Fig. 20 The ion map for m/z 281 obtained from the slices of the
spinal cord for 9AA matrix with different concentrations of matrix (7
mg/ml 9AA and 5 mg/ml 9AA) and solvent: A - 70% EtOH, 50% EtOH; B -
70% MeOH and 100% MeOH).
Such solvent-related, matrix-related effects must be taken into
consideration during the interpretation of the results. If a single ion
seems to be desorbed from a single structure with the aid of exact
solvent/matrix mixture, it does not mean that using the other solvent,
or matrix will give us the same result. Thus, it should be clearly
stated that comparing the ion maps obtained from different matrices and
solvents, must be done with extreme caution.
2. Technical considerations about wet interface matrix
deposition.
Wet-interface matrix deposition devices allow to control the number of
applied matrix layers and the nozzle height over the sample. Our article
points the necessity of proper optimization of the process of sample
preparation, since it could help to obtain a substantial improvement of
the analysis parameters.
2.1. The number of applied matrix layers
In the positive ionization mode for DHB and norharmane matrices, the
peaks overall intensity gets higher with the higher number of matrix
layers, but only up to some optimum. After that point, the number of
layers does not increase the peaks intensity or does it insignificantly.
In the case of DAN, the higher number of layers may even be responsible
for losing the discrimination between the white and gray matter in the
discussed case. Thus, it is highly advisable to check this parameter
during sample preparation optimization.
In the negative ionization mode for 9AA and norharmane, the peaks
intensity increases with the number of the matrix layers, and we did not
observe the “saturation effect.” Nevertheless, the quality of the
obtained ion map could be reduced. Thus, this parameter must be
optimized before the final analysis. For NEDC matrix and lipid analysis,
the effect is very similar, except for small molecules such as glucose.
In this case, the lower number of matrix layers gives better results.