Confirmation of 40Ar+ related product ions in the octopole reaction cell of an ICP-QMS/QMS with 18O2 enriched oxygen as the reaction cell gas

Abstract

⁴⁰Ar⁺ related product ions in an octopole reaction cell (ORC) were observed with an inductively coupled plasma tandem quadrupole mass spectrometer (ICP-QMS/QMS). In order to simplify the ion composition of the plasma, the carrier gas was directly connected to the torch injector, i.e. without introduction of the sample solution. The m/z of the first QMS was set to 40 permitting the passage of ⁴⁰Ar⁺. Isotope enriched oxygen gas (¹⁸O₂) was used as the reaction gas in order to separate the spectra of O₃⁺ and Ar₂O²⁺. The intensities of the ions produced in the ORC were measured by the second QMS. As the products of the reactions among ⁴⁰Ar⁺ ions and ¹⁸O₂ molecules in the ORC, the following ions were observed: ¹⁸O⁺, ¹⁸O₂⁺, ¹⁸O₃⁺, ¹⁸O₄⁺, ⁴⁰Ar¹⁸O⁺, ⁴⁰Ar¹⁸O²⁺, ⁴⁰Ar₂¹⁸O²⁺, and ⁴⁰Ar¹⁸O₂²⁺. At a reaction gas flow rate of 0.3 mL min⁻¹, the ionic species with the highest signal intensity was ¹⁸O₂⁺, followed by ⁴⁰Ar⁺ and ¹⁸O⁺. The maximum signal intensities of different product ions were obtained at different flow rates of the reaction gas. A lower flow rate of the reaction gas benefited the production of ⁴⁰Ar¹⁸O⁺ and ¹⁸O₃⁺, while a higher flow rate of the reaction gas benefited the production of ⁴⁰Ar¹⁸O²⁺ and ⁴⁰Ar₂¹⁸O²⁺. The maximum transfer efficiency from ⁴⁰Ar⁺ to the resulting ionic species was 91%, 2.0%, and 0.12% for ¹⁸O₂⁺, ¹⁸O⁺, and ⁴⁰Ar¹⁸O⁺, respectively. The maximum transfer efficiency to the other investigated ionic species was less than 0.023%. By using ¹⁸O₂ as the reaction gas, the mechanisms of ⁴⁰Ar⁺ resulted ionic species interfering with the measurement of ³²S¹⁶O⁺ were confirmed. The results showed that the contributions of ¹⁶O₃⁺ and ⁴⁰Ar₂O²⁺ were almost equivalent to each other.