Evaluation of an Inductively Coupled Air-Argon Plasma as an Ion Source for Mass Spectrometry

Abstract

An inductively coupled air–argon plasma (air–Ar ICP) has been developed using a modified torch and a 40.68 MHz generator (maximum rf power 4 kW), and evaluated as an ion source for mass spectrometry (MS). The outer and aerosol carrier Ar flows were completely replaced with air; however, 1.5 l min^-1 of Ar was used as the intermediate gas to maintain a stable plasma discharge at low rf power. The optimized sampling depth and the aerosol carrier air flow rate for maximum analyte signals were found to be 10 mm above the load coil and 0.9 l min^-1, respectively. The analyte signal increases with rf power, but 2 kW was sufficient for the stable discharge and the acceptable analytical sensitivity. In the mass spectra obtained under the optimized conditions, N⁺, O⁺ and NO⁺ were clearly observed, but the signal for Ar⁺ was weak, which is similar to that for an N₂ ICP. The ion signals for N₂⁺ and O₂⁺ were relatively large, compared with N₂ and O₂ ICPs operated with Ar added to the outer gas. The analytical sensitivity of the proposed air–Ar ICP is superior to an Ar ICP using the same equipment for elements with low first ionization potentials (IP) of <6.5 eV, but inferior for elements with high first IPs (>6.5 eV). The secondary discharge increases the average kinetic energy of the analyte ions, the distribution of which is wider in the air–Ar ICP than in the Ar ICP. The ratios of monoxide ion to singly charged ion remain almost constant along the plasma axis. Space charge effects from co-existing elements are also discusssed.

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