A Mass Spectrometry Experiment on the Degrees of Freedom Effect

An experiment is described that allows students to investigate the physical chemistry involved in unimolecular fragmentation and to explain their experimental results using Rice–Ramsperger–Kassel (RRK) theory. For the experiment, students prepare a solution of a series of homologous analytes and collect mass spectra of those analytes using GC-MS. All the analytes undergo the same three-step fragmentation in the mass spectrometer. The relative peak intensities from the ions in the fragmentation scheme are plotted as a function of the vibrational degrees of freedom of the molecular ions' R group. Trends in these plots can be explained using concepts from RRK theory and highlight the "degrees of freedom effect" in mass spectrometry. The experiment is suitable for a variety of upper-level, undergraduate chemistry laboratories.