spectra. The gift of Temazepam and Nordiazepam from J. A. F. de Silva, Hoffmann La Roche Inc., as well as I4C oxazepam and N1,03-dimethyloxazepam from Hans W. Ruelius and K*Agersborg’ Jr” wyeth are gratefdy acknowledged.
LITERATURE CITED D. M. Halley, J. chromatogr., 98, 527 (1974). M. A. Brooks and J. A. F. de Silva, Talanta, 22, 849 (1975). J. M. Clifford and W. Franklln Smyth, Ana/yst(London),99, 241 (1974). J. A. F. de Silva, M. A. Schwarz, V. Stefanovic, J. Kaplan, and L. DArconte, Anal. Chem., 36, 2099 (1964). (5) J. A. F. de Silva and C. V. Pugllsl, Anal. Chem., 42, 1725 (1975). (6) J. A. F. de Sihra, I. Behersky, C. V. Pugllsl. M. A. Brooks, and R. E. Welnfeld, Anal. Chem., 48, 10 (1976). (7) J. Vessman, S.Stromberg, and G. Ritz, Acta pharm. Suec. 7 , 363 (1970). (8) A. Frlgerio, K. M. Baker, and G. Bebedere, Anal. Chem., 45, 1846 (1973). (9) H. Ehrsson and A. Tilly, Anal. Lett., 6, 197 (1973).
(1) (2) (3) (4)
(10) J. A. F. de Silva, C. V. Pugllsl, and N. Munno, J. fharm. Sci., 63, 520 ( 1974). (11) M. Ervlk and K. Gustavli, Anal. Chem., 46, 39 (1974). (12) R. Modln and G. Schlll, Acta fharm. Suec., 4, 301 (1967). (13) H. Ehrsson, Acta fharm. Suec., 8, 113 (1971). (14) A. Brandstrom, “Preparative Ion Pair Extraction”, Apotekarsocleteten and Hassle Lakemedel, Stockholm, 1974, p 93. (15) J. Benett, W. Franklin Smyth, and I. E. DavMson, J . Pharm. Pharmacol., 25, 387 (1973). (16) R. 8. Hagel and E. M. Debesls, Anal. Chim. Acta, 78, 439 (1975). (17) J. Vessman, G. Freij, and S. Stromberg, Acta pharm. Suec., 9, 447 (1972). (18) T. Nordgren and R. Modin, Acta fharm. Suec., 12, 407 (1975). (19) H. Ehrsson, Anal. Chem., 46, 922 (1974). (20) M. Wretllnd, A. Pllbrant, A. Sundwall, and J. Vessman, “The Pharmacoklnetlc Profile of Oxazepam”, Acta Pharmacol. Toxicol. Suppl., 28 (1977).
for review February 11, 1977* Accepted June 13, 1977.
Ion-Exchangers for Gas-Solid Chromatography Roland F. Hirsch”’ Chemistry Department, Seton Hall University, South Orange, New Jersey 07079
Courtenay S. G. Phllllps Merton College, Oxford, England
Lightly-sulfonated porous polymers are efflclent and selective packlngs for gas-solid chromatography. They are easy to prepare and do not show the talled peaks observed when conventional macroreticuiar ion-exchange reslns are used in GSC. The degree of sulfonatlon can be varled, and wlth It the extent of speciflclty for compounds formlng complexes wlth the metal counterlon on the packlng. Reactlons catalyzed by the packlngs were also observed.
Ion-exchange materials are finding increased use in gas chromatography because they offer a great range of potential selectivity through variation of the ionic form of material (1-5). Two difficulties have prevented more widespread use of ion exchangers in GC: their chromatographic separation efficiencies are poor, and they are such strong adsorbants that many substances cannot be eluted without raising the temperature above the decompostion point of the ion exchanger or of the sample. One solution to these problems has been to prepare bound-monolayer cation exchangers, in which the ion-exchange group is covalently bound to a porous silica support (5). Impressive separations of cis-trans isomers of olefins were obtained with these packings. However, their synthesis is time-consuming and requires special handling techniques for some of the reagents, and the matrix must be deactivated by silanization after the ion exchanger has been prepared. We wish to report on the use of lightly-sulfonated porous polymers as ion exchangers for gas-solid chromatography. These materials, which have previously found application in liquid chromatography (6-8), allow efficient separations a t ‘On sabbatical leave at the Inorganic Chemistry Laboratory, Oxford,
1975-76.
moderate temperatures. They are easy to prepare and require no special treatment prior to use. The extent of sulfonation of the polymer determines the degree of enhancement of retention of specifically-adsorbed substances; hence it is possible to tailor the packing to the needs of a particular separation problem by choosing the proper synthesis conditions.
