New mass spectrometric methods for studying the atmospheric impact of hydrocarbons

Mass spectrometry has been used successfully to study polar organic compounds in air samples, but nonpolar compounds remain a challenge for this type of analysis. Two current developments address this problem by investigating the non-polar hydrocarbons in synthetic air: using membrane introduction mass spectrometric (MIMS), and using new atmospheric pressure reactors. Selectivity enhancement of the MIMS was achieved by application of an ozone chemical pretreatment and by using nitric oxide chemical ionization. The basis of the ozonolysis was the exclusion of the polar ozonolysis products by the membrane. Nitric oxide chemical ionization was applied to produce strong unique ion signals for alkanes and alkenes. The quantitation of the methods was evaluated by constructing calibration curves for all of tested hydrocarbons. The nature and the yields of the ozonolysis products were analyzed by newly designed interchangeable atmospheric pressure reactors coupled to an ion trap mass spectrometer (ITMS). The O3-tetramethylethene (TME) reaction was used as a sample case. A static reactor was used to distinguish between different reaction products. A flow-tube reactor was used to study the kinetics of the products in different time ranges, as well as the product yields.; Ozone reacted to suppress at least 95% of the MIMS signal from beta-pinene in a mixture of toluene and beta-pinene, and the signal from cyclohexene in a mixture of cyclohexene and cyclohexane. The ozone modulation was useful for removing alkenes from alkane/alkene mixtures. NO CI showed the ability to produce detectable signals for all test hydrocarbons. The calibration curves of the two methods showed good linearity (R2 ∼ 0.97) with 10% relative standard deviation. The detection limits of most tested compounds were in the range of parts-per-billion by volume (ppbv). The yields of acetone and hydroxyacetone in the O3/TME reaction were 98 and 18% respectively, in agreement with prior literature results. These results indicate that MIMS with ozone pretreatment and NO could be useful for environmental analytical application provided the monitored compound was at elevated levels. These new reactors could be used to study other types of gas phase reactions.