Mass resolution secondary ions using IMS 1280 Multicollection system is determined by the choice of three exit slit positions that are designed to move together for all detectors. When the instrument was installed in WiscSIMS lab, widths of three slits were made to be 500µm, 250µm, and 150µm, which correspond to Mass Resolving Power of 2500, 5000, and 8000 (assuming zero entrance slit width), respectively. Multicollectors are on the linear trolley that is not exactly along the focal plane of the mass spectrometer. Therefore, ions that are detected at the positions far from optical axis would be defocused at the exit slit plane. In the case of sulfur four (32, 33, 34, 36) isotope analyses, mass resolution of >4000 is required to separate 33S from 32SH interference by using slit #2 (250µm). However, by using 250µm exit slit width, 32S and 36S secondary ions do not show mass spectrum with wide flat top. For this reason, we replaced exit slit plates for two end detectors (L2p and H2p) by those having wider slit width for #2 (500µm). Mass resolution of 32S and 36S are set to ~2,200 with a wide flat top peak shape after the modification (Ushikubo et al. 2014) without any significant interference peaks.

multi-collector exit slit

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