Raster mode continuous-flow liquid microjunction mass spectrometry imaging of proteins in thin tissue sections

Mass spectrometry imaging by use of continuous-flow liquid microjunction sampling at discrete locations (array mode) has previously been demonstrated. In this Letter, we demonstrate continuous-flow liquid microjunction mass spectrometry imaging of proteins from thin tissue sections in raster mode, and discuss advantages (a tenfold reduction in analysis time) and challenges (suitable solvent systems, data interpretation) of the approach. Visualisation of data is non-trivial, requiring correlation of solvent-flow, mass spectral data acquisition rate, data quality and liquid microjunction sampling area. The latter is particularly important for determining optimum pixel size. The minimum achievable pixel size is related to the scan time of the instrument used. Here we show a minimum achievable pixel size of 50 µm (x-dimension) when using an Orbitrap Elite; however a pixel size of 600 µm is recommended in order to minimise the effects of oversampling on image accuracy.