Fig. 13 The relationship between the chosen peaks’ intensities and signal-to-noise ratio (s/n) in terms of the number of matrix layers (A) and the spraying nozzle height (B) for the negative ionization mode rat spinal cord tissue slices (matrix: 6 mg/ml norharmane (chloroform:MeOH:H2O))
Generally, for the negative ionization mode, norharmane gives the spectra of higher intensity for the lipids around m/z = 728 compared to 9AA and then with the higher m/z range their intensities are going lower (see: Fig. 21). Additionally, there is no signal observed for low m/z ions like m/z = 255, or m/z = 283. It seems, that only the lipids of higher molecular weights are able to ionize with this matrix.
N-(1-naphthyl) ethylenediamine dihydrochloride (NEDC) – negative ionization mode
NEDC (7 mg/ml, 70% MeOH) is quite destructive when sprayed from the low position of the spraying nozzle. This effect is seen in Fig. 14. Spraying nozzle heights 25 and 15 gave quite intensive spectra, but the problem with matrix crystallization could be clearly observed in the picture. Therefore, spraying nozzle height at a value 5 seems to be optimal. It gives quite good peak intensity for small molecules and lipids and the s/n ratio is also acceptable (Fig. 15).