In-tube collision-induced dissociation for selected ion flow-drift tube mass spectrometry, SIFDT-MS: a case study of NO(+) reactions with isomeric monoterpenes

Proton affinities of aldehyde molecules determined from the forward and backward gas-phase proton transfer reactions in a selected ion flow-drift tube

Proton affinity (PA) and gas-phase basicity (GB) are important thermodynamic properties that provide insights into ion-molecule interactions. Aldehydes play a significant role in biology, the environment, and industry, but their PAs remain unknown for those with more than 5 C atoms. This study aims to experimentally determine PAs and GBs of hexanal, heptanal, and octanal using pentanal as a reference. A selected ion flow drift tube (SIFDT) was used to study proton transfer reactions among all possible combinations of protonated and neutral molecules from this set. Rate coefficients (k), equilibrium constants (K), and effective temperatures (Teff) were used to calculate Gibbs free energy changes (ΔG) and enthalpy changes (ΔH). PAs and GBs were then determined relative to the known values of pentanal. Experimental PAs were found to increase with aldehyde chain length: pentanal 796.6 kJ mol-1 < hexanal 809.6 kJ mol-1 < heptanal 813.4 kJ mol-1 < octanal 824.0 kJ mol-1. Theoretical enthalpies and entropies were obtained via density functional theory (DFT) B3LYP/6-311++G(d,p) with D4 dispersion correction for both open and bent protonated structures, allowing comparison with experimental data. The theoretical calculations for open structures underestimate the observed PAs, while the bent structures align more closely with experimental trends, indicating that larger protonated aldehydes may have bent and cyclic shapes. These findings contribute to bridging the gaps in knowledge about protonated aldehydes, providing a better understanding of their ion chemistry.

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.

The potential of NO+ and O2 +• in switchable reagent ion proton transfer reaction time-of-flight mass spectrometry

Selected ion flow tube mass spectrometry (SIFT-MS) and proton transfer reaction mass spectrometry with switchable reagent ion capability (PTR+SRI-MS) are analytical techniques for real-time qualification and quantification of compounds in gas samples with trace level concentrations. In the detection process, neutral compounds-mainly volatile organic compounds-are ionized via chemical ionization with ionic reagents or primary ions. The most common reagent ions are H₃ O⁺ , NO⁺ and O₂ ^+• . While ionization with H₃ O⁺ occurs by means of proton transfer, the ionization via NO⁺ and O₂ ^+• offers a larger variety on ionization pathways, as charge transfer, hydride abstraction and so on are possible. The distribution of the reactant into various reaction channels depends not only on the usage of either NO⁺ or O₂ ^+• , but also on the class of analyte compounds. Furthermore, the choice of the reaction conditions as well as the choice of either SIFT-MS or PTR+SRI-MS might have a large impact on the resulting products. Therefore, an overview of both NO⁺ and O₂ ^+• as reagent ions is given, showing differences between SIFT-MS and PTR+SRI-MS as used analytical methods revealing the potential how the knowledge obtained with H₃ O⁺ for different classes of compounds can be extended with the usage of NO⁺ and O₂ ^+• .

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.

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

RATIONALE

Monoterpene hydrocarbons play an important role in the formation of secondary aerosol particles and in atmospheric chemistry. Thus, there is a demand to measure their individual concentrations in situ in real time. Currently, only the total concentration of monoterpenes C₁₀ H₁₆ can be determined by chemical ionization mass spectrometry techniques using reagent ions H₃ O⁺ , NO⁺ and (C₆ H₆ )_n ^+• without gas chromatographic separation.

METHODS

The styrene cation C₈ H₈ ^+• was investigated as a reagent for chemical ionization of monoterpenes. The modified selected ion flow drift tube, SIFDT, technique was used to characterize the differences in product ion distributions between α-phellandrene, α-pinene, γ-terpinene, β-pinene, ocimene, sabinene, 3-carene, (R)-limonene, camphene and myrcene.

RESULTS

The monoterpene molecular cation C₁₀ H₁₆ ^+• is the main product (about 90%) for all isomers except (R)-limonene and camphene with an efficient channel of C₈ H₈ ^+• C₁₀ H₁₆ adduct formation and γ-terpinene with unexpectedly significant product ions at m/z 134 and 135 corresponding to losses of H₂ and H.

CONCLUSIONS

Utilization of the styrene cation for the ionization of monoterpenes is beneficial due to the very low fragmentation of the product ions. Specific association product ions for camphene and (R)-limonene and fragment product ions for γ-terpinene allow them to be distinguished from other isomers that produce mostly the molecular cation.

Quantification of volatile compounds released by roasted coffee by selected ion flow tube mass spectrometry

RATIONALE

The major objective of this exploratory study was to implement selected ion flow tube mass spectrometry, SIFT-MS, as a method for the on-line quantification of the volatile organic compounds, VOCs, in the headspace of the ground roasted coffee.

METHODS

The optimal precursor ions and characteristic analyte ions were selected for real-time SIFT-MS quantification of those VOCs that are the most abundant in the headspace or known to contribute to aroma. NO⁺ reagent ion reactions were exploited for most of the VOC analyses. VOC identifications were confirmed using gas chromatography/mass spectrometry, GC/MS, coupled with solid-phase microextraction, SPME.

RESULTS

Thirty-one VOCs were quantified, including several alcohols, aldehydes, ketones, carboxylic acids, esters and some heterocyclic compounds. Variations in the concentrations of each VOC in the seven regional coffees were typically less than a factor of 2, yet concentrations patterns characteristic of the different regional coffees were revealed by heat map and principal component analyses. The coefficient of variation in the concentrations across the seven coffees was typically below 24% except for furfural, furan, methylfuran and guaiacol.

CONCLUSIONS

The SIFT-MS analytical method can be used to quantify in real time the most important odoriferous VOCs in ground coffee headspace to sufficient precision to reveal some differences in concentration patterns for coffee produced in different countries.

Ion chemistry at elevated ion–molecule interaction energies in a selected ion flow-drift tube: reactions of H3O+, NO+ and O2+ with saturated aliphatic ketones

Rate coefficients and product ion branching ratios determined for proton transfer, association and charge transfer reactions provide insight into reaction mechanisms.