ToF-SIMS was used to identify a foreign crystalline material which appeared in a drug solution after storage. Only a microscopic amount of crystallite was present, insufficient for bulk analytical techniques. The material was collected on a fine filter and a small clump of crystallites on the filter surface was analysed.

The measurements involved the latest technology whereby RAW data were acquired from an area containing some of the crystallites of interest. Retrospectively, mass spectra were created for blank filter and crystallite regions providing, by comparison, the mass spectral fingerprint for the crystallite. This is illustrated below where the same partial mass range is plotted off and on the crystalline material.

The information was enhanced by retrospective chemical mapping which clearly showed the crystallites (red; using a peak from the mass spectral fingerprint) on top of the filter (green; using a signal which is filter-specific). This work indicated that a polymer additive had leached out of a bottle stopper, into the drug solution and crystallised.

RAW data: A selected area on a sample is scanned and a mass spectrum is acquired and stored at every pixel point. Retrospectively, the following operations can be performed: Region-of-Interest (ROI) analysis: For defined regions (any shape and size) within the selected analysis area, it is possible to sum the mass spectra at all of the pixel points within each defined region thus generating area-specific mass spectra. Imaging analysis : For the selected analysis area, it is possible to generate chemical maps (images) for any of the spectral peaks. For very weak signals, where there is not enough individual peak intensity to produce meaningful images, it is also possible to add peak intensities thereby improving image statistics.