NIR Raman spectrometry

Since the 1970s, Raman spectrometry has been using visible laser lines as the most common excitation sources. It was generally believed that NIR laser lines were not suitable for obtaining high-quality Raman spectra, though since Hirshfeld and Chase [58] and Fujiwara et al. [59] reported successful NIR Raman spectrometry measurements in 1986, this technique has advanced dramatically and NIR Raman spectrophotometers are now commercially available. 

In light of the fact that Fourier transform instrumentation was largely responsible for expanding Raman spectroscopy into the analytical laboratory, it is perhaps interesting to consider why Raman spectroscopy is so popular today but Fourier transform Raman does not play the dominant role. After a discussion of the poor sensitivity of NIR Raman spectrometry using a scanning monochromator with PMT detection in Section 18.1, it was stated To improve this situation, either a multichannel or multiplex measurement was needed and the multiplex measurement came first. Multichannel measurements came very shortly afterward, however, and instruments based on polychromators with silicon-based charge-coupled-device (CCD) array detectors have become more popular than FT-Raman spectrometers. In this section we compare the performance of FT- and CCD-Raman spectrometers. 

C.A. McGill, A. Nordon, D. Littlejohn, Comparison of in-line NIR, Raman and UV-visible spectrometries, and at-line NMR spectrometry for the monitoring of an esterification reaction. Analyst, 127, 287-292 (2002). 

McGill, C.A. Nordon, A. Littlejohn, D. Comparison of In-Line NIR, Raman and UV-Visible Spectrometries, and At-Line NMR Spectrometry for the Monitoring of an Esterification Reaction Analyst 2002, 127, 287-292. 

Any interferometer that is capable of at least 2-cm resolution at the excitation wavelength (1064 nm) can be used for FT-Raman measurements. Thus just about every interferometer that is sold today could be used for this pmpose. Because the throughput should be optimized, however, it is better to use an interferometer with a 4- or 5-cm-diameter beamsplitter than a smaller size. Since NIR Raman measurements with a Nd YAG laser are made between about 9300 and 6000 cm , the optimal beamsplitter should be one with a quartz substrate. While Cap2 and extended-range KBr beamsplitters have been used for FT-Raman spectrometry, it is usually advisable to purchase a beamsplitter that has been optimized for these measurements, as the higher the beamsplitter efficiency, the higher will be the signal-to-noise ratio of the spectrum. 

Two types of FT-Raman spectrometer are commercially available. The first is an instrument that can be operated as a conventional FT-NIR spectrometer and modified to measure FT-Raman spectra the second is a dedicated FT-Raman spectrometer. Examples of the two types of instruments are shown in Figures 18.8 and 18.9. Provided that all the considerations discussed above are accounted for properly, acceptable Raman spectra can be measured on both types of instruments. However, since the dedicated instrument has been designed explicitly for Raman spectrometry, the optical efficiency is usually superior. If an instrument is going to be used primarily for Raman spectrometry, we recommend that a dedicated instrument be purchased. If only a few Raman measurements are expected to be needed each week and NIR transmission and refiection measurements are also needed, it may well be financially advisable to purchase the more versatile instrument. 

For mid-IR, NIR and Raman spectrometry, the instrumentation is different, but the main components of spectrometers are all required. 

Advanced techniques like molecularly imprinted polymers (MIPs), infrared/near infrared spectroscopy (FT-IR/NIR), high resolution mass spectrometry, nuclear magnetic resonance (NMR), Raman spectroscopy, and biosensors will increasingly be applied for controlling food quality and safety. 

High performance spectroscopic methods, like FT-IR and NIR spectrometry and Raman spectroscopy are widely applied to identify non-destructively the specific fingerprint of an extract or check the stability of pure molecules or mixtures by the recognition of different functional groups. Generally, the infrared techniques are more frequently applied in food colorant analysis, as recently reviewed. Mass spectrometry is used as well, either coupled to HPLC for the detection of separated molecules or for the identification of a fingerprint based on fragmentation patterns. ... 

One indication of the developing interest in PATs in the pharmaceutical area is the number of book chapters and review articles in this field that have appeared in the last few years. Several chapters in The Handbook of Vibrational Spectroscopy3 are related to the use of various optical spectroscopies in pharmaceutical development and manufacturing. Warman and Hammond also cover spectroscopic techniques extensively in their chapter titled Process Analysis in the Pharmaceutical Industry in the text Pharmaceutical Analysis.4 Pharmaceutical applications are included in an exhaustive review of near-infrared (NIR) and mid-infrared (mid-IR) by Workman,5 as well as the periodic applications reviews of Process Analytical Chemistry and Pharmaceutical Science in the journal Analytical Chemistry. The Encyclopedia of Pharmaceutical Technology has several chapters on spectroscopic methods of analysis, with the chapters on Diffuse Reflectance and Near-Infrared Spectrometry particularly highlighting on-line applications. There are an ever-expanding number of recent reviews on pharmaceutical applications, and a few examples are cited for Raman,7 8 NIR,9-11 and mid-IR.12... 

Techniques that apply to in situ analysis of the dosage form, its precursor granulations, or powders are discussed. Applications of solid-state NMR, FTIR microspectroscopy, visual and scanning electron microscopy, Raman spectroscopy, NIR analysis, thermal techniques, mass spectrometry, and imaging techniques are presented. 

IR spectrometry as described above is a really powerful tool to determine the structure of the H-bond network established by H2O molecules that are embedded in macromolecules. It is most certainly due to become a basic tool, as it gives unique information on this network. It is nevertheless not the only method that will be used in the future. Two other kinds of spectroscopic methods will certainly also be of interest. They are simpler methods NIR (near infrared) and Raman spectroscopies. These are routine methods that are complementary to IR spectrometry. 

In spite of these considerations MIR proved to be by far the most effective of a range of techniques studied in a wide ranging assessment of recognition methods that also considered NIR, FT Raman spectroscopy. Pyrolysis Mass Spectrometry, Pyrolysis Infrared Spectroscopy and Laser Induced Emission Spectral analysis [6]. 

In principle, all kinds of spectroscopic techniques lend themselves to on-line measurements. Only a very few are practical. Although low-field NMR has been used to measure various material properties by applying empirical relationships, NMR is still not a realistic proposition for on-line measurements. Ironically, ETIR spectroscopy suffers from too much sensitivity. Typically, good spectra can be obtained only from very thin polymeric films (or solutions). Attenuated total reflection (ATR) probes, in which only a fraction of the IR light penetrates a very short distance into the sample, reduce the problem of excessive sensitivity. However, they aggravate the problems of variations in the baseline and nonlinear response. The latter problem also obstructs the use of UV spectrometry for monitoring polymerization reactions. Of the remaining options, near-infrared (NIR) and Raman spectroscopy are the most attractive. 

---