Advantages of SRI-

Proton-Transfer-Reaction mass spectrometry (PTR-MS) using H₃O⁺ reagent ions is an established low-pressure chemical ionization method for real-time monitoring and direct quantification of volatile organic compounds (VOC). While this method covers the vast majority of organic compounds, further primary selective reagent ions (SRI) like NO⁺, O₂⁺, NH₄⁺ can significantly increase our understanding of the chemical composition of complex samples. Recently, IONICON introduced a next-generation PTR-MS, the FUSION PTR-TOF, boosting the sensitivity to unprecedented levels while still conserving the genuine, predictable ion chemistry. IONICON now introduces SRI^-, extending the instrument capabilities to the negative ion mode. Owing to the incorporation of the novel Fusion RF reaction chamber, reagent ions like CO₃^- become accessible. These enable the detection of inorganic compounds like SO₂, HNO₃, H₂SO₄, other inorganic acids and organic acids. Switching requires no hardware modifications and can be done fully automated. Hence, the bipolar FUSION PTR-TOF offers maximum flexibility to capture both quantitative H₃O⁺ and CIMS-like CO₃^- data in real-time with one single instrument.

Characterization of SRI^-

CO₃^- ionization is most selective to acids. A large number of organic acids are detected predominantly via H⁺ abstraction; some compounds like larger perfluoroalkyl carboxylic acids (PFCAs) also show an increasing degree of A.CO₃^- adduction ionization. An intercomparison with H₃O⁺ proton transfer reaction reveals a simplified detection scheme with significantly reduced fragmentation.

Ambient detection of PFCAs

Perfluoroalkyl carboxylic acids (PFCA) are fluorine containing organic acids and present a subgroup of per- and polyfluoroalkyl substances (PFAS). Similar to PFASs, PFCAs are of concern for the environment and human health as they can accumulate in the environment and in living organisms like animals and humans. Today, these synthetic compounds have been found all around the globe. PFCAs can be detected using CO₃^- ionization. Two of them, C₂F₃O₂^- and C₄F₇O₂^-, are exemplary shown from a dataset of ambient air in Innsbruck, Austria. Trifluoroacetic acid (TFA, C₂HF₃O₂) is typically used in analytical chemistry but has also been detected in beverages like beer and tea or in the environment. Perfluorobutanoic acid (PFBA, C₄HF₇O₂) is a product from the oxidation of other PFAS. 

Ambient detection of SO₂

In addition to acids, inorganic compounds like SO₂ can finally be detected via SRI^-. SO₂ is commonly emitted by burning of fossil fuels and certain industrial processes and is harmful to health and the environment. Ionization via SRI^- results in a linear response > 150 ppbV and limits of detection in low double digit pptV levels. Ambient measurements reveal the high variability of SO₂ in Innsbruck in April.

Get SRI^- for FUSION PTR

Have a look at the FUSION PTR-TOF Series and consider optional bipolar TOF option for your instrument. Get in touch with us for your instrument quote!