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.