Selectivity of some ion chromatography stationary phases for small anions in solvent-water mixtures with hydroxide

The selectivity of two hydroxide-selective ion chromatography stationary ph ases was examined with varying solvent composition. The solvents acetonitri le 2-propanol and dimethylsulfoxide were found to diminish retention of the se anions. However, retention times were found to increase for small anions with increasing methanol composition. For somewhat larger anions such as p erchlorate and tetrafluoroborate, moderately decreasing or stable retention times were observed with increasing methanol content. The loss of retentio n is attributed to hydroxide form resins preferentially imbibing water from the solution phase, effectively increasing thr elution strength of hydroxi de because of its greater hydration energy. In methanol-water mixtures to 4 0%, these types of anion exchangers are known to swell to nearly the same e xtent (volume) as in water. Thus, hydration energy differences between hydr oxide and these anions may not be as pronounced relative to weaker solvatio n in methanol. Run time for common inorganic anions on a high-capacity anio n-exchange column can be reduced from 30 to 10 min through the use of aceto nitrile. In addition, as the acetonitrile content is increased, retention r eversals such as bromide-nitrate and chloride-nitrite occur. Four divalent species (sulfite, maleate, sulfate, fumarate) that are not separated on Dio nex IonPac AS11 with hydroxide can be separated on the same column easily i n 50% methanol. (C) 1999 Academic Press.