Measuring long chain alkanes in diesel engine exhaust by thermal desorption PTR-MS

Abstract

Abstract.A method using thermal desorption sampling and analysis by proton transfer reaction mass spectrometry (PTR-MS) to measure long chain alkanes (C 12 -C 18 ) and other larger organics associated with diesel engine exhaust emissions is described. Long chain alkanes undergo dissociative proton transfer reactions forming a series of fragment ions with formula C n H 2n+1 . The PTR-MS is insensitive to nalkanes less than C 8 but displays an increasing sensitivity for larger alkanes. Fragment ion distribution and sensitivity is a function of drift conditions. At 80 Td the most abundant ion fragments from C 10 to C 16 n-alkanes were m/z 57, 71 and 85. The mass spectrum of gasoline and diesel fuel at 80 Td displayed ion group patterns that can be related to known fuel constituents, such as alkanes, alkylbenzenes and cycloalkanes, and other compound groups that are inferred from molecular weight distributions such as dihydronapthalenes and naphthenic monoaromatics. It is shown that thermal desorption sampling of gasoline and diesel engine exhausts at 80 Td allows for discrimination against volatile organic compounds, allowing for quantification of long chain alkanes from the abundance of C n H 2n+1 fragment ions. The total abundance of long chain alkanes in diesel engine exhaust was measured to be similar to the total abundance of C 1 -C 4 alkylbenzene compounds. The abundance patterns of compounds determined by thermal desorption sampling may allow for emission profiles to be developed to better quantify the relative contributions of diesel and gasoline exhaust emissions on organic compounds concentrations in urban air.

PTR-MS fragmentation patterns of gasoline hydrocarbons

“…We have previously reported on C 6 -C 16 n-alkane fragmentation patterns and PTR-MS response factors [8]. The proton affinity for n-alkanes increases with carbon number and we observed increasing sensitivity with carbon number at 120 Td up to C 14 , but response factors are still about a third of that expected for compounds that react at the collisional rate limit with H 3 O + , similar to that observed for the dimethyl and trimethyl cyclohexanes.…”

“…Compound loss to the cold trap is a function of compound vapor pressure, mass flow rate, and conditioning time [16]. For high concentration mixtures (>10 ppbv) C 11 and C 12 compounds are transmitted without significant loss [8]. Test gas mixtures were prepared using a dynamic dilution system consisting of a syringe pump (Harvard Apparatus) to continuously dispense precise molar flows of liquid chemical compounds [9,8].…”

“…For high concentration mixtures (>10 ppbv) C 11 and C 12 compounds are transmitted without significant loss [8]. Test gas mixtures were prepared using a dynamic dilution system consisting of a syringe pump (Harvard Apparatus) to continuously dispense precise molar flows of liquid chemical compounds [9,8]. Chemicals (Sigma Aldrich, Chem Service) were used as received and had a stated purity of 96% or better.…”

“…For simplicity we used sensitivity factors relative to toluene. These have been reported for n-alkanes [8], and we assumed that iso-alkanes have the same sensitivity as n-alkanes of the same carbon number. For cycloalkanes we used the relative response factors reported here.…”

“…The fragmentation of alcohols, aldehydes, aromatics, alkanes, alkenes, ketones, esters, and monoterpenes has been documented in the literature [4,35,15,19,3,8]. The chemical composition of urban air is principally a result of vehicle emissions which contribute alkanes, cycloalkanes, alkenes, aromatics, and aldehydes as major constituents ( [29,30,27,23]).…”