MALDI-TOF mass spectrometry: Obtaining reliable mass spectra for insoluble carbonaceous pitches

Abstract

The insolubility of high molecular weight (MW) carbonaceous pitches (e.g. mesophases) in even aggressive solvents has always been a significant obstacle to their MW characterization. By using matrix-assisted, laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry with the matrix 7,7,8,8-tetracyanoquinodimethane (TCNQ), we are now able to obtain reproducible, highly resolved mass spectra for insoluble pitches. A solvent-free sample preparation technique was developed in which the matrix and pitch sample are combined and ground together to form a fine powder. We then use the high surface tension of water to create a thin uniform coating of the matrix–pitch mixture on the target cell. Laser power and pulsed ion extraction delay were both found to significantly alter the spectra obtained, and a procedure for optimizing each is described. Three carbonaceous pitches were evaluated: an anthracene pitch produced by thermal polymerization, a petroleum pitch similar to A-240 and produced by heat-soaking decant oil, and a high MW cut of this pitch produced by supercritical extraction. The anthracene pitch contained enough lower MW species to serve as a self-matrix, so no TCNQ addition was required. For the high MW cut of petroleum pitch, the use of TCNQ or dithranol revealed the presence of trimers and tetramers at MWs up to 1250 Da.