Displaced dual-mode imaging with desorption electrospray ionization for simultaneous mass spectrometry imaging in both polarities and with several scan modes.

Displaced dual-mode imaging (DDI) is introduced as a method for simultaneous imaging in positive and negative-ion mode on the same sample with desorption electrospray ionization imaging, as well as a method for simultaneous imaging in full-scan and tandem mass spectrometry (MS/MS) mode. DDI is performed by using a smaller row distance in the y-direction than the desired image resolution and recording for example every second row in positive-ion mode and the other half of the rows in negative-ion mode, thus resulting in two separate images. This causes some degree of oversampling, which is thus utilized to obtain complementary mass spectrometric of the sample. Imaging with both polarities is exemplified on an imprint of a Hypericum perforatum leaf containing secondary metabolites which ionize in both polarites and a mouse kidney containing phospholipids which ionize in positive or negative mode only. Simultaneous full-scan and MS/MS imaging was demonstrated on the same mouse kidney, as the mouse had been given a relatively low dose of the antidepressive drug amitriptyline. While the full-scan data allowed imaging of the endogenous phospholipids, the drug and its metabolites were only visible in the MS/MS images. The latter approach is useful, for example in whole-body imaging experiments where the full-scan data gives an overview of the tissue, and the MS/MS mode provides the sensitivity to image trace amounts of drugs and metabolites.