Laser-desorption mass spectrometry of standard polynuclear aromatic hydrocarbons and fullerenes

Laser-desorption mass spectrometry (LD-MS) has been used to investigate the mass spectra of a suite of standard polynuclear aromatic hydrocarbons and a sample of fullerenes (mixed C₆₀ and C₇₀), as a part of a study of high molecular mass ions in tars and liquid extracts derived from coal. Positive-and negative-ion LD-MS spectra of these standards were compared with electron impact spectra to elucidate the mechanism of laser desorption. The data indicate that ion-molecule reactions occur during the desorption step giving hydrogenated molecular ions via mechanisms analogous to self-chemical ionization; fullerenes gave no hydrogenated molecular ions. Carbon cluster ions (both positive and negative ions) are formed by the laser energy with fewer carbon atoms than the original molecule, suggesting their formation to result from the thermal destruction of parent molecules. However, none of the standard compounds gave cluster ions greater than the hydrogenated molecular ions. This finding provides confirmation that large molecular mass materials identified in coal-derived liquids originated from the sample itself and did not form from smaller molecular mass compounds under the power of the laser.