Raman spectroscopy process

Principles and Characteristics As already indicated in Chp. 1.2.3, Raman scattering induced by radiation (UV/VIS/NIR lasers) in gas, liquid or solid samples contains information about molecular vibrations. Raman specfioscopy (RS) was restricted for a long time primarily to academic research and was a technique rarely used outside the research laboratory. Within an industrial spectroscopy laboratory, two of the more significant advances in recent years have been the allying of FT-Raman and FTIR capabilities, coupled with the availability of multivariate data analysis software. Raman process control (in-line, on-line, in situ, onsite) is now taking off with various robust commercial instrumental systems equipped with stable laser sources, stable and sensitive CCD detectors, inexpensive fibre optics, etc. With easy interfacing with process streams and easy multiplexing with normal (remote) spectrometers the technique is expected to have impact on product and process quality. 

In situ measurements in industry must be extrapolated to on-plant monitoring. The feasibility of using fibre optic coupling between the Raman experiment and the FT interferometer has been demonstrated. For on-line use special designed probes can withstand up to 300°C and 15,000 psi. Because Raman light can remotely be focused, it is even possible to measure in a non-invasive mode (for example through a specified reactor window). A portable process Raman analyser enables both in-line and at-line measurements. 

Choosing a suitable Raman spectrometer for on-line process analysis requires different criteria from laboratory analysis. Some key considerations are laser safety, ruggedness, repeatability, long-term and environmental stability, high uptime, calibration ttansferability, ease of operation and maintenance, smart diagnostics for analyser performance, and industry-standard communication. Many processes require the analysis to be performed 

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. 

Process monitoring using Raman spectroscopy (mainly in its NIR Fourier transform variant) is proposed for QA/QC purposes, on-line polymer analysis, in situ cure kinetics, emulsion polymerisation, non-invasive analysis of physical parameters (in situ crystallinity determination, etc.) and reactor compositions, real-time measurements, molecular interactions, and components in aqueous solutions. 

---

One of the now-classic examples of process Raman spectroscopy is monitoring the efficiency of converting the anatase form of titanium dioxide to the rutile form by calcining [67-69]. The forms have dramatically different spectra, enabling simple univariate band ratio models to be used. The uncertainty in the mean... 

Raman spectroscopy was discovered over 75 years ago but has only been a viable process tool for 10-15 years. However, there has been an astounding increase in process Raman spectroscopy examples in the last five years. The United States Food and Drag Administration s (US FDA) endorsement of process analytical technology clearly set off an explosion of activity. Problems that sometimes sidelined Raman in the past, such as fluorescence or highly variable quantitative predictions from samples that were too small to be representative, are being re-examined and leading to new technology. In turn, that helps open, or perhaps reopen, new application areas. The availabihty of easy to use Raman instrumentation at many prices also helps with that. 

I. R. Lewis, Process Raman spectroscopy, in Handbook of Raman Spectroscopy From the Research Laboratory to the Process Line, I.R. Lewis and H.G.M. Edwards (Eds), Practical Spectroscopy Series 28, Marcel Dekker, New York, 2001. 

NIR, Raman would be expected to offer advantages such as ease of use for quantitative analysis. The reason for less widespread use of process Raman spectroscopy is due in part to more expensive equipment, relative to NIR. A broader implementation of process Raman spectroscopy in the pharmaceutical industry has previously also been hampered by inherent weaknesses in sampling in remote measurements on solids. This is discussed further in Section 10.3. 

United States patent 5,684,580 by Cooper etal. of Ashland Inc. discusses monitoring the concentration of benzene and substituted aromatic hydrocarbons in multiple refinery process streams and using the results for process control.56 Xylene isomers can be differentiated by Raman spectroscopy, making it technically preferable to NIR. This patent is a good example of process Raman spectroscopy and subsequent process control. 

What is the state of process Raman spectroscopy right now Is it still slowly building Is it a niche market and unlikely to expand Is it thriving and flourishing Since almost all industrial applications will use a commercial instrument, the instrument vendors are the only ones who can really answer these questions but are likely to be bound by confidentiality agreements with their customers. In the absence of publications or other public disclosures, it is useful to examine the factors that may be limiting or appearing to limit the use of Raman spectroscopy for process control. 

Educated inferences suggest that the field is thriving despite the limited publications on full process control installations. The number of Raman vendors is increasing, along with mergers and acquisitions activity. Additionally, the list of companies that have or are willing to publicly acknowledge their use of process Raman spectroscopy or advocate its utility is impressive. Some of these are listed in Table 5.2. 

Table 5.2 Companies publicly acknowledging or willing to acknowledge their use or advocacy of process Raman spectroscopy...

For others, Raman spectroscopy is a new or exotic technique. Compared to other process analytical techniques, there are still relatively few published examples of process Raman spectroscopy, and no published studies, as yet, of its long-term performance. This kind of information is imperative for new users to be able to accurately assess the role Raman spectroscopy could play in their organizations. It also helps supplement and balance information received from vendors. While the technique is long past its infancy, the community of potential users may perceive the situation differently. 

X-ray diffractograms for pristine and PEI-wrapped arc-SWCNTs are presented in Figure 10.6. The peak corresponding to SWCNT bundles (appearing at low diffraction angles (43), 20 = 6°) disappear after the purification and wrapping processes. Raman spectroscopy can also be applied to characterize these dispersions. The G/D ratio decreases drastically for the shrouded carbon nanotubes after the wrapping process (see Table 10.1), attributed to an increase in the... 

Scattering spectroscopy measures the light that a sample scatters at a certain wavelength, incident angle, or polarization angle. This technique is somewhat similar to emission spectroscopy, but scattering occurs much more quickly than the absorption/emission process. Raman spectroscopy is the most useful example of this type. 

IR Lewis. Process Raman Spectroscopy. In IR Lewis and HGM Edwards, eds. Handbook of Raman Spectroscopy. New York Marcel Dekker, 2001, pp. 919-973. 

The Raman scattering effect differs from ordinary scattering in that part of the scattered radiation suffers quantized frequency changes. These changes are the result of vibrational energy level transitions that occur in the molecules as a consequence of the polarization process. Raman spectroscopy is discussed in Chapter 18. 

---