Raman applications

In practical application, Raman sensors exclusively use frequency-stabilised laser sources to compensate for the low intensity of the Raman radiation. For Raman sensors, prevalently compact high-intensity external cavity laser diodes are used, operated in CW (continuous wave) mode. These diode lasers combine high intensity with the spectral stability required for Raman applications and are commercially available at various wavelengths. 

In the past, process Raman applications largely were developed and implemented by formally trained Raman spectroscopists, aided by process engineers and other personnel. However, there are far fewer Raman spec-troscopists than chemical, process, or production engineers or other types of personnel. Increasingly, these... 

Major categories of process Raman applications include reaction monitoring, in-process quality checks, and mobile or field point measurements. Quality control laboratory applications often are converted to a continuous process monitoring approach, or could simply be viewed as part of a larger production process. 

There is a fast growing number of Raman applications in the pharmaceutical industry reported in the literature. Still, the vast majority of these reports are on lab-based measurements for determination of physical form changes and fairly little is reported about the use of Raman in pharmaceutical manufacturing. In particular, the use of Raman in secondary manufacturing is quite uncommon. In this review we focus on papers published after year 2000. 

Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields... 

Selected Successful Raman Applications, http //www.kosi.com/raman/analyzers/ramanrxn2.html. 

Thus far, we have reviewed basic theories and experimental techniques of Raman spectroscopy. In this chapter we shall discuss the principles, experimental design and typical applications of Raman spectroscopy that require special treatments. These include high pressure Raman spectroscopy, Raman microscopy, surface-enhanced Raman spectroscopy, Raman spectroelectro-chemistry, time-resolved Raman spectroscopy, matrix-isolation Raman spectroscopy, two-dimensional correlation Raman spectroscopy, Raman imaging spectrometry and non-linear Raman spectroscopy. The applications of Raman spectroscopy discussed in this chapter are brief in nature and are shown to illustrate the various techniques. Later chapters are devoted to a more extensive discussion of Raman applications to indicate the breadth and usefulness of the Raman technique. 

M. J. Smith, G. Kemeny, and F. Walder, Nicolet FT-Raman Application Notes, AN-9142 (1992). 

The laser frequency is an important consideration in Raman applications because resonance enhancement greatly increases both the sensitivity and the selectivity of the technique. Nonresonance Raman scattering is weak enough that sample concentrations in the molar range are often required to obtain good quality spectra, but resonance enhancement can lower the required concentration to the millimolar to micromolar ranges. Selectivity is an equally important feature since vibrational spectroscopy frequently suffers from crowding and overlap of... 

Weber, W.H. (2000) Raman applications in catalysts for exhaust-gas treatment, in Raman Scattering in Materials Science (eds W.H. Weber and R. Merlin), Springer,... 

An important qualifier is required for the statement that etendue remains constant through the system. The usable etendue is determined by the minimum AS2 product for the system. This minimum could be determined by the laser spot size, the entrance slit, or the detector area and is often related to the limiting aperture applied to the definition of //. For many Raman applications, the etendue is determined by the spectrometer/detector combination (e g., the // of a spectrograph and the area of a CCD pixel). Increasing the Af2 product has provided much of the motivation for building Raman spectrometers with lower // and maximum etendue. 

Table 7.1 are continuous wave (CW), not pulsed. Second, frequency stability to < 1 cm" is important to assure Raman shift precision and avoid line broadening. Although the Raman shift axis is usually calibrated periodically, the laser frequency must remain stable between calibrations. Third, lasers vary significantly in output linewidth, from hundreds of reciprocal centimeters to much less than 1 cm". For the majority of samples of analytical interest, a laser linewidth below 1 cm" is sufficient. Laser linewidths are often quoted in terms of frequency rather than wavenumber, in which case 1 cm" equals 30 GHz. Lasers are available with < 1 MHz linewidths (< 10 em ), but such lasers would be unnecessarily narrow for most analytical Raman applications. Fourth, lasers differ in their output of light at wavelengths other than the laser line itself. Gas lasers (Ar+, Kr+, He-Ne) emit atomic lines (plasma lines), and solid-state lasers luminesce, both of which can interfere with Raman scattering. Essentially all lasers require a bandpass filter or monochromator to reduce these extraneous emissions. 

Beam divergence is a less common laser consideration of importance to Raman applications. No optical beam is perfectly collimated, and diffraction... 

Since the He-Ne is based on emission of gas-phase atoms, the frequency accuracy is excellent, and the output linewidth is sufficiently narrow for most Raman applications without special accessories such as an intracavity etalon. Like ion lasers, He-Ne lasers exhibit a variety of atomic emission lines from the DC discharge that must be filtered out before reaching the sample. 

The list below defines most of the detector properties important to Raman applications, particularly for dispersive spectrometers. Specifications of particular devices will be discussed in subsequent sections. More specific definitions are provided for CCD detectors in Section 8.5.2. 

Several terms of importance to Raman applications will be defined in this section. The list is not intended to be comprehensive. 

Various Dispersive Raman applications for surface analysis Table 13.6... 

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