Sensitive detection of <i>n</i>-alkanes using a mixed ionization mode proton-transfer-reaction mass spectrometer

Abstract

Abstract. Proton-transfer-reaction mass spectrometry (PTR-MS) is a technique that is widely used to detect volatile organic compounds (VOCs) with proton affinities higher than water. However, n-alkanes generally have a lower proton affinity than water and therefore proton transfer (PT) by reaction with H3O+ is not an effective mechanism for their detection. In this study, we developed a method using a conventional PTR-MS to detect n-alkanes by optimizing ion source and drift tube conditions to vary the relative amounts of different primary ions (H3O+, O2+, NO+) in the reaction chamber (drift tube). There are very few studies on O2+ detection of alkanes and the mixed mode has never been proposed before. We determined the optimum conditions and the resulting reaction mechanisms, allowing detection of n-alkanes from n-pentane to n-tridecane. These compounds are mostly emitted by evaporative/combustion process from fossil fuel use. The charge transfer (CT) mechanism observed with O2+ was the main reaction channel for n-heptane and longer n-alkanes, while for n-pentane and n-hexane the main reaction channel was hydride abstraction (HA). Maximum sensitivities were obtained at low E ∕ N ratios (83 Td), low water flow (2 sccm) and high O2+ ∕ NO+ ratios (Uso = 180 V). Isotopic 13C contribution was taken into account by subtracting fractions of the preceding 12C ion signal based on the number of carbon atoms and the natural abundance of 13C (i.e., 5.6 % for n-pentane and 14.5 % for n-tridecane). After accounting for isotopic distributions, we found that PT cannot be observed for n-alkanes smaller than n-decane. Instead, protonated water clusters of n-alkanes (M ⋅ H3O+) species were observed with higher abundance using lower O2+ and higher water cluster fractions. M ⋅ H3O+ species are probably the source for the M + H+ species observed from n-decane to n-tridecane. Normalized sensitivities to O2+ or to the sum of O2++ NO+ were determined to be a good metric with which to compare sensitivities for n-alkane detection between experiments. Double hydride abstraction was observed from the reaction with O2+. Sensitivity to CT increased with carbon chain length from n-pentane to n-dodecane, sensitivity to HA increased from n-heptane to n-dodecane and sensitivity to PT increased from n-decane to n-tridecane. Sensitivity to CT exponentially decreased with molecular ionization energy, which is inversely related to the carbon chain length. We introduce a calibrated fragmentation algorithm as a method to determine the concentrations of n-alkanes and demonstrate its effectiveness using a custom n-alkane mixture and a much more complex oil example representing perhaps the most difficult mixture available for application of the method. We define optimum conditions for using the mixed ionization mode to measure n-alkanes in conventional PTR-MS instruments regardless of whether they are equipped with switchable reagent ion (SRI) capabilities.

Characterizing sources and emissions of volatile organic compounds in a northern California residence using space‐ and time‐resolved measurements

“…Alkanes constitute a major class of VOCs that do not react with H 3 O + and hence cannot generally be measured by PTR‐ToF‐MS. However, for some highly abundant alkanes, reactions with impurity reagent ions (ie, O 2 + and NO + ; <5%) might also produce a high enough signal to allow for detection by the instrument …”

“…However, for some highly abundant alkanes, reactions with impurity reagent ions (ie, O 2 + and NO + ; <5%) might also produce a high enough signal to allow for detection by the instrument. 35 The instrument was operated at a drift-tube pressure of 2.3 mbar, a drift-tube temperature of 75°C, an E/N ratio of 120 Td, and an inlet temperature of 70°C. The instrument background was determined a few times per day using zero air generated by passing filtered ambient air through a platinum catalyst heated to 350°C.…”

Identification and quantification of VOCs by proton transfer reaction time of flight mass spectrometry: An experimental workflow for the optimization of specificity, sensitivity, and accuracy

“…A similar mechanism mixed ionization mode. 23 The presence of this reaction channel must be considered when using O 2 + to measure phenols at low E/N values in water-rich matrices; its contribution is estimated as follows:…”

VOC emission rates over London and South East England obtained by airborne eddy covariance

“…Due to O 2 + and NO + impurities in PTR-MS ($1-5%), elevated concentrations of butane and higher alkanes are going to be detected at the most abundant ion fragments such as at m/z 57 and 71, which will contribute to the ion signals of MTBE and MVK/MACR. 22 In PTR-MS several major vehicle exhaust emission components are expected to contribute to the ion signal at m/z 57, including MTBE and C 4 H 8 (butene) isomers. The butenes react to form protonated molecular ions at this mass while greater than 95% of the MTBE proton transfer reaction products fragment to the mass 57 ion.…”