Sample preparation for infrared spectroscopy

A less aggressive sample preparation method is a mull in which the sample is dispersed in a liquid matrix, most commonly used is the mineral oil Nujol but other liquids may also be used (e.g. fluorolube and hexachlorobutadiene). The sample is ground and mixed with the oil (Nujol) and then sandwiched between two IR transparent (metal halide) plates. Although the sample is less likely to be altered using this technique, the oil has strong absorbance bands which obscure some regions of the spectmm (by using alternatives to Nujol 

Diffuse reflectance or DRIFTS (diffuse reflectance infrared Fourier-lransform spectroscopy) allows the sain)le to be analysed neat, ot diluted in a non-absorbing matrix (e.g. KCl or KBr at 1-5% w/w analyte). DRIFTS also may be used to obtain the spectrum of a solute in a volatile solvent by evaporating the solution onto KBr. When the IR radiation interacts with the powdered sample it will be absorbed, reflected and diffracted. The radiation which has been diffusely reflected contains vibrational information on the molecule. This technique allows non-destructive testing of neat materials and is suited to quantitative analysis, although care must be taken to ensure that a consistent particle size is used. 

FT-IR spectrometers may be interfaced with an optical microscope allowing analysis of very small sample quantities and simultaneous chemical and physical investigation of the sample [12]. The spectrum may be collected from an area as small as lOxlOmiaons, the effective diffraction limit of the IR radiation. These instruments may be used to collect the spectrum in reflectance, transmission or ATR modes the choice will be determined by the sample thickness and transmission properties. 

As for IR spectra, the frequency of the vibrational modes is characteristic of functional groups and so the molecular stmcture can be deduced from interpretation of the spectrum. Unfortunately, the availability of comprehensive correlation tables for Raman, lags behind that of IR. Tables that correlate both IR and Raman frequencies are available [4, 6]. 

Thorough descriptions of instrumentation can be found elsewhere [15-17] and only brief details are given here. Instrumentation for Raman spectroscopy falls into two distinct categories FT instruments and dispersive instruments. Both types of instrument have useful application in the pharmaceutical industry and a comparison is summarised in Table 6.1. 

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