EXPERIMENTAL All chromatographic experiments were carried out in PyeUnicam Series 104 gas chromatographs with flame ionization detectors. Nitrogen was the carrier gas. Glass columns (1/4-inch 0.d.) contained the packing materials. Commercial ion-exchangeresins were washed with 2-propanol, water, 1 M NaOH, 1 M HC1, and water, dried at 110 “C for at least 4 h, and sized into the 40-60 mesh range using standard screens. The resins were converted into the silver form by washing with 1M AgN03, to the sodium form by washing with 1 M NaOH, to the nickel form by washing with 0.2 M Ni(N0J2, and to the cadmium form by washing with 0.5 M CdC12,followed in all cases by washes with water, and drying at 110 “C for at least 4 h. Porapak Q (80-100mesh) was sulfonated by suspending a 10-g portion of the porous polymer in about 50 mL of concentrated sulfuric acid (6, 9). The mixture was swirled vigorously for the prescribed time, and then about 50 mL of 50% aqueous sulfuric acid was added to quench the reaction. The resin was washed with dilute sulfuric acid and water. It was then converted to the desired metal ion forms in the same way as already described for the commercial ion exchangers. The degree of sulfonation was determined by controlling the temperature and duration of the sulfonation step, as discussed below. It was measured by titration of a portion of the washed resin with sodium hydroxide solution. The silver ion content of the silver form was determined by atomic absorption spectrometry of an acid-digested portion of the resin.
RESULTS AND DISCUSSION For use in gas chromatography, an ion-exchange resin must have a permanent pore structure, as there is nothing present ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1549
D
TIME
Flgure 2. Separations of 2-pentene isomers on short columns of sulfonated Porapak Q. ( a )Resin I, 13-cm column, 230 OC. ( b ) Resin 11, 13-cm column, 230 O C . (c)Resin 111, 11-cm column, 170 O C . (A) trans-2-Pentene. (e) cis-2-Pentene. Ag+ form of resins
TIME
Figure 1. Stopped-flow gas chromatography on a commercial ionexchange resin. Ni2+ form of Amberlyst 15: 2,2dimethylbutane, 180 O C , 10 mL/min N2 carrier: ( A ) flow of carrier gas stopped, (6) flow restarted, (C) main peak, ( D ) stopped-flow peak ...!.l - 1........ 1.....!..- - . l - --l..___ _____11 (as in 1:iiquia cnromacograpny using polar soivenw LO ~ w e i i the resin to allow the sample access to the ionic groups inside the beads of resin. The macroporous (or macroreticular) type of resin is therefore necessary for gas chromatography (2). Rohm and Haas Amberlyst 15 macroreticular cation-exchange resin, which was used in a previous study (2), gives a satisfactory range of retention for many samples, but the efficiency (in terms of the height per theoretical plate) is not adequate. On further study of this resin as a GSC packing, it was discovered that a small but distinct tailing is present, regardless of the type of hydrocarbon being tested or the metal ion form of the resin. Other macroporous cation-exchange resins were obtained and tested in their sodium and silver ion forms. Among the resins studied were Rohm and Haas Amberlite 200 and Amberlite 252, Permutit Zerolit 625, and Dow Chemical Dowex MSC-1. All were unsatisfactory in their poor efficiency and lack of freedom from tailing. This tailing was present regardless of the sample size used (over a range from 1ng to greater than 1Mg) and the retention time of the peak did not change until the largest sample sizes were reached. The tailing therefore is not of thermodynamic origin. To prove that it is a kinetic phenomenon, several samples were chromatographed using flow stops before and after elution of the main portion of the sample (10). In all cases, new peaks appeared after each flow stop period on the trailing edge of the main peak, indicating that a slow process had proceeded during the stop, releasing some of the sample for elution. An example is shown in Figure 1. Since this behavior occurred with all samples on all of the resin forms, it was concluded that a chemical reaction was not responsible for the peak after each flow stop. In other words, the stopped-flow peaks correspond to the same chemical substance which was originally injected. Rather, a slow mass transfer process within the stationary phase must be the source of these observations. Although the resins are all of the macroporous type, their internal volume contains small (“micro”) as well as large (“macro”) pores (11). Slow diffusion of sample molecules within the small pores would explain the results obtained with the resin packings. Elimination of the small pores should eliminate this kinetic tailing. Several unsuccessful attempts were made to do so. A sample of Amberlyst 15 was heated in concentrated sulfuric acid a t 110 “C for 1 week in the hope that sulfonation of the relatively small number of unsulfonated aromatic rings would occur under these extreme conditions, making the material I.-
!..
