Atmospheric pressure plasma assisted reaction chemical ionization for analysis of chlorinated compounds separated by liquid chromatography

Abstract

We report development of an atmospheric pressure plasma assisted reaction chemical ionization (PARCI) source with liquid sample introduction, enabling high sensitivity detection of chlorine in LC-separated compounds. In this novel approach, analytes are introduced into an argon inductively coupled plasma for high-temperature reactions including atomization. The plasma reaction products are then guided into an atmospheric pressure reaction tube where formation of negative ions is enhanced at lower temperatures. Cl⁻ ions emerging from the reaction tube are then detected by an atmospheric sampling single quadrupole mass spectrometer without any modifications. We demonstrate that Cl⁻ ions are generated in PARCI with similar efficiencies between inorganic and organic chlorine-containing compounds, confirming the elemental nature of the ionization for quantitative measurements. We further demonstrate that the ionization reactions within the reaction tube can be controlled by addition of ionization reagents to the plasma. Specifically, we find that addition of low ionization potential elements such as sodium in conjunction with methanol leads to significant enhancements in sensitivity. Chlorine sensitivity and detection limits using LC-PARCI-MS compare favorably with those obtained using state-of-the-art LC-ICP-MS/MS. Operation of PARCI in negative mode alleviates the isobaric interferences for chlorine detection and obviates the need for complex mass analyzers. Notably, the simplicity of PARCI and compatibility with atmospheric sampling mass spectrometers make this approach readily adoptable for integration of elemental quantification with molecular characterization using other ion sources.