<title>On-line monitoring by Raman spectroscopy: instrument control and calibration</title>

Abstract

We have demonstrated the utility of a fiber-optic Raman spectrometer with a charge coupled device (CCD) detector for measurements on a variety of chemical systems. Applications have included remote characterization of nuclear wastes, monitoring the heterogeneously catalyzed conversion of quadricyclane to norbornadiene, and monitoring emulsion and suspension polymerization reactions. Instrument control and calibration are essential elements for successful on-line monitoring. We wish to be able to compare measurements made with on- line systems with reference measurements made on laboratory instruments. Raman spectra are fundamental properties of molecular species. With suitable control and calibration of the measurement systems, spectra taken on different systems can be directly compared. We have built into our measurement system both hardware and software to provide automated wavenumber and intensity calibration and correction. The wavenumber axis calibration is based on measurement of the spectrum of atomic lines due to neon. The neon spectrum is provided by an inexpensive and compact lamp connected to the spectrometer by a dedicated optical fiber. In using atomic lines for calibration we must be able to determine peak positions reliably with an uncertainty much less than the sampling interval in the digitized spectrum. We describe a way of doing this as part of our calibration procedure. Intensity axis calibration involves correction for pixel-to-pixel and wavelength-dependent sensitivity variation of the detector and wavelength- and time-dependent throughput variations of the spectrograph and optical fiber probe. The data needed for these corrections are obtained by measurements with a calibrated white light source and with a standard sample. The white light source is a compact tungsten halogen bulb with its own dedicated optical fiber. The standard sample, a stable, readily available material, is incorporated into the measurement system at the sample end of the fiber optic probe. A computer-controlled, stepper motor driven positioner places the neon, white light and sample fibers at the focus of the spectrometer entrance optics as required.