Anal. Chem. 1 9 8 0 , 52, 15R-27R (43) Lee, R . T.; Cascio, S.;Lee, Y. C. Anal. Biochem. 1979, 95, 260-269. (44) Bueneman, H.; Mueller, W. Nucleic Acids Res. 1978, 5 , 1059-1074. (45) Jaworek. D.; Botsch, H.; Maier, J. Methods Enzymol. 1976, 44,195-201. (46) Hoiejgl, V. Blochim. Siophys. Acta 1979, 577, 389-393. (47) Wright, C. L.;Warsy, A . S.;Holroyde, M.J.; Trayer, I . P. Biochem. J . 1978, 775, 125-135.
(48) Hoiroyde, M. J.; Chesher, J. M. E.; Trayer, I . P.; Walker, D. G. Biochem. J . 1976, 153, 351-361. (49) Rosemeyer, H.; Seela, F. Carbohydr. Res. 1979, 74, 117-125. (50) Dickerman, H. W.; Ryan, T. J.; Bass, A . I.; Chatterjee. N. K . Arch. Biochem. Siophys. 1978, 186, 218-234. (51) Carlsson, J.; AxBn, R.; Unge. T. Eur. J . Biochem. 1975, 59, 567-572.
Ion Exchange and Liquid Column Chromatography Harold F. Walton Box 2 15, University of Colorado, Boulder, Colorado 80309
This review includes journals available t o December 31, 1979. T o report the last two years of liquid chromatography is a formidable task. No area of chemical analysis has advanced so fast. I have heeded the request of ANALYTICAL CHEMISTRY’S editors and emphasized principles and methodology, citing enough representative examples to compile Table I and 11, tables t h a t readers have found useful, but which make no pretense to be comprehensive. Many of the papers cited in the tables illustrate new techniques and interesting points of theory. Readers concerned with specific applications should consult the book list ( A l - A 8 ) or the reviews (Bl-B14). There have been several meetings and international symposia devoted wholly or in part to liquid chromatography. Volumes 158 and 165 of the J o u r n a l of Chromatography comprise the proceedings of two international conferences on chromatography; volume 149 of the same journal reports the proceedings of the Third International Conference on Liquid Column Chromatography, held in Austria in 1977. The Fourth International Conference was held in Boston in 1979, and its proceedings appeared in the Journal of Chromatography, Vol. 185. A U S - J a p a n Seminar on Advanced Techniques of Liquid Chromatography was held in Boulder in 1978; it was informal; several of the papers have appeared in the open literature and are cited in this review, and a summary of the seminar, by A. P. Graffeo and N. H. C. Cooke, was published in t h e J o u r n a l of Chromatographic Science [ 1979,17, 2021.
ION EXCHANGE General. Ion-exchan e chromatography is becoming more efficient. Advantage is i e i n g taken of conventional gel-type resins in small, uniform particles for the chromatography of inorganic ions (123,161,421)as well as for organic compounds; here, t h e advantages of low crosslinking (287, 51 7) and the effect of inorganic counterions on the retention of uncharged organic solutes (517 ) are noted. Macroporous and surfacesulfonated resins are being used (421);their preparation and properties are described (162,460). “Ion chromatography”, which uses resins whose ion-exchange function resides in a thin surface film, is very popular (A7,B10,118,161,186,271, 292,320). The method is especially useful for anions, and very low concentrations can be measured, down to parts per billion if concentrator columns are used (523). In its orthodox form, “ion chromatography” depends on conductometric detection, preceded by a “suppressor column” to remove ions of the eluent. The suppressor column can be eliminated if a suitable ion-selective detector is used (161). Detectors t h a t respond to some ions and not others make ion-exchange chromatography more practical. Anion-exchange separation of transition-metal ions in hydrochloric acid, with a concentration gradient and atomic absorption detection, is a good example (209). Studies of the distribution of metal ions between resins and various com lexing solutions continue. Cation exchange in concentratefHC1-HC10, (359) provides means of separating easily hydrolyzed ions like titanium and zirconium; in tartrate 0003-2700/80/0352-15R$Ol .OO/O
and succinate (90, 91) many elements are separated; anion exchange has been studied in HBr-HN03 mixtures (4611, thiocyanate (447),and malonate media (61),and cation exchange in oxalic-hydrochloric acid mixtures has useful possibilities (365). A new aspect of ion exchange is the widespread use of paired-ion chromatography, in which the stationary phase is a hydrophobic packing and the mobile phase contains longchain, hydrophobic ions of opposite charge t o the ions being separated. I t is now clear that highly hydrophobic counterions are incorporated into the surface of the packing, making i t essentially an ion exchanger. Paired-ion chromatography, developed for or anic ions, has now been used for inorganic species like halijes, azide ions and oxy anions of nitrogen, sulfur, and the halogens (404). Paired-ion chromatography will be discussed a t greater length below. New ion-exchanging materials, organic and inorganic, continue t o be reported, but less frequently than before. I n o r g a n i c Exchangers. Among the new materials are bismuth tungstate (402),antimonates of thorium (103),nickel and cobalt (399,401),arsenate and vanadoarsenate of tin(1V) (400, 481), and zirconium arsenophosphate (448). Older materials have been studied more intensively, particularly crystalline zirconium phosphate (B3, 73, 81, 1 2 7 ) and crystalline antimony pentoxide (2-4). On t h e latter, selectivity orders are reported for alkali and