Raman techniques limitations

Most chemists tend to think of infrared (IR) spectroscopy as the only form of vibrational analysis for a molecular entity. In this framework, IR is typically used as an identification assay for various intermediates and final bulk drug products, and also as a quantitative technique for solution-phase studies. Full vibrational analysis of a molecule must also include Raman spectroscopy. Although IR and Raman spectroscopy are complementary techniques, widespread use of the Raman technique in pharmaceutical investigations has been limited. Before the advent of Fourier transform techniques and lasers, experimental difficulties limited the use of Raman spectroscopy. Over the last 20 years a renaissance of the Raman technique has been seen, however, due mainly to instrumentation development. 

Fig. 9.5. Raman chemical images of tablet formulations with different API form compositions to demonstrate technique limit of detection of 0.01% w/w for form D...

From a practical standpoint, the use of the glancing angle X-ray method, while powerful, requires a synchrotron source and therefore, due to the constraints of beam time, is necessarily limited in the number of systems that can be studied in a given time period. Of the optical methods listed, the fluorescence and resonance Raman techniques directly measure spectra of an embedded... 

The use of Raman spectroscopy in the lumber/paper industry has been found to be feasible using the FT-Raman technique. Earlier results using a visible laser were limited due to the laser-induced fluorescence created with most wood samples. Measures to circumvent fluorescence were time-consuming, and the signal-to-noise (S/N) ratio was poor. With most wood samples, using a near-IR laser excitation, fluorescence essentially was eliminated. 

The Raman techniques combined with AEM microscopic imaging, as for instance TERS (tip-enhanced Raman scattering) spectroscopy [27], allow to analyze surface nanostructures beyond the diffraction limit, but the cost of the instrumental apparatus is not affordable for any research laboratory. Therefore, in this chapter, the results obtained with those techniques will not be presented, though they increased Raman enhancement factors by up to lO, with the possibility of single-molecule detection. Conversely, confocal micro-Raman apparatus is affordable to every research group allowing SERS investigations with more comparable results. 

Vibrational spectroscopy provides an excellent tool for examining interfacial properties. Experiments have been carried out using both the infrared and Raman techniques [12-14]. Discussion is limited here to Fourier transform infrared spectroscopy (FTIR) in the reflection mode. It is important to understand how the infrared radiation interacts with dipolar adsorbates at the interface. Consider an electromagnetic wave travelling in the (x, z)-plane, which strikes the interface located in the (x, y)-plane at an angle 0 with respect to the interface (see fig. 10.7). The electrical field vector associated with the wave can be resolved into two components, one oscillating in the (x, z)-plane (the parallel or p-component)... 

Neutron inelastic scattering techniques have been widely applied to the study of vibrational and rotational dynamics in hydrogenous molecular systems.1 The bulk of this research has been concerned with the study of intermolecular and interionic motions in solids, but a limited yet significant amount of effort has been directed toward the study of large-amplitude intramolecular vibrations, most notably torsional vibrations and hydrogen-bond modes.2 The present paper is restricted primarily to a discussion of the application of neutron scattering to the study of torsional vibrations and barriers to rotation of methyl groups in molecules. We will present several examples in which neutron spectra have provided information complementary to that obtained by the more widely available and applicable infrared and Raman techniques. We will also discuss in simple terms some limitations and pitfalls of the neutron technique and the interpretation of neutron spectral results. 

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... 

One way to directly measure rotational transitions in non-polar molecules such as these is by rotational Raman spectroscopy, which operates on the same principle as other Raman techniques (see Section 6.3). A rotational Raman transition connects initial and final rotational levels within the same vibrational state, so only the rotational quantum number changes. However, this technique is limited in precision by the uncertainties in the photon energies of the incident and scattered light. The scattering intensity increases dramatically with photon energy. 

One of the limiting factors for the application of the Raman technique, however, becomes evident by comparing the intensity of the laser source and the scattered radiation [9,15,19-21]... 

Fourier transform infrared and FT-Raman methods for the quantitation of polymorphs of cortisone acetate were compared by Deeley et al. [32]. The Raman analysis provided similar standard errors of prediction to the diffuse reflectance FTIR method of around 3.0-3.5%. Better precision and accuracy was reported in the same article for a Raman quantitative analysis of a novel research drug with a standard error of prediction of around 2.5%. The authors also outlined some of the advantages of the FT-Raman technique for quantitative analysis, primarily the minimal sample preparation that may alter polymorphic forms and that handling of the samples is unnecessary—spectra can be obtained through glass vials. Limitations of the technique were also described, notably intensity changes... 

An alternative approach to improving the sensitivity of the Raman technique with potential for environmental applications arose in 1974 [11] with the discovery of surface-enhanced Raman spectroscopy (SERS) in which detection limits can typically be lowered by a factor of approximately 10 -10 relative to NRS, with SERS having the advantage of fluorescence suppression over the RRS. (Considerable research has been conducted for determining the sources of enhancement, and the reader is referred to the reviews of Vo-... 

Dinh [12] and Garrell [13]). Very recently, enhancement factors on the order of lO -lO for compounds adsorbed on metal nanoparticles or colloids have been reported [14-16]. Such decreases in detection limits have generated claims of single-molecule detection by the Raman technique [14-17],... 

The fact that the resolution of the nonlinear Raman techniques is limited only by the laser line widths gives the stimulated Raman techniques particular appeal under conditions where interference from background luminescence is problematic or in situations where very high resolution is required. The main disadvantage of these techniques, however, is that they are quite sensitive to laser noise. The latter requires high stability in laser power. 

In addition, the Raman technique has proved to be particularly valuable in the study of single crystals where the infrared technique has greater limitations on sample size and geometry. Polarization data obtained from Raman spectra allow unambiguous classification of fundamentals and lattice modes into the various symmetry classes. Although Raman spectroscopy will never challenge X-ray diffraction as a tool for quantitative structural analysis, it is the preferred technique when qualitative information is sufficient because it is faster and less expensive. 

Infrared spectroscopy has been a late addition to the spectroscopic inventory of the membrane biophysicist. The reason has been the presence of water. Biological membranes not only encompass a range of widely different molecular structures, but like most biological structures require an aqueous environment whereby water is not only the solvent of choice, but often part of the molecular structure itself. Water which does not impair spectroscopic measurements using the NMR, ESR, UV or Raman techniques is a strong infrared absorber, a fact which has precluded or severely limited the application of conventional infrared spectroscopy to the study of biological systems. 

The aforementioned samples, which were analyzed by the FT-Raman technique, were also subjected to a complementary FTIR analysis (see Fig. 15.2). The spectra in Fig. 15.2 were identified as EG (A), CA (B),TEOS + CA + EG (molar ratio 1 2 1) reacting at 60 C for several hours (C),TEOS + CA + EG + Li (molar ratio 1 2 1) (D), and TEOS + CA + EG + Na also reacting at 60 °C for several hours (E). For this reason, the FTIR study was limited to the short range (1800-1600 cm" wavelength, highlighted area in... 

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