Halogen Determinations using a Liquid Sampling-Atmospheric Pressure Glow Discharge (LS-APGD) Microplasma Ion Source Coupled to a Commercial Mass Spectrometer

Description:
Though halogens are among the most reactive chemical species, identification of these species via mass spectrometry (MS) is a challenge due to the limited ionization efficiencies of traditional ionization sources i.e., electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and inductively coupled plasma (ICP). Halogens possess very high (>1000 kJ/mol) positive ionization energies, with electron capture being energetically more favorable. While a natural thought due to its high ionization temperature, ICP performs comparatively poorly in the positive ion mode, with negative ionization having low efficiency as well. Therefore, an ionization source capable of transitioning generally non-volatile halide salts into the gas phase, with the ability to make either cations and anions, for detection by MS is needed. For this aim, a liquid sampling-atmospheric pressure glow discharge (LS-APGD) is evaluated for potential applications in the detection of halogen elements. The LS-APGD ionization source has the potential to ionize polar, nonpolar, and elemental cohorts, which, when coupled with MS, provides an alternative for the detection of many challenging species. The capabilities of the LS-APGD source to ionize a diverse group of halogen salt species is presented here. A high flow rate (500 mL min-1) of He sheath gas permitted the isolation of halogen anions in negative ion mode. In comparison, lower He flow rates (200 mL min-1) resulted in the halogens associated with a number of readily identifiable adducts when operated in positive ion mode. Response curves, taken in the positive ion mode, using 50 µL injections revealed a detection limit of 0.70 µg mL-1 NaF, 0.38 µg mL-1 NaCl, 0.31 µg mL-1 NaBr, and 0.27 µg mL-1 NaI for the LS-APGD ionization source. The use of the LS-APGD as the ionization source is anticipated to provide a sensitive method of halogen determination without the need for sample preparation.

Speaker: Cameron Stouffer - Clemson University