An intensity-modulated 257-nm pump laser beam is used to induce refractive index changes inside of a capillary tube employed as an on-line detection system for capillary electrophoresis. These refractive index changes inside the capillary are monitored by demodulating the periodic lateral shifts of the interference fringes, generated by a probe laser beam oscillating at 633 nm, with a phase-sensitive amplifier. The use of capillaries with an internal diameter of 25 μm results in a twofold benefit in electrophoretic separations in conjunction with thermo-optical detection. The pumped heat is efficiently quenched at the capillary wall, and high electrical fields can be applied along the separation tube. The combination of both effects leads to highly efficient separations and fast modulations of the pump source. This is demonstrated by the separation of a test mixture of three dansylated amino acids in less than two minutes. Only 350-pg amounts of total sample are injected onto the column, and the detection volume is less than 10 pL.

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