Quantification and evaluation of Joule heating in on-chip capillary electrophoresis

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Abstract
We present the use of a novel, picoliter volume interferometer to measure, for the first time, the extent of Joule heating in chip‐scale capillary electrophoresis (CE). The simple optical configuration for the on‐chip interferometric backscatter detector (OCIBD) consists of an unfocused laser, an unaltered silica chip with a half‐cylinder channel and a photodetector. Using OCIBD for millidegree‐level noninvasive thermometry, temperature changes associated with Joule heating (≥2.81°C above ambient) in on‐chip CE have been observed in 90 νm wide and 40 νm deep separation channels. The temporal response of Joule heating in isotropically etched channels was exponential, with it taking an excess of 2.7 s to reach equilibrium. Buffer viscosity changes have also been derived from empirical on‐chip thermometry data, allowing for the determination of diffusion coefficients for solutes when separated in heated buffers. In addition, OCIBD has allowed the reduction in separation efficiency to be estimated in the absence of laminar flow and due to increased molecular diffusion and lower buffer viscosity. A 7% reduction in separation efficiency was determined for a high current drawing buffer such as Tris‐boric acid under an applied field of just 400 V/cm. Results indicate that heating effects in on‐chip CE have been underestimated and there is a need to readdress the theoretical model.