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Scientists at the Science and Technology Facilities Council (STFC) Rutherford Appleton Laboratory in Oxfordshire, UK, have developed an effective laser-based method for the characterisation of the bulk chemical content of pharmaceutical capsules. The unusual aspect of the method is that the capsules can be analysed without being opened.

In close collaboration with teams from leading pharmaceutical company Pfizer, the researchers in STFC's Lasers for Science Facility succeeded in quantifying the presence of the active pharmaceutical ingredient in production line relevant capsules to a relative error of one per cent. Other established non-invasive methods were unable to reach the same level of accuracy with the same sample.

The technique holds great potential for a range of process control applications in the pharmaceutical industry. The results of the collaborative study are reported in the Journal of Pharmaceutical and Biomedical Analysis.

The development stems from research into a novel Raman spectroscopy method, Spatially Offset Raman Spectroscopy (SORS) which is under development at STFC for a wide range of applications. These include the detection of explosives in non-metallic containers, the detection of counterfeit drugs through opaque packaging and the non-invasive diagnosis of bone disease and cancer (Fig.1).

The concepts, which are relatively simple to implement, were developed through experiments involving STFC's large-scale facilities which provided insight into photon transport processes.

The development is being carried out in close collaboration with STFC's knowledge technology transfer arm (CLIK) and the new techniques are planned for commercialisation through STFC's spin-out company LiteThru.

The work stems from earlier research efforts of a wider collaborative team including the laser team at the Rutherford Appleton Laboratory, the University of Michigan, ICI PLC, the Royal Veterinary College, University College London and the Gloucestershire Royal Hospital focused on non-invasive spectroscopy of powders and the diagnosis of bone disease and cancer.

For its part, the Central Laser Facility at the STFC Rutherford Appleton Laboratory is one of the world's leading laser facilities providing scientists from universities in the UK and Europe with an unparalleled range of state-of-the-art laser technology.

The SORS revolution

Described by its inventors as a 'revolutionary technique capable of investigating tissue beneath patients' skin', SORS enables safe, non-invasive characterisation of tissue in vivo or the analysis of powder layers at depths of several millimetres. This is by at least an order of magnitude deeper than with the conventional Raman method.

The SORS method is based on a simple concept that has gone unnoticed in Raman spectroscopy until now. Instead of following classical Raman spectroscopy textbooks and collecting Raman light from the regions of sample illuminated by the laser beam where the maximum signal is present, a counterintuitive step is invoked.

The Raman scatter is collected from regions laterally offset away from the excitation laser spot. This has a dramatic effect on the composition of the detected Raman signals. The offset Raman signal contains much lower relative Raman and fluorescence contributions from the surface layer than those observed with a 'perfectly optimised' instrument.

The Raman effect is a process by which light is inelastically scattered from molecules and as a consequence its wavelength altered. This change gives a unique spectral signature specific to each chemical species under investigation.