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Increased applications of fluorochemicals have prompted development of elemental methods for detection and quantitation of these compounds. However, high-sensitivity detection of fluorine is a challenge because of difficulties in excitation and ionization of this element. Recently, a new approach has emerged to detect F as a diatomic ion (BaF+) in inductively coupled plasma mass spectrometry (ICP-MS). However, formation of this species in the high-temperature plasma is inefficient, leading to low sensitivities. Here, we introduce a post-ICP chemical ionization approach to enhance analytical performance for F detection in liquid samples. Solutions of fluorochemicals are introduced into an ICP leading to formation of HF in the afterglow. Subsequently, reagent ions from nanospray of sodium acetate and barium acetate electrolytes are utilized to ionize HF to Na2F+ and BaF+, respectively, via post-plasma ion-neutral reactions. Both ions provide substantially better sensitivities compared to that of BaF+ formed inside the plasma in conventional ICP-MS methods. Notably, post-plasma BaF+ offers a sensitivity of 280 cps/ppb for F, near two orders of magnitude higher than that of conventional ICP-MS methods. Compound-independent response for F from structurally diverse organofluorines is confirmed by monitoring BaF+ and a limit of detection (LOD) of 8–11 ng/mL F is achieved. Importantly, isobaric interferences are substantially reduced in chemical ionization, leaving F background as the main factor in LOD determination. Insights into BaF+ formation via experimental and computational investigations suggest that BaNO2+ and Ba(H2O)n +2 serve as reagent ions while nonreactive BaCH3CO2+ is the dominant ion produced by nanospray. The facile development of effective post-plasma ionization chemistries using the presented approach offers a path for further improvements in F elemental analysis.
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Elemental fluorine detection by dielectric barrier discharge coupled to nano-ESI mass spectrometry for non-targeted analysis of fluorinated compounds
The growing use of fluorochemicals has elevated the need for non-targeted detection of unknown fluorinated compounds and transformation products. Elemental mass spectrometry coupled to chromatography offers a facile approach for such analyses by using fluorine as an elemental tag. However, efficient ionization of fluorine has been an ongoing challenge. Here, we demonstrate a novel atmospheric-pressure elemental ionization method where fluorinated compounds separated by GC are converted to Na2F+ for non-targeted detection. The compounds are first introduced into a helium dielectric barrier discharge (DBD) for breakdown. The plasma products are subsequently ionized by interaction with a nano-ESI plume of sodium-containing aqueous electrolytes. Our studies point to HF as the main plasma product contributing to Na2F+ formation. Moreover, the results reveal that Na2F+ is largely formed by the ion-neutral reaction between HF and Na2A(NaA)n+, gas-phase reagent ions produced by nano-ESI where A represents the anion of the electrolyte. Near-uniform fluorine response factors are obtained for a wide range of compounds, highlighting good efficiency of HF formation by DBD regardless of chemical structure of the compounds. Detection limits of 3.5 to 19.4 pg fluorine on-column are obtained using the reported GC-DBD-nano-ESI-MS. As an example of non-targeted screening, extractions from oil-and-water-repellent fabrics are analyzed via monitoring Na2F+, resulting in detection of a fluorinated compound on a clothing item. Notably, facile switching of the ion source to atmospheric-pressure chemical ionization with the exact same chromatographic method allows identification of the detected compound at the flagged retention time.
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- Award ID(s):
- 1904835
- PAR ID:
- 10169161
- Date Published:
- Journal Name:
- Analytical Chemistry
- ISSN:
- 0003-2700
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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 HNO 3 in the post-plasma flow, shifting ionization reactions toward BaNO 3 + 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 HNO 3 improves in the order Al 3+ ≈ Sc 3+ > La 3+ > Mg 2+ > Ba 2+ as the metal center in the reagent ions, consistent with the stronger metal–F bond in the series. Sc-based ionization resulting in ScNO 3 F + 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.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.analchem.0c02141
