A study of small volatile organic compounds with PDMS

Abstract

Macfarlane first made the intriguing observation that fission fragment impacts on solid organics can desorb ionized nonvolatile molecules for mass analysis. This technique is called plasma desorption mass spectrometry (PDMS). A region of high charge density called the infratrack is created around the path of the fission fragment in the solid sample. The high charge density within the infratrack produces a Coulomb explosion which causes the track to rapidly expand and then contract. This process, in turn, creates a pressure pulse or a shock wave within the material. This pulse or wave propagates through the material and transfers kinetic energy to surface species in the surrounding ultratrack. The aim of this study was to further elucidate the volatilizationionization process for a complete mass distribution of secondary ions, i.e. covering emission from the infratrack and ultratrack. To handle a complete mass distribution, simple organic compounds were chosen. The organic samples were continuously or intermittently condensed onto a metal plate during the measurements. Four regions of the observed mass distribution are discussed. A variety of unlabeled compounds and one labeled volatile organic compound were used. The results of a coincidence counting study validated the accuracy of the observations. The fragments observed in the mass spectra are indicative of the sample of interest. A gas phase collisional model for the production of Hn' species is proposed and discussed by using hydrogen cluster peak ratios on samples with varying hydrogen densities. Recombination in the infratrack region is suggested by the trends in the formation of CH,,' and C2HI' ions from samples with varying hydrogen densities. The possibility of near-equilibrium conditions within the infratrack is considered and the thermodyanmic model used is discussed. The molecular ion region and the associated hydrogen loss peaks are shown to depend upon the molecular weight, bonding and degree of saturation of the compounds. Additionally, the infratrack appears to affect the formation of these ions which arise from the ultratrack. Aggregate (cluster) emission appeared to be from the expulsion of chunks of the surface. No chemical reactions appeared to occur in aggregates of the studied molecules.