Fibre-Optic Raman Instrumentation

All these applications have different instrumental configurations and in many cases the laser can be delivered to the analysis point via fibre optics. This provides an implementation challenge as laser exposure and eye safety are paramount concerns particularly in a manufacturing environment. Although mentioned briefly here Raman PAT applications are likely to be an area of significant growth over the coming years. 

The Raman technique has been readily adapted for on-line process analysis, especially in the pharmaceutical industry ". It has the benefits of mid IR, e.g. the ability to identify compounds from the vibrational fundamentals, without the constraints of mid IR, e.g. the limitations of the optical materials that can be used. Its popularity is also due in part to the excellent throughput of optical fibres for the radiation required for Raman, i.e. in the Vis and NIR regions. This use of optical fibre probes (Figure 9.14) facilitates easy in-line analysis because the sample can be remote from the instrumentation, even to hundreds of metres in distance. Fibre optic multiplexers are also available, allowing many samples to be analysed sequentially. Small laser diode sources and CCD detectors can be attached to the optical fibres and changed as required, rendering the overall device small and flexible. Radiation from the laser diode light source is transmitted to the sample by optical fibre... 

The availability of Raman spectrometers with fibre optic probes facilitates coupling with other analytical instruments. The use of Raman spectroscopy coupled with DSC and variable temperature/humidity X-ray powder diffraction [88] has been demonstrated. 

Modern Raman systems are ideally suited for at- or near-line analysis. Fibre-optic probes, which can be interfaced to CCD-Raman spectrometers with greater ease than to FT-Raman instruments, have greatly expanded the utility of Raman spectroscopy by taking the measurement capability to the sample [374], It is also relatively simple to interface Raman spectrometers to other techniques, such as chromatography, light scattering, XRD, DSC, etc. but this is not yet an active area of research. Everall [375] has reported off-line LC-Raman (LC-Transform) interfacing. 

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. 

Infrared and Raman instrumental advances, microspectroscopic techniques and fibre optics and new sampling methods have made possible many biological and medical applications. Correction for background and interference is automatically performed by most modern instruments. The use of statistical techniques and of derivative spectra for the examination of subtle differences in cases where bands overlap have been very useful. The direct examination of cells and tissues by infrared " can provide useful information on cellular composition, packing of cellular components, cell structure, metabolic processes and disease. Near infrared and Fourier Transform techniques may be applied to the study of food. ... 

Another attachment that can be effected to a Raman spectrometer is a remote probe. This consists of a fibre optic cable that passes the laser beam to a sample outside the conventional sampling chamber. This can be of use in many different applications, the most obvious of which is where the sample is too large or complex to fit into the instrument. In vivo biological studies utilize a fibre optic probe for the investigation of human tissue. Industrial process monitoring uses a Raman spectroscopic probe for online quality control during manufacture. 

Remote sampling in IR and Raman is a big development area - from measnrements of atmospheric air to sample probes attached by optical fibres to the rest of the instrument at some distance away. Another growing area is that of IR microscopes and spectral imaging (Section 4.5), which will be discnssed at the end of the section on imaging techniqnes. 

---