Degenerate four-wave mixing is demonstrated as an effective and sensitive laser analytical spectroscopic method for circular dichroism measurements. A forward-scattering degenerate four-wave mixing optical setup is used to obtain simple optical alignment, highly efficient wave mixing, and very effective use of low laser power. This nonlinear laser-based circular dichroism method offers many advantages, including easy and efficient optical signal collection, use of very short analyte path lengths (e.g., 0.1 mm), and excellent detection sensitivity that is comparable or better than conventional laser-based or non-laser-based circular dichroism methods. Using an analyte path length of only 0.1 mm, and a probe volume of 98 pL, a circular dichroism mass detection limit of 0.68 pg or 2.8 fmol is reported for (+)Co(en)3(3+).

A novel four-wave mixing technique for the measurement of circular dichroism in optically active liquid samples is demonstrated. When two cross-polarized continuous-wave laser beams are crossed at a small angle in a circular dichroic liquid, a weak thermal grating is produced with a phase depending on the sign of the circular dichroism. The polarization of one of the beams can be modified to allow coherent interference with an intensity grating-induced thermal grating. A probe beam scattering from the composite grating results in a coherent signal beam that reveals the sign and the magnitude of analyte circular dichroism. The use of this technique to optimize the signal-to-noise ratio in the presence of scattered light and laser noise is discussed.

A fiber-optic degenerate four-wave mixing (D4WM) probe for the measurement of small absorptions in liquid-phase samples is described. Laser D4WM is a nonlinear laser spectroscopic technique that has proven to be highly sensitive for the detection of trace analytes in condensed-phase media. A significant improvement in the forward-scattering optical arrangement of D4WM is demonstrated by using optical fibers for both laser light input and output. There is considerable flexibility inherent in the design since the system may be used in three configurations: (1) the simplest case of transmitting the signal radiation by optical fiber to the detection electronics, (2) the case of guiding the excitation beams to the analyte by polarization-maintaining optical fibers, and (3) the combination of both. The optical fiber-based D4WM system is shown to be an effective and sensitive laser analytical spectroscopic method for trace analysis, offering advantages such as detection in very small probe volumes, remote and in situ analysis, and convenient and efficient optical alignment enhancements obtained by the use of optical fibers.

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A circular dichroism (CD) detector based on laser four-wave mixing (FWM) is demonstrated using separate injections of analyte enantiomers onto a standard silica-based microbore high-performance liquid chromatography (HPLC) column. Using the chiral column, a preliminary "detected" mass detection limit of 180 pg is determined inside a laser probe volume of 200 pL, corresponding to a circular dichroism detection limit, DeltaA, of 2.2 x 10(-5) for (-) camphorquinone. Detection sensitivity levels are dramatically improved when our FWM-CD detector is interfaced to a microbore system due to the lower mobile-phase flow rates and the smaller sample concentrations required for the analysis. Using the microbore column, a preliminary circular dichroism detection limit, DeltaA, of 1.6 x 10(-6) and a preliminary concentration detection limit of 4.1 x 10(-4) M are determined for camphorquinone. This corresponds to a "detected" mass detection limit of 33 pg for the chiral compound. Laser wave mixing offers better detection limits than conventional circular dichroism detection methods and, hence, offers many promising applications.