NEGATIVE THERMAL IONIZATION MASS-SPECTROMETRY OF MAIN GROUP ELEMENTS .2. 6TH GROUP - SULFUR, SELENIUM AND TELLURIUM

A systematic investigation of the formation of negative ions for the 6th main group elements using negative thermal ionization mass spectrometry (NTI-MS) is presented. A double-filament ion source with BaO on the ionization filament has been applied to reduce the work function of the rhenium filament material. S.-, Se.- and Te'- were produced as most abundant ions. Low intensities of SeO.-, SeO2.-, TeO.- and TeO2.- have also been detected. Although the electron affinity of SO2 is low, high ion currents of SO2.- have been observed from BaSO4 samples. This may be due to an electron capture process of this molecule rather than to a thermal ionization process. A silica gel suspension mixed with the sample enhanced the Se.- ion current by a factor of about 40 and the Te.- intensity by a factor of about 10. However, the silica gel showed no enhancing effect on the S.- ion current. An improvement in the precision of the selenium and tellurium isotope ratio measurements by a factor of up to 10 was obtained when using the silica gel technique as compared with previous NTI investigations. The data of the selenium isotope abundance measurements were accepted as "best measurements" by the IUPAC.