Ion stopping experiment to determine the origin of background ions in icp-ms

Abstract

Ion stopping experiments have been performed using a triple grid placed between the ion extraction and collector lenses of a modified lens stack. Stopping voltages for ions extracted from the plasma were between 6.6 and 13.1 V (Be⁺), 9.8–13.1 V (Co⁺, In⁺, Ce⁺, Pb⁺), and 11.8–14.4 V (U⁺). Controlled contamination of the ion extraction interface was performed, and the stopping voltage for Be⁺ ions contributing to the subsequent background signal was determined to be between 1.5–5.9 V. This suggests that these ions originated downstream of the ion sampling interface, possibly due to deposition of Be on the skimmer cone, which was subsequently re-volatilised. The low stopping voltage of these ions meant that they would be prevented from entering the quadrupole by applying a small retarding potential on one of the lens elements (or possibly the quadrupole itself), while still admitting those ions with higher energies which were extracted from the plasma.