Abstract

A highly sensitive and selective laser-based circular dichroism detector for flowing liquid systems, specifically liquid chromatography (LC), is presented. This detector is quite useful for the analysis of biologically related molecules, since optical activity is detected for low nanogram (ng) quantities of samples. Both transmission detected (TDCD) and fluorescence detected circular dichroism (FDCD) were developed for LC. For LC-TDCD, an argon ion laser is operated at 488 nm and 20 mW, with high-frequency polarization modulation employed to reduce laser amplitude noise to1 part in 10⁶, i.e., a signal-to-noise ratio of 10⁶. This is in contrast to a signal-to-noise ratio of 10² for an argon-ion laser before any effort is made to stabilize the power fluctuations. Also, detection selectivity is demonstrated since optically inactive, yet absorbing, molecules do not produce an appreciable detector response. Presently, the optimum modulation frequency range is from 500 to 700 kHz for our system. Similarly, for LC-FDCD, a He-Cd laser is operated at 325 nm, with ~8-mW power, at 150 kHz. Specifically, riboflavin is detected in LC-FDCD at subnanogram levels with optical activity information.

© 1986 Optical Society of America