An experimental and theoretical investigation into the use of H₂ for the simultaneous removal of ArO⁺ and ArOH⁺ isobaric interferences during Fe isotope ratio analysis with collision cell based Multi-Collector Inductively Coupled Plasma Mass Spectrometry

Summary, in English

Hydrogen in a hexapole collision cell is used with varying success in multi-collector Inductively Coupled Plasma Mass Spectrometry to reduce the plasma derived interferences ⁴⁰Ar¹⁶O⁺ and ⁴⁰Ar¹⁶OH⁺ that are isobaric with ⁵⁶Fe⁺ and ⁵⁷Fe⁺ respectively. The reactions of ArO⁺ and ArOH⁺ with H₂ in the hexapole of a multi-collector Inductively Coupled Plasma Mass Spectrometer were studied practically and theoretically to better constrain possible reduction mechanisms. Addition of H₂ into the hexapole caused the signal of ArOH⁺ to increase (+ 30%) suggesting its formation there. Reactions in the hexapole cell become dominant over transmission at a lower r.f. setting for ArOH⁺ than for ArO⁺, indicating that ArOH⁺ reacts more efficiently within the hexapole. Increasing H₂ flow rate caused a decrease in background equivalent concentrations of both ArO⁺ and ArOH⁺ with a lower ArOH⁺ decrease rate due to formation from ⁴ArO⁺:ArO⁺ + H₂ → ArOH⁺ + H followed by ArOH⁺ + H₂ → Ar + H₂ O⁺ + H. Ab initio calculations show ArO⁺ to have two low lying spin states; a quartet (⁴ArO⁺) and low lying excited doublet (²ArO⁺) that is likely to be metastable. Although highly exothermic (- 538 kJ mol^{- 1}), reaction of ⁴ArO⁺ with H₂ to form H₂O⁺ is spin forbidden. Formation of ArOH⁺ from ArO⁺ is exothermic (- 26 kJ mol^{- 1} and - 51 kJ mol^{- 1} from ⁴ArO⁺ and ²ArO⁺ respectively) and spin-allowed, supporting the formation of ArOH⁺ from ArO⁺ in the hexapole. The reaction ArOH⁺ + H₂ → Ar + H₂O⁺ + H (- 39 kJ mol^{- 1} and - 61 kJ mol^{- 1} from ¹ArOH⁺ and ³ArOH⁺ respectively) is likely the mechanism of ArOH⁺ removal. For ¹ArOH⁺ (possibly produced from ²ArO⁺) there may be a kinetic barrier for removal, giving a possible further explanation to the persistence of ArOH⁺.

• Author: T. Arnold; J. N. Harvey; D. J. Weiss
• Publishing year: 2008-06
• Language: English
• Pages: 666-672
• Publication/Series: Spectrochimica Acta - Part B Atomic Spectroscopy
• Volume: 63
• Issue: 6
• Links: http://dx.doi.org/10.1016/j.sab.2008.04.009
• Document type: Journal article
• Publisher: Elsevier
• Keywords: Argon based interferences; Collision cell; Hexapole reaction cell; Hydrogen reaction gas; Iron isotope analysis; MC-ICP-MS; Potential energy surface
• Status: Published
• ISBN/ISSN/Other: ISSN: 0584-8547