Process monitoring Raman techniques

Raman spectroscopy is particularly well suited for use in process monitoring and conttol. This chapter discusses Raman spectroscopy s attractive features as well as alerts the reader to aspects that may present ehallenges. The fundamental principles of the technique are reviewed. The reader will learn about instrumentation and options in order to make the most appropriate choices. Special aspects of performing quantitative Raman spectroscopy are discussed since these are required in many installations. Apphcations from many diverse fields are presented. The reader is encouraged to examine aU of the areas since there are good lessons and stimulating ideas in aU. 

A tunable pulsed laser Raman spectrometer for time resolved Raman studies of radiation-chemical processes is described. This apparatus utilizes the state of art optical multichannel detection and a-nalysis techniques for data acquisition and electron pulse radiolysis for initiating the reactions. By using this technique the resonance Raman spectra of intermediates with absorption spectra in the 248-900 nm region, and mean lifetimes > 30 ns can be examined. This apparatus can be used to time resolve the vibrational spectral o-verlap between transients absorbing in the same region, and to follow their decay kinetics by monitoring the well resolved Raman peaks. For kinetic measurements at millisecond time scale, the Raman technique is preferable over optical absorption method where low frequency noise is quite bothersome. A time resolved Raman study of the pulse radiolytic oxidation of aqueous tetrafluoro-hydroquinone and p-methoxyphenol is briefly discussed. 

NIR-Raman has found applications in the pharmaceutical industry. McCreery and colleagues reported the use of the technique for identification of pharmaceuticals inside amber vials. Even with the signal attenuation through the glass, adequate spectra were obtained for determination of vial content with 1-60 s integration times. Using a library of spectra, identification of the pharmaceuticals in the vials was performed and identification was found to display accuracy between 88% and 96%. This work demonstrated the potential of NIR-Raman for online process monitoring. 

Raman spectroscopy has been used to monitor a variety of industrial processes to improve product quality and process understanding [18,19]. The same features that make Raman spectroscopy a useftd technique for traditional process monitoring, such as the short analysis time and ease of optical sampling, mean that it can also be a useful tool for analyzing and understanding chemical reactions performed in continuous microreactors. 

Over the past years MIR, NIR, and Raman spectroscopy have been further developed to a point where each technique can be considered a potential candidate for industrial quality-control and process-monitoring applications. However, adding up the specific advantages and disadvantages of the individual techniques, NIR spectroscopy is certainly the most flexible and advanced alternative. 

Raman Spectroscopy Unlike conventional process monitoring techniques such as UV-Vis, chromatography, or NIR, Raman spectroscopy approach provides high chemical selectivity, requires no sample preparation, is unaffected by water, and is nondestructive. In the case of some of the materials of interest here, such as SBR,... 

Kazarian et al. [281-283] have used various spectroscopic techniques (including FUR, time-resolved ATR-FHR, Raman, UV/VIS and fluorescence spectroscopy) to characterise polymers processed with scC02. FTIR and ATR-FTIR spectroscopy have played an important role in developing the understanding and in situ monitoring of many SCF processes, such as drying, extraction and impregnation of polymeric materials. 

IR is one of three forms of vibrational spectroscopy that is in conunon use for process analytical measurements the other two being near-lR (NIR) and Raman. Each one of these techniques has its pros and cons and the ultimate selection is based on a number of factors ranging from sample type, information required, cost and ease of implementation. The sample matrix is often a key deciding factor. NIR has been the method of choice for many years within the pharmaceutical industry, and sample handling has been the issue, especially where solid products are involved. IR is not particularly easy to implement for the continuous monitoring of solid substrates. However, often there is no one correct answer, but often when the full application is taken into account the selection becomes more obvious. In some cases very obvious, such as the selection of IR for trace gas analysis - neither NIR nor Raman is appropriate for such applications. 

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