Discussion
1. Different matrices, different pictures
Collecting such a great amount of analysis allows us to notice an interesting observation: the obtained ion map of a particular ion could differ dramatically regardless of the used matrix, the solvent composition, or even the number of matrix layers.
In the positive ionization mode, we have observed this effect for the norharmane and DAN matrices in comparison with the classical DHB matrix. Norharmane produces a strong signal from lipids. Nevertheless, the lipids seem to be located in a whole tissue area, and the discrimination between the white and the gray matter is lost. For example, the ion map for m/z 810 and 766 for the DHB matrix suggests that those substances are present mainly in the white matter, but in the case of norharmane, these peaks were present all over the tissue slice (see Fig. 18). This effect occurs probably because the chloroform is used as a solvent.