Preparation and application of methacrylate-based cation-exchange monolithic columns for capillary ion chromatography

Polymer-based strong canon-exchange monolithic capillary columns with different capacities were constructed for ion chromatography by radical polymerization of glycidyl methacrylate (GMA) and ethylene dimethacrylate in a 250-mum-i.d. fused-silica capillary and its subsequent sulfonation based on ring opening of epoxides with 1 M Na2SO3. The cation-exchange capacities can easily and reproducibly be controlled in the range of up to 300 muequiv/mL by changing the immersion time of the epoxy-containing polymer in the Na2SO3 solution. The chromatographic performance of the produced monolithic capillary columns was evaluated through the separation of a model mixture of common cations such as Na+, NH4+, K+, Mg2+, and Ca2+. As an example, these cations could be well separated from one another on a 15-cm-long cation-exchange monolithic column (column volume, 7.4 muL) with a capacity of 150 muequiv/mL by elution with 10 MM CuSO4. The pressure drop of this 15-cm column was similar to1 MPa at a normal linear velocity of 1 mm/s (a flow rate of 3 muL/min), and the numbers of theoretical plates for the cations were above 3000 plates/15 cm. This GMA-based cation-exchange monolithic column could withstand high linear velocities of at least 10 mm/s. Over a period of at least two weeks of continuous use, no significant changes in the selectivity and resolution were observed. The applicability of a flow rate gradient elution and the feasibility of direct injection determination of major cations in human saliva sample were also presented.