Forward-scattering degenerate four-wave mixing detection in micro-column liquid chromatography using fused-silica capillary flow cells

In order to have as simple a chromatographic setup as possible, laser-based forward scattering degenerate four-wave mixing (F-D4WM) detection is appli ed directly to fused-silica capillary dow cells that can be used in micro-c olumn liquid chromatography (mu LC). Compared to the previously used specia lly designed flow cells (optimized from an optical point of view), such cyl indrically shaped cells ha,le obvious disadvantages such as background scat ter and signal beam distortion. However, a laser beam can be focused easily in a fused-silica capillary, yielding a laser-probed volume of less than 1 00 pL, which is attractive for miniaturized chromatographic systems. Parame ters such as capillary dimensions (internal and outer diameters), capillary configuration in relation to eluent flow direction, flow rate, and various background suppression techniques are considered from a theoretical and a practical point of view and subsequently optimized. Interestingly, the dire ction of the eluent flow is critical; it should be in the plane formed by t he two laser beams. mu LC/F-D4WM is used to analyze spiked river Rhine wate r.