Near-infrared Stokes and anti-Stokes Raman spectrometry of explosives.

摘要

The analysis and identification of explosive threats has become of great concern to law enforcement personnel over the last 10 years. During the same period of time, Raman spectroscopic instrumentation has experienced a significant increase in performance and a decrease in size and complexity. In this dissertation the use of several instrumental approaches to the measurement of fluorescence-free Raman spectra is described with an aim of developing a field-portable explosives analyzer.; Anti-Stokes Raman spectra of 27 explosive materials were obtained with 1064-nm excitation using fiber-optic sampling and a dispersive spectrograph equipped with a charge-coupled device (CCD) array detector, over a range of −250 to −1650cm−1. Fiber-optic probe sampling permitted spectral acquisition within twelve meters and 240s. The utility of an anti-Stokes correction routine was demonstrated which allowed anti-Stokes spectra measured with 1064-nm excitation to be successfully searched and identified against libraries of Stokes spectra obtained using an FT Raman system equipped with a 1064-nm Nd:YAG laser.; A NIR laser (1064-nm) dispersive Raman spectrometer with germanium array detection was used to measure the Stokes-shifted Raman spectra of explosives. By using a germanium array detector, spectral features from 250 to 1800cm −1 could be observed for the majority of compounds studied. Spectra were obtained within 240s. The utility of this instrument is compared to other previous identified instrumental approaches, with particular regard to fieldability of a Raman-based explosive analyzer.; Stokes Raman spectra of 23 explosives with 785 and 830-nm excitation, and 1064-nm anti-Stokes Raman spectra, were compared using fiber-optic sampling and a no-moving parts spectrograph. 830-nm excitation offers better fluorescence rejection than 785-nm, which is key for the analysis of fluorescent explosives. With a high-powered 1064-nm laser, anti-Stokes measurements did not yield spectra of as high quality as 785-nm and 830-nm Stokes Raman measurements. FT-Raman remains the preferred method for fluorescence-free analysis of these explosive materials in the laboratory, but 830-nm excitation is preferred for a field-portable instrument.; This work demonstrates that the use of an 830-nm based fiber coupled Raman spectrometer coupled with post-processing baseline correction, and library searching is the preferred approach for the measurement of Raman spectra in the field.