Fiber optic scanning Raman spectrometer

Fig. 3.5-10 c shows another sample arrangement which makes use of a fiber-optical connection from the laser to the sample and back to the spectrometer. It is specially designed for the scanning of surface layers, e.g., of precious prints or paintings. The half spheric concave mirror reflects the portion of exciting radiation and Raman radiation back to the sample which has been. scattered by the sample and is not collected by the optical fiber. Thus the mirror as a component of a multiple reflection system enhances the observed intensity of the Raman lines by a factor of 2 to 8, depending on the properties of the sample. 

With the ever-increasing need to improve quality and productivity in the analytical pharmaceutical laboratory, automation has become a key component. Automation for vibrational spectroscopy has been fairly limited. Although most software packages for vibrational spectrometers allow for the construction of macro routines for the grouping of repetitive software tasks, there is only a small number of automation routines in which sample introduction and subsequent spectral acquisition/data interpretation are available. For the routine analysis of alkali halide pellets, a number of commercially available sample wheels are used in which the wheel contains a selected number of pellets in specific locations. The wheel is then indexed to a sample disk, the IR spectrum obtained and archived, and then the wheel indexed to the next sample. This system requires that the pellets be manually pressed and placed into the wheel before automated spectral acquisition. A similar system is also available for automated liquid analysis in which samples in individual vials are pumped onto an ATR crystal and subsequently analyzed. Between samples, a cleaning solution is passed over the ATR crystal to reduce cross-contamination. Automated diffuse reflectance has also been introduced in which a tray of DR sample cups is indexed into the IR sample beam and subsequently scanned. In each of these cases, manual preparation of the sample is necessary (23). In the field of Raman spectroscopy, automation is being developed in conjunction with fiber-optic probes and accompanying... 

The range of applications of Raman spectroscopy has also been extended by several important recent developments, such as Raman microscopy, which makes it possible to study extremely small samples. One can also analyze the surface of an extended inhomogeneous sample to obtain very high spatial resolution, or scan across a surface using fiber optics. It is also possible to use specially developed interference filters or holographic notch filters in certain applications as an alternative to a dispersing spectrometer, provided that one suppresses the fluorescence that would otherwise interfere with the measurements. 

Commercial analyzers mainly intended for laboratory use have been adapted with optical fibers to most spectrophotometers (Photonetics Sodety). Assemblies of fibers on spec-trofluorimeters [82,199] and Raman [167,177,193,200], infrared (IR), near-IR [189] mid-IR [201], and Fourier transform (FT) IR [202,203] spectrometers have already proved their value for remote spectroscopic sensing. However, these units are only partially adequate for continuous in situ process control because of their weak dynamic range after coupling of fibers and a maladjusted numerical aperture with respect to optical fibers. Photodiode-array instruments, without spectral scanning, are nevertheless the best for on-line control. This class includes the series of Hewlett-Packard HP 8450-8452 spectrophotometers and the Fluo 2(X)1 spectrometer (Dilor Society) [175] for fluorimetric and Raman determinations. 

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