Capillary gap flow cell as capillary-end electrochemical detector in flow-based analysis

A gap flow cell (gap-FC) as a capillary-end electrochemical (EC) detector, incorporating exchangeable screen-printed electrodes, that facilitates adjustment of the gap distance between the capillary outlet and working electrode down to 30 μm was investigated. The analytical performance of the gap-FC was assessed in terms of detector response (current), the efficiency of EC conversion of ferrocyanide, and effective cell volume, as a function of flow rate and gap distance, within a range of 0.1-200 μL min^-1 and 30-100 μm, respectively. As a result, an efficiency of EC conversion within the range of ∼1.5-11% (∼3-16 times higher than wall-jet FC), at an effective cell volume of ∼30-75 nL, was achieved. The gap-FC was applied as a liquid chromatography EC detector, coupled with standard and capillary format liquid chromatographs, where the analytical performance of gap-FC was compared with that of a UV detector. With the standard LC, the sensitivity for ascorbic acid, 2,3-dihydroxybenzoic acid and pyrocatechol, was greater than with the UV detector, with limits of detection (LODs) one order of magnitude lower for the gap-FC (0.10 μM, 0.09 μM, 0.19 μM, respectively, RSD 1.35-3.8%, n = 9, linearity r² ∼0.99 for concentration range 1-100 μM). LODs of test electroactive solutes using the gap-FC were ∼2 times lower than LODs reported in existing literature using standard EC detectors, including wall-jet and thin-layer FCs.