Atmospheric Pressure Dark-Current Argon Discharge Ionization with Comparable Performance to Direct Analysis in Real Time Mass Spectrometry

Gas Chromatography-Atmospheric Pressure Inlet-Mass Spectrometer Utilizing Plasma-Based Soft Ionization for the Analysis of Saturated, Aliphatic Hydrocarbons

Soft ionization by a chemical reaction in transfer (SICRIT) is applied to couple gas chromatography (GC) to a high-resolution atmospheric pressure inlet mass spectrometer. These instruments are generally used in combination with liquid chromatography systems (LC-MS). Ionization of alkanes is not possible here with conventional electrospray ionization. Alternatively, separate GC-electron ionization (EI)-MS is employed for the analysis of nonpolar substances like alkanes, however, with the inherent challenge of strong fragmentation. In the case of alkanes, the determination of molecular masses becomes nearly impossible in complex hydrocarbon mixtures because of the wealth of similar fragment ions and the absence of the molecular ion signal. SICRIT, a soft ionization technique based on dielectric barrier discharge (DBDI), produces characteristic oxidized cations from alkanes that can be directly correlated to their molecular mass. Isotope labeling experiments reveal an ionization mechanism via hydride abstraction and reaction with water. Soft ionization can be achieved for iso- and n-alkanes, with very little fragmentation, enabling the determination of their molecular mass. Calibrations for n-alkanes from C10 to C30 were performed exhibiting high linearity, reproducibility, and sensitivity with an average LOD of 69 pg (on column). Measurements of diesel fuel samples are compared to traditional GC-EI-MS. The presented method combines sensitivity and easy handling of a GC-EI-MS with the determination of molecular mass commonly only achieved with field ionization (FI)-MS, while using existing and highly optimized mass spectrometers commonly coupled with LC. Additionally, many other analytes such as (alkylated-) PAHs could be detected simultaneously in the diesel sample.