alkdne-earth ions and for transition-metal ions; they depend on the degree of loading. Tin dioxide exchanges both anions and cations; its mode of action is peculiar (233-235), and it is specially selective for scandium (86)and lithium (218). Activated carbon impregnated with tin dioxide combines the selectivity of an inorganic exchanger with the physical, hydraulic properties needed in chromatography (218). Most inorganic exchangers are either slow to react, or are soft and powdery, or disintegrate easily; hence, their use in columns is limited. Other exchangers whose special uses are noted are hafnium and thorium phosphates (25, 104), cerium(II1) oxalate (81, tungsten(V1) oxide and zirconium tungstate (102),zirconium molybdate (165),and copper(I1) ferrocyanide (2611. Aluminosilicate molecular sieves were used to separate glucose and fructose (531),and hydrous oxides of aluminum, titanium, and zirconium removed traces of heavy-metal ions from water (306). C h e l a t i n g a n d S p e c i a l Resins. A polymer with 8hydroxyquinoline functionality, previously described, has been used for several metal separations and is specially selective for vanadium (505). Starting with macroporous crosslinked polystyrene, amide groups (374, 390), arsonic acids (1461, dithiocarbamate (26),thio lycolate (385),oxime (264,463)and nitroresorcinol units (4637 have been attached; t,he products have the selectivities expected from those of the parent substances in solution. Oxime and thioglycolate resins, for example, are selective for mercury and copper; nitroresorcinol is selective for iron(III), copper, and cobalt. A cellulose-based exchanger carrying salicylic acid absorbs iron(II1) and uranium(VI), and one with hydroxyphenylazo-2-naphthol units @ 1980 American Chemical Society
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I O N EXCHANGE AND L I Q U I D COLUMN CHROMATOGRAPHY
absorbs uranium(V1) very strongly (57). Instead of being chemically grafted to polymers, complex organic molecules can be held as strongly absorbed counterions in ion-exchange resins; a mercaptoazobenzenesulfonate, held in an anion-exchange resin, absorbs mercury(I1) (65);an anion exchanger loaded with substituted 8-hydroxyquinoline sulfonate absorbs many metal ions (297). The chelating agent may also be placed in the mobile phase, using a nonionic reverse-phase support, which essentially becomes a cation exchanger (35). Controlled-pore glasses bonded with EDTA and 8hvdroxvauinoline are commercial Droducts. Their selectivitv f& tran“si6on metals has been studikd and they have been used in column chromatography ( 174,244);however, the columns give broad bands. A new high-capacity cation exchanger (0.5 mequiv/g) based on porous silica is descrihed (524). By treating commercial silica-based, bonded anion and cation exchangers with octadecyltrichlorosilane, sorbents were prepared that combined the properties of reverse-phase packings with those of ion exchangers (305);they simulated, in fact, reverse-phase packings that carry ion-pairing reagents. They were applied to the chromatography of nucleotides. Dithiocarbamate and diketone bonded phases on silica, carrying copper ions, were used in ligand-exchange chromatography (68),as were diamine bonded phases (82). Crown ethers bonded to silica (43,44, 67) have interesting chromatographic uses, one of which is the determination of water in salt hydrates by selective elution with methanol (44). Perfluorinated hydrocarbons with sulfonic acid groups not only make excellent ion-exchanging membranes, but, cut and ground into small particles, give excellent chromatography of simple inorganic ions (537). Their selectivity is related to hydration energies, the polymer matrix having minimal effect. T h e synthesis of solvent-modified, porous methacrylate polymers was described (149).
Solvent-Modified Polymers; Asymmetric Resins. “Macroporous” resins are made by polymerization in the presence of a solvent that selectively dissolves the monomers and precipitates the polymer. I t is important to realize that through the use of solvents, crosslinked polymers can be made, a n d from them ion-exchange resins, that have a continuous range of pore distributions. Some solvents preferentially dissolve the polymer, yielding open, “isoporous” products. The choice of crosslinking agents multiplies the possibilities, which are clearly expounded in a chapter by Davankov and his colleagues ( R 5 ) . Highly permeahle “isoporous macronet” polymers were used by these workers to make a family of asymmetric ion exchangers by bonding one of four l-amino acids, proline, hydroxyproline, allohydroxyproline, and azetidine to the styrene polymer. These exchangers, loaded with copper(I1) ions, are stationary phases for chromatographic resolution of dl-amino acids in solution, using ammonia solution as t h e eluent. Their properties, and t h e degrees of resolution of some 30 amino acids, are described in four important papers (98, 99). Other workers have made asymmetric resins by grafting l-proline onto polymers (248, 298, 299) or onto silica (17.3). Other fixed chiral ligands have been used (287,282,315,384). Natural pectic acid separates optical isomers (394). Usually t h e chiral cation exchanger is loaded with copper(I1) ions, which then hold t h e second, mobile ligand in a fixed orientation. Nonionic chiral stationary phases that bind dissolved molecules by charge transfer (326,364) were used to separate dl-sulfoxides (
