Anal. Chem. 1980, 5 2 , 15-19
H. Klevens, J . Phys. Colloid Chern., 54, 283 (1960). D. Arnold, C. Plank, and E. Erickson, Chem. fng. Data Ser., 3, 253 (1958). P. Wauchope and F. Getzen, J . Chem. fng. D,rta. 17, 38 (1972). C. McAuliffe, J . Phys. Chem., 70, 1274 (1966). F. P. Schwarz. J . Chem. €ng. Data 22, 273 (1977) D. Mackay and Wan Ying Shiu, J . Chem. €ng. Data, 22, 399 (1977). W. May, S . Wasik, and D. Freeman, Anal. Chern., 50, 997 (1978). R . Brown and S . Wasik. J . Res. Nat!. Bur Stand. Sect A . (45) 78. 453 (1974). R. S. Stearns, H. Oppenheimer, E. Simon and W. Harkins, J . Chem. Phys., 15, 496 (1947). "Handbook of Chemistry and Physics", 41st ed., Chemical Rubber Publishing Company, Cleveland, Ohio 1960. J. Calvin Giddings, "Dynamics of Chromatography, f'ari 1, Principles and Theory", Marcel Dekker, New York, 1965, p 277. A. E.van Arkel and S. E.Vles, Recl. Trav. Chim. Pays-Bas, 55, 407 (1936). A. Michaelis, Ber. Dtsch. Chem. Ges.. 46, 3683 (1913). R. J. Laub and R. L. Pecsok. "Physicochemical Applications of Gas Chromatography", John Wiley and Sons, New York, 1978, p 33.
was unaffected by the chemical composition of the solute, this method is apparently nonspecific. However more determinations would have to be made to test this implication. If the method is nonspecific then t h e chemical composition of the solute need not be known for the application of this method. Furthermore, the method is amenable to those solutes which may be toxic; the columns can be easier filled in a hood, sealed u p and weighed if necessary, and then run in a hood. To extend the application of the method to solutes with indices of refraction less than 1.4, other methods of zone differentiation could be used such as visible fluorescence excitation. The method also avoids several disadvantages of other methods (1-15). The formation of emulsions is a problem with those methods relying on vigorous shaking of the water-solute mixture to ensure saturation ( I , 2,4412). Furthermore despite the vigorous shaking, the achievement of saturation is not always assured. The accuracy of the method is independent of the solubility of the solute as with other methods (1-15). This elution chromatography method provides a simple, inexpensive, and direct means to measure the solubilities of a wide variety of organic compounds.
RECEIVED for review July 30. 1979. Accepted September 24, 1979. This work has been supported by the Office of Environmental Measurements a t the hational Bureau of Standards. Certain commercial equipment, instruments, and materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material, instruments, or equipment identified are necessarily the best available for t h e purpose.
LITERATURE CITED (1) (2) (3) (4) (5) (6)
15
A. Rex, 2 . Phys. Chem., 55,355 (1906). A. Klemenc and M. Low, Red. Trav. Chim. Pays-Bas, 49, 629 (1930). V. Rothmund, 2 . Phys. Chern., 26, 433 (1898). H. Fuhner. Chem. Ber., 57, 510 (1924). P. M. Gross and J. H. Saylor. J . A m . Chem. Soc.. 53, 1745 (1931). R. L. Bohon and W. F. Claussen, J . Am. Chem. SOC.,73, 1571 (1951).
Liquid Chromatography Columns of Microparticle Amberlite XAD-2 Robert G. Baurn,' Rolf Saetre,2 and Frederick F. Cantwell" Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Small particle size cuts of the nonionic adsorbent Amberlite XAD-2 have been obtained by grinding, sieving, and solvent elutriation. Columns, 15 cm long, packed by a stirred slurry packing technique with 3.6-8.4 pm XAD-2 yield efficiencies of greater than 20 000 plates per meter. Although permeabilities are smaller than those available with commercial reverse-phase bonded packings, necessitating slower solvent flow rates, the XAD-2 provides separation times adequate for most analyses and it can be used at all pH values including dilute sodium hydroxide.
Amberlite XAD-2 resin is a macroporous styrene-divinyl benzene copolymer which has been shown to be a versatile liquid chromatographic packing. It is a hydrophobic adsorbent, since it is not wetted with water ( I ) , with a specific surface area of about 330 m 2 / g ( 2 ) . Although XAD-2 is nonionic, it adsorbs both neutral and ionic species (3-5). Columns of XAD-2 have been used to analyze preservatives (3) and active drugs ( 4 ) in pharmaceutical syrups and to separate a variety of organic acids (61, bases (7), nitro- and P r e s e n t address: Searle Laboratories, Box 5110. Chicago, Ill.
