Raman techniques quantitative

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. 

The FT-IR technique using reflection-absorption ( RA ) and transmission spectra to quantitatively evaluate the molecular orientation in LB films is outlined. Its application to some LB films are demonstrated. In particular, the temperature dependence of the structure and molecular orientation in alternate LB films consisting of a phenylpyrazine-containing long-chain fatty acid and deuterated stearic acid (and of their barium salts) are described in relation to its pyroelectricity. Pyroelectricity of noncentrosymmetric LB films of phenylpyrazine derivatives itself is represented, too. Raman techniques applicable to structure evaluation of pyroelectric LB films are also described. 

Since a larger sample volume is presumed to be probed, the use of transmission mode has led to simpler, more accurate models requiring fewer calibration samples [50]. Scientists at AstraZeneca found that with a transmission Raman approach as few as three calibration samples were required to obtain prediction errors nearly equivalent to their full model [42]. For a fixed 10-s acquisition time, the transmission system had prediction errors as much as 30% less than the WAI system, though both approaches had low errors. It is hoped that this approach in combination with advanced data analysis techniques, such as band target entropy minimization (BTEM) [51], might help improve Raman s quantitative sensitivity further. 

M. Kim, H. Chung, Y. Woo and M.S. Kemper, A new non-invasive, quantitative Raman technique for the determination of an active ingredient in pharmaceutical liquids by direct measurement through a plastic bottle. Anal. Chim. Acta, 587, 200-207 (2007). 

Dramatic improvements in instrumentation (lasers, detectors, optics, computers, and so on) have during recent years raised the Raman spectroscopy technique to a level where it can be used for species specific quantitative chemical analysis. Although not as sensitive as, for example IR absorption, the Raman technique has the advantage that it can directly measure samples inside ampoules and other kinds of closed vials because of the transparency of many window materials. Furthermore, with the use of polarization techniques, one can derive molecular information that cannot be obtained from IR spectra. Good starting references dealing with Raman spectroscopy instruments and lasers are perhaps [34-38]. 

In 1967, Hasegawa identified the solid-state photochemical transformation of distyrylpyrazine 0 as a four center polymerization to a crystalline polymer with cyclobutane rings. Extensive crystallographic and mechanistic studies of this process have been reported (50). This type of four center photopolymerization has been extended to give a quantitative asynunetric induction (51). Laser Raman techniques have been used to study monomer-to-polymer conversion of these photopolymerizations and other processes as well (52). 

Classical, spontaneous Raman scattering is a powerful analytical tool that allows for the investigation of the qualitative and quantitative composition of biological, pharmaceutical, and environmental samples. The following discussion of NIR-Raman spectroscopy will begin with a general review of Raman spectroscopy, followed by a description of NIR-Raman, with further discussion about instrumentation and applications of the NIR-Raman technique. 

Because of its insensitivity to quenching (the lifetime of the virtual state is -lO s), Raman spectroscopy is of considerable interest for quantitative measurements on combustion processes. Further, important flame species such as O2, N2 and H2 that do not exhibit IR transitions (Sect.4.2.2) can be readily studied with the Raman technique. However, because of the inherent weakness of the Raman scattering process (Sect.4.3) only non-lumi-nous (non-sooting) flames can be studied. 

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... 

The major building blocks for a visible absorbance microspectrometer are available in existing Raman microprobes [400]. Visible microspectroscopy does not supply as much structural information as IR and Raman techniques but is superior with regard to detection and quantitation in low-concentration situations. Samples as small as 2 /xm in diameter can be studied. 

A problem unique to Raman spectroscopy is the fact that most polymers fluoresce strongly when exposed to laser radiation. This problem can be reduced by using Fourier transform and resonance Raman techniques. Because of this and other difficulties associated with Raman spectroscopy, the quality of Raman spectra of polymers is typically less than that of IR spectra. Therefore, it is not surprising that a quick search of the literature reveals many more quantitative studies of polymers using IR than using Raman. 

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. 

Both ordinary and resonance Raman techniques have been used to characterize a diverse array of biological systems, from proteins and amino acids, lipids and fatty acids, and carbohydrates to phenolic substances, terpenoids, alkaloids, and polyacefylenes [112]. It is a nondestructive technique, which when coupled with microscopy can be very useful for qualitative and quantitative analyses. 

Examples that use Raman spectroscopy in the quantitative analysis of materials are enonnous. Technology that takes Raman based techniques outside the basic research laboratory has made these spectroscopies also available to industry and engineering. It is not possible here to recite even a small portion of applications. Instead we simply sketch one specific example. 

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

In this review recent theoretical developments which enable quantitative measures of molecular orientation in polymers to be obtained from infra-red and Raman spectroscopy and nuclear magnetic resonance have been discussed in some detail. Although this is clearly a subject of some complexity, it has been possible to show that the systematic application of these techniques to polyethylene terephthalate and polytetramethylene terephthalate can provide unique information of considerable value. This information can be used on the one hand to gain an understanding of the mechanisms of deformation, and on the other to provide a structural understanding of physical properties, especially mechanical properties. 

The techniques used in the work have generally been spectroscopic visible-uv for quantitative determinations of species concentrations and infrared-Raman for structural aspects of the polymer. Although the former has often been used in the study of plutonium systems, there has been considerably less usage made of the latter in the actinide hydrolysis mechanisms. 

Each spectroscopic technique (electronic, vibra-tional/rotational, resonance, etc.) has strengths and weaknesses, which determine its utility for studying polymer additives, either as pure materials or in polymers. The applicability depends on a variety of factors the identity of the particular additive(s) (known/unknown) the amount of sample available the analysis time desired the identity of the polymer matrix and the need for quantitation. The most relevant spectroscopic methods commonly used for studying polymers (excluding surfaces) are IR, Raman (vibrational), NMR, ESR (spin resonance), UV/VIS, fluorescence (electronic) and x-ray or electron scattering. 

Since SERS and SERRS are substance specific, they are ideal for characterisation and identification of chromatographically separated compounds. SE(R)R is not, unfortunately, as generally applicable as MS or FUR, because the method requires silver sol adsorption, which is strongly analyte-dependent. SE(R)R should, moreover, be considered as a qualitative rather than a quantitative technique, because the absolute activity of the silver sol is batch dependent and the signal intensity within a TLC spot is inhomogeneously distributed. TLC-FTIR and TLC-RS are considered to be more generally applicable methods, but much less sensitive than TLC-FT-SERS FT-Raman offers p,m resolution levels, as compared to about 10p,m for FTIR. TLC-Raman has been reviewed [721],... 

In polymer/additive deformulation (of extracts, solutions and in-polymer), spectroscopic methods (nowadays mainly UV, IR and to a lesser extent NMR followed at a large distance by Raman) play an important role, and even more so in process analysis, where the time-consuming chromatographic techniques are less favoured. Some methods, as NMR and Raman spectrometry, were once relatively insensitive, but seem poised to become better performing. Quantitative polymer/additive analysis may benefit from more extensive use of 600-800 MHz 1-NMR equipped with a high-temperature accessory (soluble additives only). 

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