Gas chromatography of volatile organic compounds using a micro-plasma ion source with a differential mobility...

A micro-plasma ion source (MPIS) was combined with a differential mobility spectrometer as a detector for gas chromatographic measurements of volatile organic compounds (VOCs). Gas phase hydrated protons (H⁺(H₂O)_n) were produced in air at ambient pressure with a first generation MPIS embodiment and used to ionize eluting VOCs through atmospheric pressure chemical ionization (APCI) reactions. Ions formed from most VOCs were shown by mass analysis to be protonated monomers (MH⁺(H₂O)_n) and proton bound dimers (M₂H⁺) and provided, with mobility analysis, an additional dimension for VOC characterization along with retention time. Quantitative response for thirty-six VOCs with 3 to 10 carbons from seven chemical classes was obtained using a second-generation MPIS embodiment. Response curves were characteristic of APCI reactions with a working range of ∼10⁴ and a linear range of 10 to 50. Response factors for oxygen containing VOCs were similar for constituents within a chemical class while differences existed among chemical classes. Response curves were best for molecules with strong dipoles where detection limits were <10 pg. While MPIS is an enabling technology with ion mobility detectors for gas chromatography (GC), these findings suggest the MPIS could be used also as a stand-alone GC detector.