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

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 238 U 1 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 21 and 10 ml min 21 , 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 236 U/ 238 U isotope ratio was improved from 2.3 6 10 25 to 6.3 6 10 28 . Abundance sensitivities as low as 3.5 6 10 28 , 6.6 6 10 28 and 2.0 6 10 28 were achieved at masses m 2 2.5 u, m 2 1.5 u and m 1 2.5 u (m~174.94 u for 175 Lu), 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 236 U and 129 I in the presence of high-abundance stable isotopes like 238 U and 127 I.

Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS for isotope analysis of long-lived radionuclides

“…The abundance sensitivity of the mass spectrometer used is restricted by ions with higher kinetic energy so that reducing the kinetic energy of ions from several eV to <0.1 eV by collisions with gas atoms (He, Xe, Ar) or molecules (H 2 , O 2 , CH 4 , NH 3 ) introduced in a collision cell of ICP-MS leads to improved abundance sensitivity. For example, the collision of analyte ions with He (at a flow rate of 10 mL min −1 ) in the hexapole collision cell results in a reduction of kinetic energy from several eV down to 1 eV and an improvement of abundance sensitivity lower than 6 × 10 −8 was achieved (e.g., for 236 U/ 238 U ratio) in quadrupole ICP-MS (Platform) with a hexapole collision cell [49]. In comparison to ICP-CC-QMS, the abundance sensitivity for 236 U/ 238 U was observed in ICP-QMS (Elan 6000), ICP-SFMS (ELEMENT) and MC-ICP-MS (Nu Plasma) as 6 × 10 −7 , 5 × 10 −6 and 3 × 10 −7 , respectively.…”

“…The abundance sensitivity, which is the contribution of the neighboring peak to the intensity of a measured isotope, influences the accuracy of isotope abundance measurements [49]. For example, the determination of 239 Pu or 236 U ultratraces in the presence of a high uranium concentration is not possible at low abundance sensitivity of the mass spectrometer [50].…”

State-of-the-art and progress in precise and accurate isotope ratio measurements by ICP-MS and LA-ICP-MS : Plenary Lecture

“…4 For example, the determination of 239 Pu or 236 U ultratraces in the presence of a high uranium concentration is impossible at the low abundance sensitivity of the mass spectrometer. 13 Further sources of uncertainty are the instrumental background; contamination due to the solution introduction system, the sampler and skimmer cones and lens system; mass scale drift effects; plasma instabilities; drifts on ion intensities; and matrix effects. For example, Galy et al…”

Direct Determination of Gold in Rock Samples Using Collision Cell Quadrupole ICP-MS

“…This is achieved by setting the collision cell bias slightly less positive than the quadrupole bias. This means that the collision product ions, having the same energy as the cell bias, are discriminated against and rejected, while the analyte ions, having a higher energy than the cell bias, are transmitted [13,[17][18][19][20][21][22][23]. Because polyatomic ions collide with collision gas more frequently than with analyte ions, most of the analytes maintain a higher energy than the collision product ions.…”