DART–MS Negative Ionization of Vinylidene Fluoride (VDF) Telomers Reveals the Importance of [M + HCO₃]⁻ and [M + HCO₄]⁻ Association

Rationale : Direct analysis in real time (DART) is an open–air ion source that does not require a sample preparation step. Only a few studies have focused on the negative‐ion formation processes for low–polarity molecules. In this study, an ω‐iodo oligo (vinylidene fluoride) telomer of known composition was investigated to elucidate the involved DART ionization mechanisms.

Methods : A DART ionization source was coupled to a Fourier transform ion cyclotron resonance mass spectrometer (FT–ICR MS), whose high m/z measurement accuracy enabled the unambiguous assignment of molecular formulas to the detected ions. Tandem MS experiments were performed to confirm these assignments. Data interpretation and visualization were carried out using Kendrick plots.

Results : Numerous VDF telomer distributions were identified, each attributed to C₆F₁₃(C₂H₂F₂)_nI or C₆F₁₃(C₂H₂F₂)_nH, either in their deprotonated form or forming adducts or associations with I⁻, O₂^•−, NO₂⁻, NO₃⁻, HCO₂⁻, HCO₃⁻, CO₃^•−, and HCO₄⁻. The [M + HCO₃]⁻ and [M + HCO₄]⁻ anions were the most abundant, highlighting the crucial role of carbonate–linked anions in the DART ionization of PVDF. This observation was further supported by neutral losses corresponding to H₂CO₃ and H₂CO₄ in tandem mass spectrometry experiments. Furthermore, the oxidative properties of HCO₄⁻ were emphasized. These results demonstrated the need to reinterpret data obtained in a previous DART‐MS study on PVDF samples with unknown end groups.

Conclusions : The complex and diverse ionization processes observed in the negative DART ionization of fluorinated telomers underline the need for careful data interpretation. The uncommon [M + HCO₃]⁻ and [M + HCO₄]⁻ adducts were found to play a significant role in the ionization of iodinated PVDF.