Negative ionization processes of osmium for isotopic measurements

Abstract

Negative ion mass spectrometry (N-TIMS) for many elements requires O 2 gas, thermal electrons, and hot filament temperatures. These requirements are partly in conflict. For example, O 2, essential for volatilization of refractory metals, has “poisoning” effects on electron emission. Hot filament temperatures enhance reactions, but greater thermal electron emission distorts the source ion optics. Our work suggests an ionization scheme where Os is oxidized to gaseous OsO 3, with this neutral molecule ionized to OsO 3 −, primarily by electron capture very close to the hot filament. Lesser, but significant, numbers of ions are formed by electron exchange with O − away from the filament, which results in the commonly observed asymmetric peak shape. Repeated neutralization–ionization produces ions with low potential energies, which cause significant tailing of peaks on the high mass side. Low energy ions account for problems reported in N-TIMS by previous workers, including high background base lines, asymmetric peaks and poor reproducibility of isotopic ratio measurements. This interpretation is not in accord with the prevailing view that both positive and negative ionization, with a low energy spread, takes place on the filament surface. It is supported by observations previously reported, such as the widely recognized “memory effects” in N-TIMS. Declining ion intensities at higher filament temperatures, reported as puzzling maxima by previous workers, are also consistent with the proposed model. Excess electrons play a significant role in the source ion optics.