Microfluidic membrane suppressor module design and evaluation for capillary ion chromatography

- A microfluidic membrane suppressor was developed for capillary ion chromatography.
- Effective ion suppression achieved during gradient separations up to 70 mM KOH.
- Double-sided chip design allows operation at flow rates up to 10 μL/min.

A microfluidic ion-suppression module for use in ion-exchange chromatography has been developed and evaluated. The device consists of an ion-exchange membrane clamped between two polymer chips featuring a 200 × 100 μm (width × depth) eluent channel (l = 60 mm), and a 300 × 150 μm regenerant channel (60 mm), respectively. The suppression efficacy using a Nafion membrane was compared with that of a styrene-sulfonate grafted fluorinated ethylene propylene (FEP) membrane. The latter was found to outperform Nafion in terms of lowest attainable background signal (suppression efficacy) and dynamic suppression range. Increasing the suppressor temperature or the sulfuric acid regenerant concentration led to an extension of the operational suppression range, this however at the cost of an increased background signal due to enhanced diffusion, inducing sulfate bleed. Under optimized operating conditions, the microfluidic suppressor provided a dynamic capacity of 0.35 μEq./min, being compatible with gradient separations applying up to 70 mM KOH in combination with 400 μm i.d. capillary columns operated at the optimal flow velocity. The applicability of the miniaturized suppressor is demonstrated for both isocratic and gradient separations of mixtures of inorganic anions. Band-broadening characteristics of the suppressor were optimized with respect to a commercial capillary hollow-fiber suppressor, yielding comparable overall system efficiency, e.g., 8500 plates for nitrate recorded on a 150 mm long capillary column. A second chip device was also constructed, featuring suppression at both sides of the eluent flow path. This double-sided suppressor allowed to increase sample throughput and operate at eluent flow rates of 10 μL/min, while maintaining efficient suppression characteristics.