INSTRUMENTATION AND APPLICATIONS FOR DIODE ARRAY SPECTROMETERS (CHEMICAL SPECTROSCOPY, IMAGE DETECTOR)

This thesis describes several new instrumental advances and novel applications for diode array detectors in chemical spectroscopy. The instrumental aspects include an optical feedback method for stabilizing a diode array spectrometer, and an efficient method for rapid data acquisition. The novel applications include processing methods for data collected with the diode array spectrometer as a monitor for liquid chromatography and multicomponent quantification. In the optical feedback experiments, a beam splitter is used to divert a fraction of source intensity to a phototransistor, the signal from which is used to control the output from a programmable power supply that controls source intensity. Any tendency for source intensity to change with time is compensated by the feedback system. Results with the optical feedback system indicate that instrument stability is improved by more than an order of magnitude in terms of relative standard deviation in percent transmittance, making stability comparable to that for dual beam systems. The approach to data acquisition involves the use of a modular memory system. This fast memory can collect 64 spectral scans in as little as 1.6 s, for an overall data rate of 40 kHz. The rate of data acquisition is limited, not by the data system, but by the rate of scanning the diode array. Data rates as fast as 1 MHz appear possible. An application to a spectroelectrochemical technique is given. Results for the combined use of liquid chromatographic separation with multiwavelength absorption detection, as applied to the quantification and identification of polynuclear aromatic hydrocarbons, indicate that significant improvements can be achieved by the use of multiple regression methods and derivative spectroscopy. Methods are developed to aid in selection of the best wavelength range, in order to most effectively curtail some of the errors associated with multicomponent quantifications