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Small-volume continuous-flow synthesis (small-vol- Conventional Device Optimized Device nme CFS) offers a number of benefits for use in small-scale chemical production and exploratory chemistry. Typically, small-volume CFS is followed by discontinuous purification; however, a fully continuous synthesis-purification combination is more attractive. Milli free-flow electrophoresis (mFFE) is a promising continuous-flow purification technique that is well suited for integration with small-volume CFS. The purification stability of mFFE, however, needs to be significantly improved before it can be feasible for this combination. One of the major sources of instability of mFFE is attributed to the ions produced as a result of electrolysis. These ions can form pH and conductivity gradients in mFFE, which are detrimental to separation quality. The severity of these gradients has not been thoroughly studied in mFFE. In this paper, we have experimentally demonstrated that detrimental pH gradients occur at flow rates of 8 mL/ min and less, and electric field strengths of 25 V/cm and greater. To decrease the pH gradients, it is necessary to evacuate H+ and OH- as soon as they are generated; this can be done by increasing local hydrodynamic flow rates. We calculated the necessary flow rate, to be applied at the electrode, which can effectively wash away both ions before they can cause a detrimental pH gradient These optimized flow rates can be attained by designing a device that incorporates deep channels. We have confirmed the effectiveness of these channels using a prototyped device. The new design allows mFFE users to work over a wider range of flow-rate and electric-field conditions without experiencing significant changes in pH. [ABSTRACT FROM AUTHOR]