FIELD-DEPENDENT [M-2H]+ FORMATION IN THE FIELD DESORPTION MASS-SPECTROMETRIC ANALYSIS OF HYDROCARBON SAMPLES

High-boiling hydrocarbons often yield abundant [M - 2H]+. ions under the conditions of field desorption mass spectrometry (FDMS). This work evaluated [M - 2H]+. formation from various saturated and aromatic compounds. The most intense [M - 2H]+. signals observed resulted from the analysis of saturated compounds containing long and branched chains. The presence of an aromatic ring in a molecule, however, strongly diminished [M - 2H]+. formation during FDMS. Experiments involving manipulation of the applied potential between the FD anode and cathode reveal the strong field dependence of this phenomenon. At a potential difference of 10 kV, strong [M - 2H]+. formation occurred for samples including squalane, hexatriacontane (C36 n-alkane) and a Polywax 655 mixture. Analyses of the same samples conducted at a potential difference of 4 kV produced only weak (if measurable) [M - 2H+. signals. The magnitude of [M - 2H]+. formation also decreased as the sample quantity decreased.