Hydrophobicity of various cation-exchange columns for ion chromatography

• Evidence of a mixed-mode behaviour of cation-exchangers was presented • Hydrophobicity of 20 commercial cation-exchange columns was evaluated • New parameters showing input of phenyl ring and OH-groups to the retention explored • Fingerprint type diagrams characteriszing IC columns were obtained The retention of organic cations including homologues of n -alkylamines, phenylalkylamines, and ethanolamines was investigated for 20 commercially produced cation-exchange columns with varying matrix, bonding chemistry, functional groups, and ion-exchange capacity. All data was obtained using 13 mM nitric acid as eluent, under constant column temperature of 25°C and employing a combination of a UV-detector followed by a non-suppressed conductivity detector. Collected data was used to calculate methylene selectivity α(CH 2 ) values from dependences log k - n CH2 with an outlook to evaluate and characterise hydrophobicity of various cation-exchange columns used in ion chromatography. According to the determined α(CH 2 ) values the studied cation-exchangers were divided into groups of low (0.195 – 0.286), moderate (0.316 – 0.525) and high (0.502 – 1.078) hydrophobicity. New parameters illustrating contribution of aromatic ring and hydroxy- groups to the retention of model compounds were explored. The effect of aromatic group can be evaluated by a comparison of separation selectivity log( k pea / k ea ) of phenylethylamine and ethylamine; and this parameter was observed to vary from 0.480 obtained for silica coated with poly(butadiene-maleic acid) to 1.644 for carboxylic cation-exchanger IonPac CG12. A contribution of hydroxyl- group in the retention was less profound with separation selectivity between monoethanolamine and ethanolamine varied from -0.55 to 0.320. A possibility of multipoint interactions of organic cations with cation-exchangers was discussed. The evidence of a mixed-mode behaviour of cation-exchangers was presented.