²⁵²Cf-plasma desorption mass spectrometry of dipetides and tripeptides

Abstract

Plasma desorption is induced by a ²⁵²Cf fission fragment penetrating a sample holder and sample. During the plasma desorption process, sample molecules become volatilized and ionized. The ions are accelerated into an electrostatic particle guide which transports the ions to an ion detector. A Time Digitizer accurately measures the time-of-flight between the start signal, generated by the complementary fission fragment, and the stop signal from the ion detector. The mass is calculated from the time-of-flight of the ion. Plasma desorption produces positive quasi-molecular ions (M+1) and negative quasi-molecular ions (M-1) of dipeptides and tripeptides. The technique also produces enough fragment ions to be useful for sequencing peptides. Fragment ions commonly produced in dipeptides are the N-imine ion, the C-terminal amino acid quasimolecular ion, and characteristic sidechain ions. In addition to the ions produced in dipeptides, more complex fragment ions are produced in tripeptides, such as a larger imine ion, a carbonium ion, and the C-terminal dipeptide quasi-molecular ion. Plasma desorption mass spectrometry produces mass spectra with intense quasi-molecular ions from samples of low volatility such as peptides. Ions including two parent molecules are also detected. Among the peptides analyzed were the dipeptides, ArgAsp, PhePhe, and TyrLeu. With the plasma, desorption technique, the leucyl and isoleucyl amino acid group can be differentiated in peptides.