Testing the performance of a VUV photoionization source on a double focussing mass spectrometer using alkanes and thiophenes

Abstract

The performance of a newly developed photoionization source in combination with a high resolution mass spectrometer is tested. The total ion currents for several linear alkanes ( n-pentane to n-decane) and for some thiophenes (2-methylthiophene, 2,5-dimethylthiophene and 2-ethylthiophene) are measured at three fixed wavelengths: the Kr I (10.03 eV and 10.64 eV), Ar I (11.62 eV and 11.83 eV) and Ne I (16.67 eV and 16.85 eV) resonance lines. These total ion currents are compared with the data for the alkanes and thiophenes obtained with low energy (10.6 eV, 11.8 eV and 16.7eV) electron impact ionization on the same mass spectrometer. The loss in ion intensity at several positions throughout this instrument is determined for the photo-ionization source and for the electron impact ionization source. One out of every 1500 ions created in the photoionization source is measured by the detector. For the electron impact ionization source, one out of 180 ions is measured. The introduction of a five-element “Heddle” lens for the transfer of the ions from the photoion source to the mass analyser resulted in an approximately 1.5 fold loss in ion current. From the measured total ion intensities, the photoionization and electron impact ionization cross-sections at energies of 10.6 eV, 11.8 eV and 16.7 eV are calculated. The photoionization cross-section values of the linear alkanes are found to be in the range of 2.5 Mbarn to 355 Mbarn, and the electron impact ionization cross-section values are between 40 Mbarn and 735 Mbarn. The photoionization cross-section values of the thiophenes range from 4 Mbarn to 31 Mbarn, and the electron impact ionization cross-section values from 81 Mbarn to 760 Mbarn.