Styrene radical cations for chemical ionization mass spectrometry analyses of monoterpene hydrocarbons

Recent developments and applications of selected ion flow tube mass spectrometry (SIFT-MS)

Selected ion flow tube mass spectrometry (SIFT-MS) is now recognized as the most versatile analytical technique for the identification and quantification of trace gases down to the parts-per-trillion by volume, pptv, range. This statement is supported by the wide reach of its applications, from real-time analysis, obviating sample collection of very humid exhaled breath, to its adoption in industrial scenarios for air quality monitoring. This review touches on the recent extensions to the underpinning ion chemistry kinetics library and the alternative challenge of using nitrogen carrier gas instead of helium. The addition of reagent anions in the Voice200 series of SIFT-MS instruments has enhanced the analytical capability, thus allowing analyses of volatile trace compounds in humid air that cannot be analyzed using reagent cations alone, as clarified by outlining the anion chemistry involved. Case studies are reviewed of breath analysis and bacterial culture volatile organic compound (VOC), emissions, environmental applications such as air, water, and soil analysis, workplace safety such as transport container fumigants, airborne contamination in semiconductor fabrication, food flavor and spoilage, drugs contamination and VOC emissions from packaging to demonstrate the stated qualities and uniqueness of the new generation SIFT-MS instrumentation. Finally, some advancements that can be made to improve the analytical capability and reach of SIFT-MS are mentioned.

Ligand Switching Ion Chemistry: An SIFDT Case Study of the Primary and Secondary Reactions of Protonated Acetic Acid Hydrates with Acetone

A study was performed of the reactions of protonated acetic acid hydrates, CH₃COOHH⁺(H₂O)_n, with acetone molecules, CH₃COCH₃, using a selected ion flow-drift tube (SIFDT). The rationale for this study is that hydrated protonated organic molecules are major product ions in secondary electrospray ionization mass spectrometry (SESI-MS) and ion mobility spectrometry (IMS). Yet the formation and reactivity of these hydrates are only poorly understood, and kinetics data are only sparse. The existing SIFDT instrument in our laboratory was upgraded to include an octupole ion guide and a separate drift tube by which hydrated protonated ions can be selectively injected into the drift tube reactor and their reactions with molecules studied under controlled conditions. This case study shows that, in these hydrated ion reactions with acetone molecules, the dominant reaction process is ligand switching producing mostly proton-bound dimer ions (CH₃COCH₃)H⁺(CH₃COOH), with minor branching into (CH₃COCH₃)H⁺(H₂O). This switching reaction was observed to proceed at the collisional rate, while other studied hydrated ions reacted more slowly. An attempt is made to understand the reaction mechanisms and the structures of the reaction intermediate ions at the molecular level. Secondary switching reactions of the asymmetric proton-bound dimer ions lead to a formation of strongly bound symmetrical dimers (CH₃COCH₃)₂H⁺, the terminating ion in this ion chemistry. These results strongly suggest that, in SESI-MS and IMS, the presence of a polar compound, like acetone in exhaled breath, can suppress the analyte ions of low concentration compounds like acetic acid thus compromising their quantification.