Improvement of abundance sensitivity in a quadrupole-based ICP-MS instrument with a hexapole collision cell

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Abstract
High isotopic abundance sensitivity is desirable for the measurement of low-abundance isotopes in the presence of neighboring high-abundance isotopes. This is of interest when analyzing ultra low concentrations of trace elements or isotopes in the presence of elements or isotopes with much higher concentrations (e.g., radionuclides included in a matrix of stable isotopes). In this study, a quadrupole-based inductively coupled plasma mass spectrometer with a hexapole collision cell (ICP-CC-QMS), Platform ICP (Micromass UK), was used to study the improvement of abundance sensitivity using uranium and lutetium as examples. The median kinetic energy of 238U+ ions was about 1.2 eV and 0.9 eV when helium was introduced as a collision gas at flow rates of 5 ml min−1 and 10 ml min−1, respectively. In the latter case, the proportion of ions with relatively high energy decreased significantly, which improved both the transmission of ions through the hexapole collision cell and mass selection by the quadrupole mass spectrometer. When introducing helium into the collision cell a reduction in peak tail of high-abundance isotopes by up to three orders of magnitude, depending on the mass analyzed, was observed. The abundance sensitivity for 236U/238U isotope ratio was improved from 2.3 × 10−5 to 6.3 × 10−8. Abundance sensitivities as low as 3.5 × 10−8, 6.6 × 10−8 and 2.0 × 10−8 were achieved at masses m − 2.5 u, m − 1.5 u and m + 2.5 u (m = 174.94 u for 175Lu), respectively. Reducing the kinetic energy also improved the mass resolution by about 10%. However, increasing the mass resolution of the quadrupole instrument had a significantly lower influence on abundance sensitivity than reducing the ion energy by collision with helium atoms. The present study of the effect of the pressurized ion guide on abundance sensitivity was limited to some extent because of the relatively high background count rate of the Daly-type ion detector in the ICP-CC-QMS system. An ion detector possessing a low background would be desirable for further studies. Improved abundance sensitivity can be useful in analyzing long-lived radionuclides like 236U and 129I in the presence of high-abundance stable isotopes like 238U and 127I.

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