1550
L
L-\
I _
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
more homogeneous and enlarging the small pores. The peak shapes were not significantly improved compared with the original resin. Several experiments were carried out using carrier gas saturated with water or a high-boilingorganicliquid (such as n-butyl benzene) in the hope that the resin would become swollen, opening up the small pores. These experiments were interesting in their own right as they represent a controlled case of competition of two species for an adsorption site, and this work is being pursued. However, the tailing was not entirely eliminated, and the conditions are somewhat awkward for routine use in gas chromatography. It was then decided to take a copolymer of styrene and divinylbenzene (the basis of most ion exchangers) and to sulfonate it lightly. By selecting a porous polymer which gave efficient separations in gas chromatography and modifying it only on the surface of the large pores, it was expected that the efficiency would be retained and tailing prevented, since no small pores would be produced. This has proved to be the case in liquid chromatography on similar materials; such ion exchangers allowed more rapid chromatographic separations than conventional, fully-sulfonated resins (6-8). Three sulfonations were carried out on the porous polymer, Porapak Q. The first (designated I) was carried out for 5 min at 75 “C, and resulted in a resin with an exchange capacity of about 1.5 mmol H+/g dry resin, and 15.5% silver content (1.4 mmol Ag+/g) for the silver ion form. The second (11)was carried out for 2 min at 25 “C, with the product having 0.9 mmol H+/g exchange capacity, and 9.0% silver content (0.8 mmol Ag+/g) after conversion to the silver form. The third (111)was carried out for about 1min at 5 “C, with the product having 0.6 mmol H+/g exchange capacity, and 5.5% silver content (0.5 mmol Ag+/g) after conversion to the silver ion form. These resins gave sharp, symmetrical peaks for a wide variety of saturated and unsaturated hydrocarbons. Efficiencies were good; plate heights were 1mm, with a value of 0.7 mm achieved for cis-2-butene on the silver form of resin I. Typical plate heights on the commercial exchangers, in contrast, were 2-6 mm (2). Tailing was not observed on the new resins, except in situations where the sample was reacting on the column, as described below. Short columns (10-15 cm) could be used to obtain good and rapid separations of isomeric compounds (see Figure 2; each chromatogram required less than 4 rnin). The retention times for saturated hydrocarbons were comparable on all three resins. The unsaturated compounds gave retention times which depended directly on the silver ion content of the resin. The retention indices for these compounds therefore increase with increasing sulfonation, as
separate peaks with retention indices of 620 and 725, respectively (at 140 "C), 1-pentene and the 2-pentenes produced a single peak with a retention index of 700 and considerable tailing and 1-hexene provided a peak with a retention index of only 760, also with substantial tailing. It appeared that the samples were undergoing a reaction catalyzed by the packing material. Stopped-flow experiments with the 2-butenes, shown in Figure 3, confirmed this. The chromatograms show that the initially pure isomers have isomerized to a significant extent while they were stopped in contact with the resin. The larger olefins may well isomerize to the most readily eluted form, providing a single peak with a tail caused by further reaction of the more strongly retained isomers taking place gradually on the column. Cadmium ion is known to catalyze these reactions (12). The partially-sulfonated porous polymer packings are easy to prepare and retain the high separating efficiency of the parent material. They would appear to offer a great deal of flexibility in meeting the selectivity requirements of a specific sample. Further work is under way to determine more completely the advantages and limitations of these packings in gas chromatography.
