Raman Spectroscopic Imaging

Raman spectroscopy is a vibrational spectroscopic technique which allows compositional analysis of liquids and solids. The technique can be employed to analyse a range of samples (organic and inorganic) to determine structural/compositional changes and identification of unknown samples.

By coupling Raman spectroscopy to light microscopy, spatial resolution down to 1 micron of compositional changes within a sample can be achieved. In addition, confocal Raman microscopy can be used to perform depth-profiling of a sample giving 3-dimensional chemical resolution.

Inelastic scattering (Stokes/ Anti-Stokes scattering) of visible light, corresponding to molecular polarizability changes due to molecular vibration (e.g. asymmetric bond stretching) result in a unique absorption spectrum for each compound of interest.

Raman is an identification technique which allows specific chemical bonds in a sample to be determined. Consequently Raman is useful as part of formulation studies or examination of unknown samples to determine the presence of compounds of interest e.g. active pharmaceutical ingredients (APIs).

In addition to determining composition the technique allows the crystallographic structure of a sample to be determined. This polymorphic information is essential in pharmaceutical studies and can also be used to investigate the crystal structures in mineral and metallurgical samples.

By combining the technique with microscopy spatial chemical resolution can be performed to determine the heterogeneity of a sample. In addition, depth profiling can be carried out to perform 3D studies to look at the penetration of a chemical through a sample e.g. transdermal drug delivery studies.

Raman spectroscopy can be carried out on both solid and bulk liquid samples though spatial resolution is only possible on solid samples. Flat sample surfaces are required in order to ensure consistent sample focus to perform spatial mapping studies. Samples should be small enough (or at least be able to be processed to be small enough) to fit on a microscope stage.