Laser Raman spectroscopy is evolving into a primary tool for the Identification of molecular water pollutants. This study pushes the limits of detectivity of carbon disulfide and benzene to ~ 20 ppm in water solutions using a high-resolution Raman spectrometer, cooled detectors, and photon counting techniques. The primary limiting factors were found to be the low throughput and the scattered light performance of the monochromator as well as insufficient laser energy.
An optomized design for a pollution-measuring instrument is suggested, and a prototype has been built which is useful with any value of excitation energy short of sample degrading. The present instrument scans spectrum windows with fixed preselecting filters followed by a small single monochromator with high throughput. No detector cooling or refinements in signal processing were attempted. The resulting detectivity with 20 mw of laser power was only 1000 ppm. However, the scattered light background or "optical noise" is unmeasurable except at the laser frequency, where it was a maximum of six percent of full scale measured against the 992 cm-1 Raman band of benzene. Equipped with an ion laser a practical field instrument capable of detectivity of 1 ppm will cost about $20,000. The instrument described herein can be built for $4,000, less laser.
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The work on which this report is based was supported in part by funds provided by the Office of Water Resources Research, United states Department of the Interior, as authorized under the Water Resources Research Act of 1964.
Bradley, Eugene B.; Frenzel, Charles A.; Reeves, John; McConnell, Robert; and Lane, Kay, "Detection and Identification of Molecular Water Pollutants by Laser Raman Spectroscopy" (1971). KWRRI Research Reports. 151.