TUNABLE DEGREE OF FRAGMENTATION OF VOLATILE ORGANIC-COMPOUNDS WITH A LOW-PRESSURE PENNING ION-SOURCE

The properties of a low-pressure ion source based on the principle of the Penning cell (Penning ion source) were investigated. At low discha rge currents, spectra similar to standard electron impact ionization ( EI at 70 eV) spectra were obtained (positive-ion-mode). However, at hi gh discharge currents, it was found by experiments carried out partly with labelled compounds that molecules are decomposed to small fragmen ts and even single atoms. Species such H-2(+), C+, CH+, CH2+, CH3+, O, OH+, H2O+, C2+, C2H+, C2H2+, C2H3+, O-2(+), Cl+ and Cl-2(+) have bee n obtained from various organic compounds. The degree of fragmentation of the molecular ion has been found to be directly dependent on the d ischarge current. These observation demonstrate that the Penning ion s ource allows one to tune the degree of framentation. In the negative-i on mode, halogenated organic compounds yielded spectra dominated by th e signals of the halogen atoms they contain. High abundances of F-, Cl -, Br- and Br-2(-) were observed, next to weak signals for O-, C-, C-2 (-), C2H-, C-3(-) and C-4(-), orginating from the CHO skeleton of the sample molecule. In contrast to the results found in the positive-ion mode, intense fragmentation, even at low discharge currents, was found .