Raman spectroscopy system calibration

These issues are not unique to Raman spectroscopy, but users may not be as aware of potential problems as for other techniques. Good understanding of the chemistry and spectroscopy of the system being modeled along with well-designed calibration experiments can help prevent problems. The power of the technique can be a hindrance if not recognized and managed. 

Bauer et al. describe the use of a noncontact probe coupled by fiber optics to an FT-Raman system to measure the percentage of dry extractibles and styrene monomer in a styrene/butadiene latex emulsion polymerization reaction using PLS models [201]. Elizalde et al. have examined the use of Raman spectroscopy to monitor the emulsion polymerization of n-butyl acrylate with methyl methacrylate under starved, or low monomer [202], and with high soUds-content [203] conditions. In both cases, models could be built to predict multiple properties, including solids content, residual monomer, and cumulative copolymer composition. Another study compared reaction calorimetry and Raman spectroscopy for monitoring n-butyl acrylate/methyl methacrylate and for vinyl acetate/butyl acrylate, under conditions of normal and instantaneous conversion [204], Both techniques performed well for normal conversion conditions and for overall conversion estimate, but Raman spectroscopy was better at estimating free monomer concentration and instantaneous conversion rate. However, the authors also point out that in certain situations, alternative techniques such as calorimetry can be cheaper, faster, and often easier to maintain accurate models for than Raman spectroscopy, hi a subsequent article, Elizalde et al. found that updating calibration models after... 

IQiowledge of parameters such as reactivity ratios, is necessary for synthesis of polymer based resists, and an accurate method of analysis should be useful in various areas associated with resist development such as quality control. Raman spectroscopy provides a convenient, absolute, nondestructive method for compositional analysis of polymer systems which, if an internal standard is present, does not require standards of known composition or ancillary calibration curves. The accuracy, with appropriate selection of experimental conditions such as slit width and integration time, is limited only by the instrumentation. 

Summary The use of the on-line FT-Raman spectroscopy for monitoring a multi-step hydrosilylation reaction combines all the advantages of an on-line analytical tool (like real time measuring results, a direct view into the reaction, and no off-line sample collection) with the requirements for the application of technology in production plants, e. g., low calibration effort within a wide temperature range, stable calibration, simple system handling for the operator, small sized equipment at the reaction vessel, and no contact with the reaction media. 

Quantitative analysis had not been as common as in IR spectroscopy until recently, due to the high cost of Raman instruments. With prices for Raman systems dropping below 40,000, and even as low as 10,000, the use of Raman spectroscopy for quantitative analysis is increasing. Quantitative analysis requires measurement of the intensity of the Raman peaks and the use of a calibration curve to establish the concentration-intensity relationship. The intensity of a Raman peak is directly proportional to the concentration ... 

Raman Scattering Spectroscopy (Renishaw 2000 system), with an Ar -ion laser (>, = 514.5 nm) in backscattering geometry, was used to investigate structural modifications of samples. The Raman shift was calibrated for the diamond peak at 1332 cm. All measurements were carried out in air at room temperature. 

In process control systems it is essential to develop rapid on-line monitoring techniques to acquire structural parameters such as crystallinity, and orientation. By controlling these structural parameters the end use properties may be influenced which are essentially defined by these parameters. In order to use laser Raman spectroscopy for such purposes, calibration systems need to be developed using an independent technique. 

Raman Spectroscopy Historically, Raman spectroscopy was never considered a sensitive technique because only 1 in 10 photons emitted from a molecule is collected. However, Raman systems have improved tremendously in the last several years. It is no longer deemed an insensitive, irreproducible, fluorescence-dominated technique. Raman is a versatile technique capable of providing information on several parameters simultaneously, such as monomer concentration and particle size. Raman is especially amenable for monomer detection in water-soluble polymers because symmetric vinylic monomer structures are good Raman scatterers and water has a weak signal. To that end, Raman is a complementary technique to FTIR and can be used to monitor monomer concentration and conversion. By employing a near-IR laser (785 nm) which removes most of the fluorescence, along with sharp monomer and polymer peaks that are often separated, monomer concentrations may be determined with univariate calibration. Additionally, since Raman is sensitive to the local molecular environment, it may be used to provide particle size information. 

See also Calibration and Reference Systems (Regulatory Authorities) FT-Raman Spectroscopy, Applications IR Spectrometers IR Spectroscopy, Theory Near-IR Spectrometers Raman Spectrometers. 

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