The analytical potential of diode laser-based intracavity absorption detection in column liquid chromatography

The possibility of on-line diode laser-based intracavity absorption detection (LIAD) in liquid chromatography is examined. Diode laser-based LIAD should, in principle, be very sensitive because of the inherent stability of the light source compared to that of other lasers. Because many diode lasers are sold in packages that also contain a photodiode for optical feedback stabilization, the detection set-up is very simple. However, contrary to earlier reports, little signal gain is observed for LIAD compared to direct absorption detection. This is mainly due to the inefficient coupling of the diode laser and the external cavity. Because LIAD is more sensitive to mechanical instabilities, while rationing with a reference signal as well as feedback stabilization is not possible, the actual detection limit achieved for the model compound mitoxantrone is 7 × 10−7 M or 1 × 10−4 absorption units. These values are about two orders of magnitude less favourable than those observed with double-beam absorption detection using the same diode lasers.