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Space-resolved tissue analysis by solid phase microextraction coupled to high resolution mass spectrometry via desorption electrospray ionization.

It is hard to overstate the tremendous utility of desorption electrospray ionization (DESI) and its various configurations for rapid and high-throughput analyses, or spatially-resolved imaging of heterogenous systems. However, there have been few attempts to employ this technique in spatially-resolved mode with solid substrates featuring extractive and analyte-enrichment properties. This study documents the development of a platform that combines solid phase microextraction (SPME) with DESI-MS for unidimensional investigation of the heterogenous distribution of compounds in semi-solid systems (i.e. depth-profiling across the fiber axis), with the ultimate end of employing it for brain tissue analysis. To this end, DESI interface and a custom holder accommodating SPME probes were built in house, with the latter contributing to reduction of mechanical sources of signal instability. The system was evaluated through the quantitative reconstruction of the laminar and radial concentration gradients of xenobiotics introduced in multi-layer gel arrangements and surrogate brain tissue models. Good quantitative capability was achieved by employing a strategy that combined signal correction via preloading internal standard onto SPME fibers, and signal integration in scan-by-scan mode. The proposed technique's suitability for characterizing more complex systems, such as rat brains ex vivo, was also evaluated. The proposed approach allows for fast and non-invasive probing of three-dimensional objects without the need for their slicing and the space-resolved mode reduces the number of required probe insertions, allowing in vivo applications. We foresee suitability of this setup for examining the spatial patterns of local drug release in the brain and the extent of the resultant physiological responses.