A sensitive absorbance measurement method based on multi- wave mixing optical method is demonstrated as an effective detector for capillary electrophoresis or liquid chromatography. The use of a single focusing lens to focus and mix the two input beams both simplifies the optical setup and utilizes the laser power very efficiently inside a small probe volume. Hence, inexpensive low-power (mW level) lasers can be used in this nonlinear multi-photon detection method, including low-power argon ion lasers, He-Ne lasers and laser diodes. The resulting small (pL range) probe volume (laser beam overlap zone) allows convenient interfacing of this detector to capillary-based electrophoresis or chromatography separation systems. Excellent signal collection efficiency (virtually 100%) for the collimated coherent signal beam, allows detection sensitivity levels similar to those of fluorescence-based methods, and yet wave-mixing detectors are applicable for both fluorescing and non-fluorescing flowing analytes. The coherent signal beam is generated by thermally-induced refractive-index spatial gratings formed by the two input beams, and hence, the signal strength also depends on some solvent properties. The wave-mixing detector is demonstrated to be effective for trace analysis, offering advantages such as detection in very small sample volumes, remote and in situ analysis, and convenient as well as efficient alignment enhancements obtained by the introduction of optical fibers into the detector optical setup.
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