Investigation of mass‐scale drift effects in the milli‐mass range: Influence on high mass resolution mode multicollector‐inductively coupled plasma mass spectrometer isotope ratio measurements

The consequences, possible origins, and prevention of mass‐scale drifts in the high mass resolution mode (HR, M /Δ M ≈ 8000) under constant conditions were investigated and simulated in case of a multicollector‐inductively coupled plasma mass spectrometer (MC‐ICP‐MS) using silicon enriched in 28 Si as the main element in this survey. A drifting mass scale strongly impairs the precise and accurate determination of isotope ratios, depending especially on the peak/plateau width. For example, 29 Si + in Si highly enriched in 28 Si has an extremely small mass plateau width of Δ M ≤ 4 × 10 −3 u, compare Δ M ( 56 Fe + ) ≈ 18 × 10 −3 u, which is to our knowledge one of the smallest plateaus routinely investigated in isotope ratio measurements, thus requiring extreme stability. During warm‐up of the double‐focusing sector field mass spectrometer, a mass drift up to Δ M /Δ t ≥ 0.006 u/hr has been observed. Long‐term studies on mass scale stability and simulations concerning fluctuations of the magnetic field B , acceleration voltage U acc and ESA voltage U ESA are reported. A change of one of these quantities of 0.01% induces changes of the mass scale of 6 × 10 −3 u, 3 × 10 −3 u, and 1 × 10 −3 u in the case of B , U acc , and U ESA , respectively. After identifying electrical charging/discharging effects in the mass spectrometer affecting the mass scale stability, the instrument was completely dismantled and carefully reinstalled. Additional stability tests using silicon, strontium, and lead finally yielded a mass drift of Δ M /Δ t ≤ 0.001 u/8 h in the case of silicon. This enhanced stability guarantees measurements of isotope ratios on smallest plateaus with lowest uncertainty. The importance of a stable mass scale is pointed out and the relevant quantities of a typical magnetic sector field mass spectrometer are discussed.