60680. *Present address: Esso Resources, 339 5 0 t h Avenue A l b e r t a T 2 G 2V3.
S.E., Calgary,
0003-2700/80/0352-0015$01 .OO/O
chlorophenols ( B ) , and aromatic compounds (9). T h e chromatographic retention of peptides and amino acids has been investigated on XAD-2 ( I O ) , and t.hat of nucleic acid hydrolysates, purines, pyrimidines, nucleosides, and nucleotides has been studied on both XAD-2 ( 1 1 ) and a closely related adsorbent, Amberlite XAD-4 (12). The dependence of retention volume of acids and bases on mobile phase p H has been well characterized and demonstrates retention of both neutral and ionic conjugate acid-base species ( 4 , 5 , 23). Amberlite XAD-2 is obtained commercially as hard, white spheres, nominally 20 to 50 mesh (14) which must be ground to a smaller particle size for use in analytical chromatography. T h e skeletal density of the highly cross-linked styrene-divinylbenzene matrix is 1.07 g/mL ((14).Swelling of the resin in water is nil. Swelling in nonaqueous solvents has not been extensively studied, although 3 % swelling has been claimed in methanol (I51 and solvent uptake studies ( I , 9) suggest a similar figure for solvents such as acetonitrile and ethanol. Other physical properties of chromatographic interest have been summarized ( 2 ) . As a chromatographic packing, XAD-2 is distinguished by its low cost, wide useful p H range (pH 0 to 14), compatibility with virtually all solvents, and relatively strong adsorbent properties. Because it possesses a hydrophobic: surface, XAD-2 finds use in many of t h e same separations that are done on reverse-phase bonded packings such as octadecylsilyl silica C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
(ODs). It can be employed for so-called “ion-pair’’ chromatography in which, for example, the chromatographic retention of an anionic sample is enhanced by the presence in the mobile phase of a large quaternary ammonium ion (12, 16). As an adsorbent the resin is well characterized. It possesses a true adsorbent surface and t h e mechanism of chromatographic retention of ionic species has recently been quantitatively explained in terms of the Stern-Gouy-Chapman Theory of t h e electrical double layer ( 2 ) . A major drawback to the use of XAD-2 columns in liquid chromatography has been the unavailability of high efficiency columns with plate numbers comparable to those of commercially available reverse-phase bonded packings. In the present work, t h e preparation and properties of such high efficiency columns of XAD-2 is described. EXPERIMENTAL Chromatographic Apparatus. Mobile phase solvents were pumped with a microprocessor-controlled dual piston reciprocating pump from a model S P 8000 chromatograph (Spectra-Physics Corp., Santa Clara, Calif.). Injections were made with a 10-pL sample injection valve, series HP (Valco Instruments Co.. Houston, Texas), and the detector was a model SF 770 Spectroflow Monitor (Schoeffel Instruments Corp., Westwood, N.J.). Chromatograms were recorded on the recorder in the S P 8000 chromatograph. Columns are described below. Chemicals and Solvents. Acetaminophen and methyl and propyl p-hydroxybenzoates were USP grade. The ethyl ester (Matheson, Coleman and Bell, East Rutherford, N.J.) and butyl ester (Eastman Kodak Co., Rochester, N.Y.) of p-hydroxybenzoic acid were reagent grade. Acetonitrile was ACS reagent grade and was freshly distilled before use. Water was deionized, distilled, and finally distilled from alkaline permanganate. Methanol used as elutriation solvent was reagent grade and was re-used after filtration and distillation. Resin Preparation, Amberlite XAD-2 resin, 20-50 mesh (Rohm and Haas Co., Philadelphia, Pa.) was first backwashed to remove any foreign particles and then washed with a 1-3 M HCI to remove metallic contaminants. After another water wash, it was ground in a triple porcelain roller mill (Enveka-Apparatebau GmbH, Chemical and Pharmaceutical Industry Co., 260 W. Broadway, New York, N.Y.) and dried in a 50 “C forced air oven. Because of its hardness, it is not practical to grind XAD-2 by hand. The portion of dried resin which passed a 325-mesh US.standard sieve (
