Trimer formation studies in polystyrene pyrolysis reveal that time-resolved mass spectrometry can be misleading

Abstract

Samples for mass spectrometric analysis may be mounted on a probe which is inserted directly into the mass spectrometric source. In ‘time-resolved’ mass spectrometry, the sample is heated, and the ion current from the resulting products is plotted as a function of time. It may be possible to resolve this curve into component curves of individual products by also recording currents corresponding to specified ions. It has been postulated that such time-resolved mass spectra may provide information about the mechanisms of pyrolysis, especially with respect to the existence of consecutive mechanisms. The pyrolysis of polystyrene and styrene/MMA copolymers has been studied using this technique. These systems were selected because of recent evidence that oligomer formation may result from the recombination of the monomer formed as a primary product. Both quartz tube and filament techniques were used to heat the samples. It was found that the time-resolved mass spectra have entirely different characteristics depending on the heating method used. Reliable time-resolved mass spectra are achievable only when filament techniques and thin samples are employed. Further work, involving comparison of monomer/trimer ratios for the copolymers and the homopolymer, has provided further evidence for the participation of monomer recombination in oligomer formation.