2 ) ,or indirect spectrophotometry ( 3 ) . A direct infrared method was reported by Abe et al. ( 4 ) ; but the method was not very sensitive, the limit of detection being 10% sodium ethyl sulfate in detergent powders. This paper describes a direct and sensitive method for the determination of ethyl sulfate by rek'ersed-phase ion Pair chromatography. C 1980 American Chemical Society
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Anal. Chem. 1980, 52, 1158-1161
The sample was eluted with a 0.010 M solution of tetra-n-butylammonium perchlorate (TBAClOJ in water/methanol (70/30 v/v) at a flow rate of 1.8 mL/min at room temperature. A calibration curve of peak areas vs. ethyl sulfate concentration was constructed using 1O-pL samples of aqueous solutions containing from 0.1 to 1.0% sodium ethyl sulfate. A straight line was obtained.
I
I 0
2
4
E
8
IO
12
Time (min) Figure 1. Chromatogram for a BQAES sample containing 1.02% ethyl sulfate, 11 % sulfate, and 2 7 % phosphate
EXPERIMENTAL Apparatus. A Waters liquid chromatograph equipped with a Model M-6000A pump, a F-Bondapak CIS column (4 mm i.d. X 30 cm), a differential refractometer, and a Rheodyne Model 7120 injector was used. Chromatographic peak areas were integrated using a Hewlett-Packard Model 3354C Lab Data System. Reagents. Methanol, distilled-in-glass grade, was obtained from Burdick and Jackson Laboratories, Inc. Deionized water further purified by the Barnstead Nanopure System was used. Tetra-n-butylammonium perchlorate was supplied by the G. F. Smith Company. Sodium ethyl sulfate, from Pfaltz and Bauer Company, was recrystallized from water-ethanol and dried over concentrated sulfuric acid ( 5 ) . The product was assayed by ignition to sodium sulfate (6). Samples of bis(dibutylethy1ammonium)hexane-l,6-bis(ethylsulfate) (BQAES), containing from 0.1 to 25% ethyl sulfate in the presence of about 30% of phosphoric acid and various amounts of sulfate, were used. BQAES is a long-chain quaternary ammonium salt of the ethyl sulfate anion. Procedure. An accurately weighed sample of BQAES, from 0.1 to 0.5 g, was diluted to 5 mL with water in a volumetric flask. Ten microliters of the diluted sample was injected into the column.
RESULTS AND DISCUSSION Ethyl sulfate was successfully separated from the sulfate and phosphate present in the BQAES samples. The retention times for phosphate, ethyl sulfate, and sulfate were 2.8, 3.8, and 4.8 min, respectively. A typical chromatogram is shown in Figure 1. Over 50 samples were analyzed. T h e relative standard deviation of t h e method was within 1 2 % for ethyl sulfate of concentration greater than 1'70. For samples containing less than 1%ethyl sulfate, the precision was within &5% relative. T h e detection limit was around 0.1% ethyl sulfate in the presence of u p to 20% sulfate. This amounts to 10 pg of ethyl sulfate. The accuracy of this method was checked against an indirect spectrophotometric method ( 3 )using BQAES samples. The agreement was within 2% relative. T h e optimum concentration of the TBAC104 solution for the chromatographic elution was found to be 0.010 10.002 M. Outside this concentration range, the ethyl sulfate and sulfate peaks were not completely resolved in the presence of high concentrations of sulfate. The effect of various diluents for the samples on the separation was studied. No difference in the chromatograms, other than the change in sign of the solvent peak, was found when water, water/methanol (70/30 ./vi, or the eluent itself was used to dilute the samples. This method should find applications in the determination of ethyl sulfate in a wide variety of samples, regardless of the size of the cations present. As was shown in this study, the concentration of the counterion used in the mobile phase was sufficient to swamp even the relatively large cation in BQAES to effect the elution of ethyl sulfate. ACKNOWLEDGMENT I thank J. N. Maloney, Jr., D. R. Senn, and N. H. Watkins for helpful discussions. LITERATURE CITED (1) Calhoun. G. M.; Burwell, R . L., Jr. J . Am. Chem. Sac. 1955, 77, 6441-6447. (2) Kurz, J. L. J . Phys. Chem. 1962, 66, 2239-2245. (3) Sheffield, W. M. Unpublished work, Monsanto Chemical Intermediates Company, Pensacola, Fla., 1974. (4) Abe, K.; Tanirnori, S.;Hashirnoto, S. Bunseki Kagaku, 1966, 15, 1364- 1368. (5) Nguyen-Quang-Trinh, Compf Rend 1946, 222, 897-898 (6) Burwell, R L , Jr J . Am Chem Soc 1949, 7 1 , 1769-1771
RECEIVED for review December 20. 1979. Accepted February 13, 1980.
Minimizing Relative Error in the Preparation of Standard Solutions by Judicious Choice of Volumetric Glassware R. 6. Lam and T. L. Isenhour" Department of Chemistry, Universify of North Carolina, Chapel Hill, North Carolina
Numerous textbooks on analytical chemistry discuss the practical details concerning determinate and indeterminate errors which may arise during the manipulation of volumetric glassware ( I , 2 ) . Other textbooks ( 3 , 4 )present tables familiar 0003-2700/80/0352-1158$01 .OO/O
to nearly all analytical chemists, regarding the relative error in different volume pipets and flasks. These tables are usually condensed from the volumetric tolerances published by the National Bureau of Standards ( 5 ) . ?? 1980 American Chemical Society