Table I. Retention of Unsaturated Compounds on Sulfonated Porapak Q Kovats Retention Index on Resin I Resin I11 (15.5% Ag) Resin I1 (9% Ag) (5.5% Ag) 225°C 230°C 1 4 0 ° C 1 4 0 ° C trans- 2-Butene cis-2-Butene 1-Pentene trans-2-Pentene cis-2-Pentene Benzene Toluene
... ...
620 720 800 720 830 830 960
900 830 930 920 1070
680 750
...
780 860 810
...
570 650 730 680 750 700 830
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
I
TIME
Figure 3. Stopped-flow studies of the 2-butenes. Cd2+ form of resin 11; 140 OC; 45 mL/min carrier gas flow; (A) flow stopped, (B) flow restarted, (C) trans-2-butene, (D) cis-2-butene. Top chromatogram
(9)
(IO) (1 1) (12)
Is for pure trans-2-butene, bottom for pure cis-2-butene injected into column
is shown in Table I. In all cases they are fully resolved from the saturated hydrocarbons, but the enhancement of retention is different for each resin. The cadmium form of resin I1 gave quite different results for the olefins. While trans-2-butene and cis-2-butene gave
K. R. K. S. P.
Ohzeki and T. Kambara, J. Chromatogr., 55, 319 (1971). F. Hirsch et al., Anal. Chem., 45, 2100 (1973). Fujimura and T. Ando, J . Chromatogr., 114, 15 (1975). Allulli et al., Anal. Chem., 48, 1259 (1976). Magidman et al., Anal. Chem., 48, 44 (1976). J. S. Fritz and J. N. Stary, Anal. Chem., 46, 825 (1974). L. C. b n s e n and T. W. Gilbert, J. Chromatogr. Sci., 12, 458, 464 (1974). T. S. Stevens and H. Small (Dow Chemical Company), U S Patent 3,966,596 (1976). H. Small, J. Inorg. Nucl. Chem., 18, 232 (1961). R. Lane, E. Lane, and C. S. G. Phillips, J. Catal., 18, 281 (1970). D. G. Howery and S. Tada, J. Macromol. Sci., Chem., 3, 297 (1969). I. Hadzistelios, F. Lawton, and C. S. G. Phillips, J. Chem. Soc., Dalton Trans., 2159 (1973).
RECEIVED for review March 18,1977. Accepted June 27,1977. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Determination of Ethyl and Methyl Parathion in Runoff Water with High Performance Liquid Chromatography Daniel C. Paschal, * Richard Bicknell, and David Dresbach
Department of Chemistry, Illinois State University, Normal, Illinois 6 176 1
High performance liquid chromatography with variable wavelength detection is described for the determination of methyl and ethyl parathion at the part per billion level in runoff water. The macroreticular resin XAD-2 was used as an adsorption medium for preconcentrationof trace organics in water by a factor of 100. Linear relationships between peak height or area and concentration were obtained in the range 0 to 120 ppb of methyl and ethyl parathion, with a lower detectlon limit of 2 to 3 ppb ( S / N = 2). Relative standard deviations In this range were 1 to 6%, with an average recovery of 99 %. Only 30 mln is required for the complete determination, and as little as 2 ng of methyl or ethyl parathion can be quantified with a 10-pL injection.
Organophosphorous insecticides enjoy wide use due to their relatively rapid decomposition and low accumulation in biological food chains. For these reasons, the organophosphorous insecticides are rapidly replacing the more persistent organochlorine agents. In fact, recent EPA restrictions have curtailed the use of several of the once widely used organochlorine pesticides such as DDT, aldrin, dieldrin, and heptachlor (1). Among the more popular replacements for these organochlorine compounds are ethyl and methyl parathion. Ethyl parathion (diethyl p-nitrophenyl phosphorothionate) and methyl parathion (dimethyl p-nitrophenyl phosphorothionate) were introduced in the 1940s. Their high, wideANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1551
