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Novel Multiwave Mixing Spectroscopic Methods for Ultrasensitive Circular Dichroism Measurements in Liquid Phase Media

出版	University of California, San Diego, 1995
URL	http://books.google.com.hk/books?id=lDZBAQAAIAAJ&hl=&source=gbs_api

註釋This thesis describes the evolution of novel laser-based multiwave mixing spectroscopic methods for sensitive measurements of the circular dichroism (CD) of chiral samples. The objective of this research is to achieve improved detectability and sensitivity compared to conventional circular dichroism methods for ultrasmall sample amounts. Degenerate four-wave mixing (DFWM) is demonstrated as an effective and sensitive laser analytical spectroscopic method for CD measurements. This nonlinear laser-based CD method offers advantages including easy and efficient optical signal collection, use of short analyte path lengths (e.g., 0.1 mm), and excellent detection sensitivity. A novel four-wave mixing (FWM) technique for the detection of CD in liquids is described that relies solely on polarization gratings for signal generation. When two cross-polarized laser beams interact in a circular dichroic sample a weak thermal grating is produced with a phase depending on the sign of the CD. The polarization of one 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 signal that reveals the sign and magnitude of the CD. Also discussed is a method that employs a pulsed laser and polarization modulation to measure CD in liquids that absorb in the ultraviolet region of the spectrum. By combining intensity and polarization gratings to detect CD in a phase-sensitive method similar to heterodyne techniques used in Raman and frequency modulation spectroscopies, it is possible to achieve signal levels for [Delta][epsilon]/[epsilon] that approach the theoretical maximum. The use of FWM-CD for detection in chiral high performance liquid chromatography (HPLC) is also discussed. Although chiral HPLC has advanced considerably, detector technology has many shortcomings, especially with respect to small sample volumes. FWM-CD offers excellent detection sensitivity, even when using very short sample path lengths. Finally, a fiber-optic DFWM probe for the measurement of weak absorptions in liquids is described. Improvements in the optical arrangement of DFWM is demonstrated using fibers for both laser light input and output. The fiber-based DFWM system is shown to be an effective and sensitive analytical spectroscopic method for trace analysis.