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

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+, O-2(+), NO+) in the reaction chamber (drift tube). There are very few studies on O-2(+) 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 O-2(+) 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 O-2(+) / NO+ ratios (U-so = 180 V). Isotopic C-13 contribution was taken into account by subtracting fractions of the preceding C-12 ion signal based on the number of carbon atoms and the natural abundance of C-13 (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 center dot H3O+) species were observed with higher abundance using lower O-2(+) and higher water cluster fractions. M center dot H3O+ species are probably the source for the M + H+ species observed from n-decane to n-tridecane. Normalized sensitivities to O-2(+) or to the sum of O-2(+) + 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 O-2(+). 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.