Nitrogen MICAP with post-plasma ionization mass spectrometry for elemental fluorine quantitation

Abstract

Recent developments in high-power nitrogen microwave plasmas have made them attractive excitation and ionization sources for elemental analysis, offering robust operation with liquid sample introduction and significant cost savings. However, nonmetal detection, particularly F analysis, remains challenging with these plasmas because of their lower excitation and ionization temperatures compared to Ar-ICP while the need for elemental F analysis continues to rise due to prevalence of fluorochemicals in pharmaceuticals and environmental contaminants. Here, we combine the advantages of a N₂ microwave inductively coupled atmospheric pressure plasma (MICAP) with those of post-plasma chemical ionization and evaluate the approach for elemental F analysis. The MICAP acts as a robust plasma reactor to break down fluorochemicals into HF which is then ionized to ScFNO₃(H₂O)_n⁺ and detected by a quadrupole MS as ScF⁺ after in-source ion activation. The approach offers a sensitivity of ∼18 cps ng⁻¹ mL of F, superior to < 4 cps ng⁻¹ mL of F achieved by BaF⁺ detection in Ar-ICP-MS/MS. An LOD of ∼40 ng mL⁻¹ of F is obtained comparable to those of Ar-ICP-MS/MS, limited by background equivalent concentration likely dominated by F contamination. F response factors are also independent of chemical species when sample introduction biases are minimized. However, broadening of flow injection peaks over time is observed, especially with high oxygen levels in the aerosol gas, denoting future areas of improvement for better analytical performance. These studies indicate a high potential of MICAP with post-plasma chemical ionization for F quantitation.