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 m ixing (F-D4WM) detection is applied directly to fused-silica capillary¯ow cells that can be used in micro-column liquid chrom atography (m LC). Compared to the previously used specially designed¯ow cells (optimized from an optical point of view), such cylindrically shaped cells have obvious disadvantages such as background scatter and signal beam distortion. H owever, 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 chrom atographic system s. Parameters such as capillary dimensions (internal and outer diameters), capillary con® guration in relation to eluent ow direction,¯ow rate, and various background suppression techniques are considered from a theoretical and a practical point of view and subsequently optimized. Interestin gly, the direction of the eluent¯ow is critical; it should be in the plane formed by the two laser beams. m LC/F-D4WM is used to analyze spiked river Rhine water.

Molecular spectrometry—milestones for the millennium

“…Whilst this technique has been of somewhat academic interest some very practical applications have recently been reported including the use of DFWM as a detector for capillary column chromatography. 9 In this experiment a laser beam (Ar ion; 1.2 W at l = 514.5 nm) is split into two pump beams, E1 and E2, which are then focused and recombined at a small angle in the capillary column which forms an interference pattern. On absorption of light by an absorbing species passing through the column a thermally-induced phase grating is produced which results in two diffracted beams, E3 and E4, which emerge in defined directions.…”

“…The intensity is inversely proportional to the thermal conductivity of the solvent and the sine of the angle between the two pump beams. In their work, de Beer et al 9 have carefully evaluated the effect of these parameters to produce a detector with a limit of detection (LOD) of 1 3 10 27 M for 1-amino-9,10-anthraquinone which has a molar absorption coefficient of 2000 M 21 cm 21 at 514 nm. This LOD represents an improvement of 50-fold over conventional absorption detection.…”