Removal of optimal cutting temperature (O.C.T.) compound from embedded tissue for MALDI imaging of lipids.

Matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI) is a common molecular imaging modality used to characterise the abundance and spatial distribution of lipids in situ. There are several technical challenges predominantly involving sample pre-treatment and preparation which have complicated the analysis of clinical tissues by MALDI-MSI. Firstly, the common embedding of samples in optimal cutting temperature (O.C.T.), which contains high concentrations of polyethylene glycol (PEG) polymers, causes analyte signal suppression during mass spectrometry (MS) by competing for available ions during ionisation. This suppressive effect has constrained the application of MALDI-MSI for the molecular mapping of clinical tissues. Secondly, the complexity of the mass spectra is obtained by the formation of multiple adduct ions. The process of analyte ion formation during MALDI can generate multiple m/z peaks from a single lipid species due to the presence of alkali salts in tissues, resulting in the suppression of protonated adduct formation and the generation of multiple near isobaric ions which produce overlapping spatial distributions. Presented is a method to simultaneously remove O.C.T. and endogenous salts. This approach was applied to lipid imaging in order to prevent analyte suppression, simplify data interpretation, and improve sensitivity by promoting lipid protonation and reducing the formation of alkali adducts.