Practical Application of Two Laser-Spectroscopic Techniques.

Abstract

Equipment design and control and data analysis programs for two laser spectroscopic techniques, inverse Raman spectroscopy and thermal lens spectroscopy, are described. An application of each method is demonstrated. Inverse Raman spectroscopy is used to obtain high quality Raman spectra rapidly. Thermal lens spectroscopy is applied to the detection of chromatographic b and s in high performance liquid chromatographic systems. Inverse Raman spectra are obtained under control of a small laboratory computer. The computer system, its functions, and capabilities are described. Several high quality, medium resolution (1-2 cm('-1)), spectra, obtained over a 1000-1500 cm('-1), range at rapid scan rates (over 300 cm('-1)/minute), are shown. Methods for obtaining higher scan rates (1000-1500 cm('-1)/minute) and wider scan ranges are suggested. Thermal lens spectroscopy is shown to be a sensitive absorption technique especially suited to application as a detector for high performance liquid chromatography. Convective effects in non-flowing systems are shown to be severe at low concentration levels. Convective effects limit the sensitivity of thermal lens detection for some time dependent studies, such as reaction kinetics. A thermal lens detector for HPLC is shown with noise levels approaching 1 x 10('-6) absorbance. The applicability to conventional analytical HPLC with detector volumes of 8 (mu)l is demonstrated. Extension to 0.5 (mu)l volume cells is demonstrated with a microbore column (25 cm x 1 mm, packed with C(,18), 10 (mu)m), 1 (mu)l sample injections, and methanol/water mobile phases at 30-50 (mu)l/minute flow rates. Experiments are presented which show that very low power lasers (under 10 mW) can successfully be used to obtain sensitive absorbance measurements with this detector.