Microfluidic membrane suppressor module design and evaluation for capillary ion chromatography

A microfluidic ion-suppression module for use in ion-exchange chromatography has been developedand evaluated. The device consists of an ion-exchange membrane clamped between two polymer chipsfeaturing a 200 × 100 µm (width × depth) eluent channel (l = 60 mm), and a 300 × 150 µm regenerantchannel (60 mm), respectively. The suppression efficacy using a Nafion membrane was compared withthat of a styrene-sulfonate grafted fluorinated ethylene propylene (FEP) membrane. The latter was foundto outperform Nafion in terms of lowest attainable background signal (suppression efficacy) and dynamicsuppression range. Increasing the suppressor temperature or the sulfuric acid regenerant concentrationled to an extension of the operational suppression range, this however at the cost of an increased back-ground 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 gradi-ent separations applying up to 70 mM KOH in combination with 400 µm i.d. capillary columns operatedat the optimal flow velocity. The applicability of the miniaturized suppressor is demonstrated for bothisocratic and gradient separations of mixtures of inorganic anions. Band-broadening characteristics ofthe suppressor were optimized with respect to a commercial capillary hollow-fiber suppressor, yieldingcomparable overall system efficiency, e.g., 8500 plates for nitrate recorded on a 150 mm long capillarycolumn. A second chip device was also constructed, featuring suppression at both sides of the eluent flowpath. This double-sided suppressor allowed to increase sample throughput and operate at eluent flowrates of 10 µL/min, while maintaining efficient suppression characteristics.