Comparison of scandium-based post-plasma chemical ionization approaches for elemental fluorine detection by nitrogen microwave-sustained inductively-coupled atmospheric-pressure plasma

Fluorine detection has been a challenge in elemental mass spectrometry because of inefficient ion generation. Polyatomic analytical ion formation has emerged as a promising strategy to address this limitation. Here, we compare several post-plasma ion-neutral reactions that leverage affinity between scandium and F atoms to form analytical ions. A nitrogen microwave-sustained inductively-coupled atmospheric-pressure plasma (MICAP) is utilized to decompose fluorochemicals. The plasma products are sampled into an interface designed to minimize their adsorption to the surfaces, and are ionized in two main ways: 1) by scandium-containing ions generated using nano-electrospray ionization (nESI); 2) by addition of scandium to the plasma coupled with a post-MICAP corona discharge. Both avenues are also evaluated in positive- and negative-ion modes, producing ions such as ScFNO₃(H₂O)_n ⁺ and ScF(NO₃)₃ ^-. In positive-ion mode, both approaches show similar fundamental characteristics, suggesting HF ionization as the main signal generation pathway. However, in negative-ion mode, nESI shows significantly lower ionization efficiency compared to that of the post-plasma corona discharge approach. The better ionization efficiency by negative-mode corona discharge ionization is attributed to production of more effective reagent ions for HF ionization, and possibility of ionizing Sc- and F-containing neutral plasma products. Importantly, the isobaric interferences are significantly decreased in negative-ion mode, improving analytical performance. Further, negative-mode ionization offers better tolerance to sodium matrix compared to that of positive-mode ionization, attributed to decreased clustering tendencies of negative ions with plasma-produced NaNO₃ upon sodium introduction. These studies provide critical insights to further improve post-plasma chemical ionization for F detection.