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

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 (U_so =  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.