HCl-selective ionization reactions for improved Cl detection robustness in post-inductively coupled plasma chemical...

Halogen detection, especially in a multielement fashion and with liquid chromatography, is challenging because of the fundamental limitations of ionization in inductively coupled plasma (ICP) mass spectrometry. Post-plasma ionization alleviates these limitations. For Cl detection, plasma-assisted HCl formation followed by chemical ionization using barium-containing reagent ions has been reported, yielding BaCl⁺ and offering simultaneous detection with other halogens (e.g. F detection using BaF⁺). However, the broad reactivity of barium-based ionization also makes it susceptible to interference from other plasma-generated species, increasing the potential for ionization suppression, sensitivity loss, and elevated matrix effects. Here, we report the development of HCl-selective post-plasma ionization reactions to improve ionization robustness and matrix tolerance in Cl detection. We evaluate a series of metal ions (M) with aqueous-phase affinity for chloride to determine their potential to form MCl(NO₃) _n ⁺ (n = 0-1) in the post-ICP region. We show that metal ions follow the order Pb²⁺, Bi³⁺, Cd²⁺ > In³⁺ > Zn²⁺, Fe³⁺ >> Ba²⁺ in terms of propensity of their reagent ions to react with HCl in the presence of interfering plasma products such as HNO₃ and HNO₂. Among the metal ions, Pb²⁺ also offers the highest sensitivity for Cl detection because of efficient reagent ion generation by electrospray ionization. The robustness of PbCl⁺ detection is also tested in P-containing matrices, revealing a 5-fold improvement relative to that with BaCl⁺ and further confirming improved matrix tolerance due to more selective ionization reactions. These studies offer fundamental insights and pave the way towards robust multielement methods for halogen detection in speciation analyses.