Inductively coupled nitrogen plasma mass spectrometry assisted by adding argon to the outer gas

Abstract

An inductively coupled nitrogen plasma (N₂ ICP) has been developed using a 40.68 MHz generator with a maximum rf power of 4 kW, and evaluated as an ion source for mass spectrometry (MS). The intermediate and central Ar flows are completely replaced with N₂; however, a certain amount of Ar should be added to the outer flow to maintain a stable plasma discharge. The sampling depth for the maximum analyte signals was found to be closer to the work coil, and the signal increases with the N₂ carrier flow rate up to 2.0 l min^–1. In the mass spectra obtained under the optimized conditions, N⁺, O⁺ and NO⁺ are strongly observed, but the signal of Ar⁺ is weak. The analytical sensitivity of the proposed N₂ ICP is almost equivalent to that of an Ar ICP for elements with low ionization potentials, but inferior for elements with high ionization potentials. The secondary discharge increases the average kinetic energy of the analyte ions, and its distribution in the N₂ ICP is wider than in the Ar ICP. A wide energy distribution also seems to lead to higher ratios of doubly charged and monoxide ions to singly charged ions. Space charge effects and ionization interference from co-existing elements are also discussed.