Sensitive absorption detection for micro-column liquid chromatography by ultraviolet forward-scattering degenerate four-wave mixing

The performance of forward-scattering degenerate four-wave mixing (F-D4WM) in the mid-ultraviolet (UV) region (351 nm) as a detection technique for mi cro-column liquid chromatography (mu LC) is studied, using nitro-substitute d polycyclic aromatic hydrocarbons (NO2-PAHs) and amino-substituted anthraq uinones (AAQs) as test compounds. When using round capillaries, the concent ration limits of detection (LOD) for the NO2-PAHs were 20-70-fold better co mpared with absorption detection using a U-shaped detector cell. The final result is influenced by photochemical degradation during 351 nm F-D4WM dete ction. Furthermore, it is demonstrated that the use of recently commerciall y available square capillaries (i.e., capillaries with both a square cross section and a square bore) instead of round ones is quite suitable for F-D4 WM detection. The square capillary (internal dimensions, 75x75 mu m(2); ext ernal dimensions 300x300 mu m(2)) did not cause significant chromatographic band broadening in mu LC. The angle of incidence of the laser Light should be 56 degrees, thus fulfilling the Brewster condition at the air-quartz bo undary. Using this approach for the AAQs, compounds not prone to significan t photodegradation under the experimental conditions, a further 4-fold impr ovement was achieved. As a result, the overall gain in concentration LOD fo r 2-amino-9,10-anthraquinone (molar absorptivity 4000 M-1 cm(-1) at 351 nm) was from 4 to 0.05 mu M, and baseline irregulations were reduced. (C) 1999 Elsevier Science B.V. All rights reserved.