Infrared Spectroscopy

Infrared spectroscopy (IR) gives a unique “chemical overview” of a sample, with all the chemicals present contributing to the spectrum produced.  It can be applied to non-crystalline materials as well as crystalline materials, and organic as well as inorganic materials. The technique allows the qualitative analysis of a large variety of samples ranging from biological samples to clay minerals. Identification of unknowns is possible through expert interpretation of spectra, in conjunction with the use of spectral libraries. Although the analysis done using IR is largely qualitative, quantitative work is also possible. Near infrared spectroscopy (NIR) is a related technique which, through calibration procedures, allows the quantification of various characteristics in a wide range of materials from foodstuffs to soil.

The section is equipped with a Fourier transform infrared (FTIR) spectrometer, an FTIR microscope and a Near infrared (NIR) spectrometer. The FTIR spectrometer can be used in conjunction with several different sample holding accessories, including Diamond Attenuated Total Reflectance (DATR) and Horizontal Attenuated Total Reflectance (HATR) attachments, giving great flexibility in the type of samples that can be analysed. The DATR allows samples to be run in their natural state and to be completely recovered afterwards. The HATR allows spectra of liquids and solutions to be readily obtained.

Typical Samples

Typically, many of the samples analysed using the FTIR spectrometer are “problem samples” which involve identification of components in complex mixtures such as scales, deposits, drilling fluids and muds from the oil industry. Characterisation of paints or the analysis of soils from contaminated land is also typical of the analyses undertaken in this section. The use of the FTIR microscope ranges from the identification of fibres to mapping of chemical composition in biological thin sections. The NIR system, which is currently under development, can be used for example in the rapid analysis of various properties of animal feedstuffs such as protein, crude fat and fibre content.


FTIR spectrum


The IR spectrum which represents a chemical fingerprint of the sample is a plot of absorption y-axis (zero absorption = 100% Transmittance) at the top and total absorption (0%Transmittance) at the bottom versus frequency in the x-axis, the unit of frequency being the wavenumber (cm-1). Absorption band intensities are determined by the strength of the change in dipole moment involved in the particular inter-atomic vibration. The stronger the dipole moment change the greater the band intensity.
Interpretation of IR spectra is achieved using functional group frequency tables, computer search of in-house and commercial IR spectral libraries and recognition of absorption band patterns gained by experience. The search facility can be very useful to identify/characterize unknown samples but care should be taken to ensure that spectral libraries are used appropriately, especially where spectra are of complex mixtures. A high match index does not necessarily mean a successful identification hence close comparison of the unknown sample spectrum and the search match spectrum by the analyst is highly recommended before a conclusion is reached. It should also be noted that all the bands have to match the reference mineral spectrum not just one or two bands for a positive identification.