T. De Beer et al., Sensitive absorption detection for micro-column liquid chromatography by ultraviolet forward-scattering degenerate four-wave mixing, ANALYT CHIM, 390(1-3), 1999, pp. 55-64
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.