Types of Raman Instrumentation

Similar to the infrared instruments, there are two general types of Raman instruments the dispersive and FT Raman spectrometers. The FT Raman instrument incorporates an interferometer that provides similar advantages to those given by the interferometer of an FTIR spectrometer (for further details, see Ref. [3]), although modem dispersive instmments with CCD detectors can measure a range of frequencies simultaneously. 

In the early 1990s, many groups purchased the new type of Raman instrument to study CVD diamond. They used HeNe excitation because this resonantly enhances the Raman signal from the graphitic, undesired, material deposited with CVD diamond. Much of the early Raman studies of DLC were done by these groups and, understandably, many of the Raman instrument purchasers in the disk industry followed their lead. 

Figure 3.5-1 Types of Raman spectrometers a scanning spectrometer with triple additive monochromators, S source, D detector b instrument with subtractive double monochromator combined with polychromator and array detector AD c Rayleigh filter RF with polychromator and array detector.

It is not in the scope of the present chapter to review all possible experimental setups for the various types of Raman scattering classical, microprobe, Fourier transform (FT), coherent anti-Stokes (CARS), surface enhanced (SERS), hyper (HRS), photo-acoustic (PARS), and so forth (see, e.g.. Refs. 29-31). A basic Raman-scattering instrument requires a laser-light source, an appropriate sample holder, a sample illumination optical unit, a scattered-light collection optical unit (these two may be combined in one system), a disperser (spectrometer) or an interferometer, a light detection unit, a recorder, and an appropriate microcomputer able to drive, control, and record all of the experimental parameters as well as the results and their processing. 

The ability of Raman instruments to collect Raman spectra directly from samples contained in either glass or polymer containers allows for analysis of both bulk and final materials. In addition, the lack of sample preparation when compared to infrared techniques can be crucial for some applications (i.e., polymorphism, where sample preparation dissolving in a solvent or grinding can obscure or eliminate critical information). The types of analysis, already successfully demonstrated and of potential interest to the pharmaceutical industry include analyses of incoming materials [110], active form, after dilution with... 

Instrumental Interface. Gc/fdr instmmentation has developed around two different types of interfacing. The most common is the on-the-fly or flow cell interface in which gc effluent is dkected into a gold-coated cell or light pipe where the sample is subjected to infrared radiation (see Infrared and raman spectroscopy). Infrared transparent windows, usually made of potassium bromide, are fastened to the ends of the flow cell and the radiation is then dkected to a detector having a very fast response-time. In this light pipe type of interface, infrared spectra are generated by ratioing reference scans obtained when only carrier gas is in the cell to sample scans when a gc peak appears. 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

Clearly, the potential applications for vibrational spectroscopy techniques in the pharmaceutical sciences are broad, particularly with the advent of Fourier transform instrumentation at competitive prices. Numerous sampling accessories are currently available for IR and Raman analysis of virtually any type of sample. In addition, new sampling devices are rapidly being developed for at-line and on-line applications. In conjunction with the numerous other physical analytical techniques presented within this volume, the physical characterization of a pharmaceutical solid is not complete without vibrational analysis. 

FT Spectrometers FT spectrometers (Figure 3) differ from scanning spectrometers by the fact that the recorded signal is an interferogram [14] (see Chapter 6.2). They can be coupled to a microscope or macrochamber with an FPA detector. FT chemical imaging systems (CISs) are available for Raman, NIR, and IR spectroscopy. However, they can only be considered as research instruments. For example, most IR imaging systems are FT spectrometers coupled to microscopes. This type of spectrometer allows the acquisition of spectra in reflection, attenuated total reflection (ATR), or transmission mode. 

In renewable energy processes, the various types of IR analyzers (including the H2-sensing Raman version) will play an important role. IR instruments were one of the first analyzers to be moved from the laboratory to the pipeline, and the technology is available for use with gas, liquid, or solid... 

Several other types of filters are discussed by Chase (5, 7). Filters are also necessary to remove the optical output of the He-Ne laser (used for referencing) because the laser has optical axes colinear with the main laser source. Since the detectors used in FT-Raman are sensitive to the He-Ne wavelength, the laser is a source of interference. Here, plasma emission filters can be used. The white light of the instrument is filtered with a near-IR cutoff filter. A final filter may be used in front of the detector. 

Recent advances in solid state detectors have led to more efficient laser Raman spectrometers. These spectrometers are based on multichannel detectors (MCD) and are at least an order of magnitude faster than the systems based on PMT detector. However, at present these systems do not match the resolution capabilities of the scanning systems. The Spex Triplemate 1877 A is a new generation instrument which can be fitted with an intensified diode array consisting of 1024 elements. Figure 4.6.3 shows spectra obtained with these two types of detectors. 

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