Elemental Fluorine Detection by Dielectric Barrier Discharge Coupled to Nanoelectrospray Ionization Mass Spectrometry for Nontargeted Analysis of Fluorinated Compounds

The growing use of
fluorochemicals has elevated the need for nontargeted
detection of unknown fluorinated compounds and transformation products.
Elemental mass spectrometry (MS) coupled to chromatography offers
a facile approach for such analyses by using fluorine as an elemental
tag. However, efficient ionization of fluorine has been an ongoing
challenge. Here, we demonstrate a novel atmospheric-pressure elemental
ionization method where fluorinated compounds separated by gas chromatography
(GC) are converted to Na2F+ for nontargeted
detection. The compounds are first introduced into a helium dielectric
barrier discharge (DBD) for breakdown. The plasma products are subsequently
ionized by interaction with a nanoelectrospray ionization (nano-ESI)
plume of sodium-containing aqueous electrolytes. Our studies point
to HF as the main plasma product contributing to Na2F+ formation. Moreover, the results reveal that Na2F+ is largely formed by the ion-neutral reaction between
HF and Na2A­(NaA)n+, gas-phase reagent ions produced by nano-ESI where A represents
the anion of the electrolyte. Near-uniform fluorine response factors
are obtained for a wide range of compounds, highlighting good efficiency
of HF formation by DBD regardless of the chemical structure of the
compounds. Detection limits of 3.5–19.4 pg of fluorine on-column
are obtained using the reported GC–DBD–nano-ESI-MS.
As an example of nontargeted screening, extractions from oil-and-water-repellent
fabrics are analyzed via monitoring Na2F+, resulting
in detection of a fluorinated compound on a clothing item. Notably,
facile switching of the ion source to atmospheric-pressure chemical
ionization with the exact same chromatographic method allows identification
of the detected compound at the flagged retention time.