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

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.