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

Abstract

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: Arnold, T. ; Harvey, J. N. and Weiss, D. J.
• publishing date: 2008-06
• type: Contribution to journal
• publication status: published
• keywords: Argon based interferences, Collision cell, Hexapole reaction cell, Hydrogen reaction gas, Iron isotope analysis, MC-ICP-MS, Potential energy surface
• in: Spectrochimica Acta - Part B Atomic Spectroscopy
• volume: 63
• issue: 6
• pages: 7 pages
• publisher: Elsevier
• external identifiers: scopus:45049088391
• ISSN: 0584-8547
• DOI: 10.1016/j.sab.2008.04.009
• language: English
• LU publication?: no
• id: b291784c-79a1-4bf6-bcd2-a15c168e108b
• date added to LUP: 2024-10-24 18:41:44
• date last changed: 2025-03-14 07:04:45