Imaging SIMS, analytical method

ToF-SIMS is a siirface-sensitive analytical method that uses a pulsed ion beam to remove molecules from the very outermost surface of the sample. The secondary ions are removed from the uppermost monolayers on the surface and then accelerated into a flight tube their mass is determined by measuring the exact time at which they reach the detector. Three operational modes are available when using ToF-SIMS, namely surface spectrometry, surface imaging, and depth profiling. 

In addition to some early applications in bioanalysis, ambient ionization mass spectrometry has been used as an imaging tool to study drug distribution in tissue sections. Most of the work reported so far involved the use of DESI as the ambient ionization method. Compared to other mass spectrometry-based tissue imaging techniques such as MALDI and SIMS, DESI allows tissue samples to be analyzed under ambient conditions without sample preparation, which simplifies the procedure and prevents the redistribution of analytes during matrix deposition. A major drawback of DESI as an imaging tool is its relatively low spatial resolution (typically 250 pm) and therefore cannot be used for cellular or subcellular imaging. 

Two variations of conventional SIMS have evolved. In one, known as matrix-enhanced SIMS, the MALDI sample preparation protocol is used to coat the sample surface with an organic acid (e.g., 2,5-dihydroxybenzoic acid) or other suitable matrix material to improve ionization efficiency [24,25], This approach has been used to image brain tissue samples from freshwater snails (Lymnaea stagnalis). Another variation employs a coating of a thin layer of gold or other metals to enhance analytical signals [26], This method, known as metal-assisted SIMS, provides images with improved spatial and chemical resolution. 

Mass spectrometry imaging with time-of-flight secondary ion mass spectrometry (ToF-SIMS) has revealed the spatial distribution of chemicals on a surface (1). When applied to biological samples, this method offers spatial information on biologically relevant small molecules (<1,000 Da). Indeed, ToF-SIMS imaging has been shown to be a powerful analytical tool for mapping the distribution of lipids on a cell membrane at the single-cell... 

This Chapter mainly deals with the big four surface analysis techniques (XPS, AES, SIMS, ISS). For spatially resolved surface analytical methodologies, cfr. Chps. 3 and 5. Various surface analysis methods provide images of elements and other information (cfr. Chp. 5.9). Eor surface studies by means of IR spectroscopies, cfr. Chp. 1. 

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