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

## TL;DR

This paper introduces a new method for detecting chlorine using selective ionization reactions that improve accuracy and reduce interference in mass spectrometry.

## Contribution

The study introduces HCl-selective post-plasma ionization using Pb2+ to enhance Cl detection robustness and matrix tolerance.

## Key findings

- Pb2+ reagent ions show higher selectivity and sensitivity for Cl detection compared to Ba2+.
- PbCl+ detection is more robust in phosphorus-containing matrices, showing a 5-fold improvement over BaCl+.
- Metal ions like Pb2+, Bi3+, and Cd2+ are more effective in forming MCl+ in the presence of interfering species.

## Abstract

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.

Tuning the metal ion in nanoESI improves Cl detection sensitivity and matrix tolerance using oxidative plasma and atmospheric-pressure post-ICP ionization.