Weak acid cation-exchange columns

A good general-purpose weak acid cation-exchange column for non-suppressed applications is the Metrosep Cation C 2 column from Metrohm. As the selection of eluents used with this column is quite varied, the reader is referred to Metrohm column literature for specific eluent recommendations. The Metrosep Cation C 2 column is prepared via encapsulation of a porous silica substrate maliec acid copolymer. 

Several papers have reported use of a weak acid/crown ether eluent in ion-exclusion/cation exchange chromatography with conductimetric detection on a weakly acidic cation-exchange column to effect the simultaneous determination of both cations and anions. Resolution was significantly improved when 18-crown-6 was present in the eluent. Detector response was positive for anions and negative for cations. [76-80] A sample chromatogram is shown in Figure 9. 

Ding, M., Tanaka, K., Hu, W., Haddad, P. R., and Miyanaga, A. (2001) Simultaneous Determination of Anions and Cations in Environmental Samples by Using a Weakly Acidic Cation-exchange Column with 2, 6-pyridinedicarboxylic Acid, Sulfosalicylic acid, and 18-crown-6 as Eluent, J. Liq. Chromatogr. Rel. Techn. 24, 3105-3117. 

D. Jensen, J. Weiss, M. A. Rey and C. A. Pohl, Novel weak-acid cation-exchange column, /, Chromatogr., 640,65,1993. 

Wash the AG-3 column with water, and apply the filtrate, along with the washes, to a weakly acidic cation exchange column or a Sephadex column equilibrated with 50% aqueous AcOH. 

One fact to keep in mind with such phases is that weak acid cation-exchange materials based on carboxylic acid functional groups are subject to esterification in the presence of alcohol containing eluents. Even thongh typical eluent conditions (i.e., weakly acidic aqneous eluents containing alcohol) do not favor ester formation, such stationary phases typically exhibit slowly declining capacity when operated in the presence of alcohol-containing eluents. Consequently, such columns are normally operated with acetonitrile, tetrahydrofuran or acetone rather than with methanol, in order to avoid this problem. 

A mixture of l-(2,3,5-tri-0-benzoyl-p-D-ribofuranosyl)-4-methylthio-l,2-dihydro-l,3,5-triazin-2-one (0.5875 g), absolute methanol (5 ml) and a normal methanolic sodium methoxide solution (1.2 ml) is stirred at room temperature with the exclusion of atmospheric moisture (a guard tube filled with potassium hydroxide pellets is fitted to the reaction vessel). The starting compound passes into solution in the course of 5 min. The resulting solution is allowed to stand at room temperature for 45 min and then the cations are removed by passage of the solution through a column packed with 10 ml of a weakly acidic cation exchange resin in the H+ form prewashed with water and methanol. The methanolic effluent (60 ml) is evaporated under reduced pressure at 30°C, the residue is dissolved in methanol (20 ml) and the solution once again is evaporated and the l-p-D-ribofuranosyl-4-methoxy-l,2-dihydro-l,3,5-triazin-2-one was obtained. 

For demineralization, a set of strong-acid and weak-acid cation-exchange resin columns, and strong-base anion-exchange and weak-base ion-exchange resin columns have been applied [6],... 

Tanaka et al. devised a method for determination of the ions in acid rain in a single run [18]. The column used contained a polyacrylate weak-acid cation exchange resin (TOSOH TSK gel OA-PAK, 300 x 7.8 mm, 5 pm particle size). Water alone as the mobile phase only separated weak acid anions from strong acid anions as a group cat-... 

The latest development in carboxylic-based weak acid cation exchangers are columns designed for the determination of hydrophobic and polyvalent amines, including biogenic amines, alkylamines, and diamines, using simple aqueous eluent and elevated column temperature. 

Table 8 Selected columns of carboxylate-based weak acid cation exchangers.

A disadvantage of weak acid cation exchangers is the dependence of the dissociation of carboxyl groups on sample pH. Polymer-coated silica columns such as the one introduced by Schomburg et al. [4] require pH adjustment of the samples with nitric acid or sodium hydroxide. In addition to the time necessary for this kind of sample preparation, the likely possibility that cationic impurities in the chemicals will contaminate the samples is problematic. Conversely, even strongly acidic samples up to pH 1.3 (corresponding to 50 mmol/... 

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