Quantification of n-heptane low temperature oxidation products by proton-transfer-reaction mass spectrometry

While studies on the low temperature oxidation of organic compounds have been in depth, fast and accurate quantitative analysis of oxidation products, especially for those without standard samples, remains challenging and requires analytical developments. This work studied n-heptane low temperature oxidation in a jet-stirred reactor (JSR) using proton-transfer-reaction mass spectrometry (PTR-MS). The oxidation products were also analyzed by a synchrotron vacuum ultraviolet radiation photoionization mass spectrometer (SVUV-PIMS) for comparison. The results showed that PTR-MS was able to measure most oxygenated intermediates compared to SVUV-PIMS. Furthermore, the method for the quantification of low temperature oxidation products was studied. By employing a series of calibration gases, the sensitivities of PTR-MS were confirmed to be linearly related to proton-transfer reaction rates, which were calculated using the trajectory parameterization theory. Mole fraction profiles of oxidation products measured by PTR-MS were compared to SVUV-PIMS and literature gas chromatography data, and in most cases, the results showed good consistency. Special attention is needed for some molecules, such as carboxylic acids, formaldehyde, and peroxides. The effect of the humidity of the reaction environment and the fragment on the quantification need to be considered. Overall, PTR-MS is a promising instrument for the quantitative analysis of combustion products and has the potential to measure the total mole fraction of complex isomers (could not distinguish isomers) and to predict the mole fraction of products without a standard sample.