R aman Spectroscopy in Process Analysis

Abstract

Abstract Raman spectroscopy is a laser light scattering measurement in which changes in the frequency of the incident light as a result of scattering from a sample are measured. Raman spectroscopy is generally used to measure vibrational energy levels of molecules and is complementary in that regard to infrared (IR) spectroscopy which measures vibrational energy levels by the absorbance of IR radiation. Although more difficult in the past to perform than IR spectroscopy, recent technological changes have made Raman spectra easier to measure and with better sensitivity. Currently, Raman spectroscopy has about the same general sensitivity as IR spectroscopy and, in many instances, much simpler and more flexible sampling requirements. Raman spectroscopy is particularly useful for molecules containing some form of unsaturation, various inorganic species, halogenated materials, and homonuclear diatomic gases (such as H 2 , N 2 , and O 2 ) which are transparent in the IR. Increased application of Raman spectroscopy for a variety of problems has led to its use for on‐line measurements in industrial manufacturing processes. These applications generally involve remote placement of the spectrometer from the process and the use of fiber optics to take light to the sampling points in the process and return scattered light to the spectrometer for analysis. The use of fiber optics is one of the strengths of Raman spectroscopy for these applications. An additional strength is in the probe designs for focusing the laser light and collecting the scattered light at the sampling point. Probes have been designed which are insertable into processes and can withstand high temperature and pressure, or which can operate from a distance through a viewing window, thus completely avoiding physical contact with the sample. This allows Raman spectroscopy to function in difficult environments where other techniques cannot gain access. Sampling intervals with Raman spectroscopy are generally in the seconds to minutes time regime, well within the range needed for meaningful process control. The disadvantages of Raman spectroscopy often stem from absorbance of the incident laser light by the sample or impurities in the sample. This can lead to sample decomposition or burning if the absorbance is significant due to the amount of power under the focused laser light. Less severe absorbance can lead to fluorescence (absorbance followed by emission) which can easily produce enough light to obscure the Raman scattering. Some samples may not have distinct enough spectral information for the problem at hand and Raman would be ruled out on that basis. Raman will compete against IR (both mid‐ and near‐) for many process applications. Mid‐IR offers a high level of spectral information, but poor options for fiber optics. Near‐IR has much less spectral information for most problems, but good choices for fiber optics. Raman offers the best of both IR approaches, having a high level of spectral information combined with good choices for fiber optics. Specific problems need to be evaluated on an individual basis as to whether Raman, mid‐IR, or near‐IR offers the best approach using knowledge about the spectral information present by each technique and the sampling options for on‐line monitoring of the process.