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

The performance of forward-scattering degenerate four-wave mixing (F-D4WM) in the mid-ultraviolet (UV) region (351 nm) as a detection technique for micro-column liquid chromatography (μLC) is studied, using nitro-substituted polycyclic aromatic hydrocarbons (NO2-PAHs) and amino-substituted anthraquinones (AAQs) as test compounds. When using round capillaries, the concentration limits of detection (LOD) for the NO2-PAHs were 20–70-fold better compared with absorption detection using a U-shaped detector cell. The final result is influenced by photochemical degradation during 351 nm F-D4WM detection. Furthermore, it is demonstrated that the use of recently commercially 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-D4WM detection. The square capillary (internal dimensions, 75×75 μm2; external dimensions 300×300 μm2) did not cause significant chromatographic band broadening in μLC. The angle of incidence of the laser light should be 56°, thus fulfilling the Brewster condition at the air–quartz boundary. Using this approach for the AAQs, compounds not prone to significant photodegradation under the experimental conditions, a further 4-fold improvement was achieved. As a result, the overall gain in concentration LOD for 2-amino-9,10-anthraquinone (molar absorptivity 4000 M−1 cm−1 at 351 nm) was from 4 to 0.05 μM, and baseline irregulations were reduced.