A reaction cell as a sample introduction portal for detection of gaseous components in ICP-MS

Abstract

The detection of gaseous elements (He, Ne, Kr, and Xe) and molecules (CO₂, CH₄, and NH₃) using inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) is described. In this study, sample gasses were introduced into a collision/reaction cell, and the sample gas components were ionised either by charge-transfer or by protonation reactions in the reaction cell through an ion reaction with the primary ions produced in the ICP. Ions originating from He and Ne, elements whose first ionisation potentials are greater than that of Ar, were not detected, whereas Kr and Xe were ionised through collision with Ar⁺. This implies that the ionisation can proceed basically by charge transfer reactions under equilibrium. The background counts for these elemental or molecular/polyatomic ions originating from either air engulfment into the ICP or from gas impurities in Ar can be effectively removed by the first quadrupole mass filter. The magnitude of softness/hardness of the ionisation can be controlled by selecting the primary ions (i.e., H⁺, Ar₂⁺, and Ar⁺, in descending order of softness), and the resulting signal intensities for [M − H]⁺, M⁺, or [M + H]⁺ ions changed as reflecting a proton affinity or electronegativity of atoms, or polarity of molecules. Overall transmission efficiencies through ionisation, ion transport, mass separation, and ion detection processes for CO₂⁺ and CH₃⁺ were about 3.6 × 10⁻¹⁰%, 1.6 × 10⁻¹⁰%, and 3 × 10⁻⁸%, respectively. The technique described here can become a rapid and on-line analysis technique for both gaseous or volatile organic compounds (VOCs).