Forward-Scattering Degenerate Four-Wave Mixing Detection in Micro-Column Liquid Chromatography Using Fused-Silica Capillary Flow Cells

Abstract

In order to have as simple a chromatographic setup as possible, laser-based forward scattering degenerate four-wave mixing (F-D4WM) detection is applied directly to fused-silica capillary flow cells that can be used in micro-column liquid chromatography (μLC). Compared to the previously used specially designed flow cells (optimized from an optical point of view), such cylindrically shaped cells have obvious disadvantages such as background scatter 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 100 pL, which is attractive for miniaturized chromatographic systems. Parameters 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 direction of the eluent flow is critical; it should be in the plane formed by the two laser beams. μLC/F-D4WM is used to analyze spiked river Rhine water.