Fluorine-selective post-plasma chemical ionization for enhanced elemental detection of fluorochemicals

Abstract

Elemental analysis of fluorochemicals has received renewed attention in recent years stemming from the increased use of fluorinated compounds. However, fundamental drawbacks of in-plasma ionization have hindered ICPMS applications in this area. Recently, we have introduced post-ICP chemical ionization for BaF⁺ formation using Ba-containing reagent ions supplied by nanospray, leading to major improvements in F detection sensitivity. Here, we present further insights into this post-plasma chemical ionization. First, we examine the effect of oxygen introduced into the plasma (a necessity for organic solvent introduction) on BaF⁺ ion formation. The results indicate that excess plasma oxygen leads to abundant HNO₃ in the post-plasma flow, shifting ionization reactions toward BaNO₃⁺ formation and suppressing BaF⁺ sensitivity. To amend this, we utilize reagent ions with other metal centers to impart selectivity toward F detection. Our investigations show that robustness of F detection in the presence of abundant HNO₃ improves in the order Al³⁺ ≈ Sc³⁺ > La³⁺ > Mg²⁺ > Ba²⁺ as the metal center in the reagent ions, consistent with the stronger metal–F bond in the series. Sc-based ionization resulting in ScNO₃F⁺ shows the best balance between sensitivity and robustness in the presence of nitric acid. Similarly, this ion shows an improved tolerance relative to BaF⁺ for a Cl-containing matrix where HCl interferes with ionization. Finally, we demonstrate a unique feature of post-plasma chemical ionization for real-time flagging of matrix effects via monitoring reagent ions. These findings provide significant improvements of post-plasma chemical ionization for elemental F analysis, particularly for online chromatographic detection where solvent gradients are utilized.