Replacing the Argon ICP: Nitrogen Microwave Inductively Coupled Atmospheric-Pressure Plasma (MICAP) for Mass Spectrometry

Abstract

We combine a recently developed high-power, nitrogen-sustained microwave plasma source—the Microwave Inductively Coupled Atmospheric-Pressure Plasma (MICAP)—with time-of-flight mass spectrometry (TOFMS) and provide the first characterization of this elemental mass spectrometry configuration. Motivations for assessment of this ionization source are scientific and budgetary: unlike the argon-sustained Inductively Coupled Plasma (ICP), the MICAP is sustained with nitrogen, which eliminates high operating costs associated with argon-gas consumption. Additionally, use of a commercial grade magnetron for microwave generation simplifies plasma-powering electronics. In this study, we directly compare MICAP-TOFMS performance with that of an argon-ICP as the atomic ionization source on the same TOFMS instrument. Initial results with the MICAP source demonstrate limits of detection and sensitivities that are, for most elements, on par with those of the ICP-TOFMS. The N₂-MICAP source provides a much “cleaner” background spectrum than the ICP; absence of argon-based interferences greatly simplifies analysis of isotopes such as ⁴⁰Ca, ⁵⁶Fe, and ⁷⁵As, which typically suffer from spectral interferences in ICP-MS. The major plasma species measured from the N₂-MICAP source include NO⁺, N₂⁺, N⁺, N₃⁺, O₂⁺, N₄⁺, and H₂O⁺; we observed no plasma-background species above mass-to-charge 60. Absence of troublesome argon-based spectral interferences is a compelling advantage of the MICAP source. For example, with MICAP-TOFMS, the limit of detection for arsenic is less than 100 ng L^–1 even in a 1% NaCl solution; with ICP-MS, ³⁵Cl⁴⁰Ar⁺ interferes with ⁷⁵As⁺ and arsenic analysis is difficult-to-impossible in chlorine-containing matrices.