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

摘要

A method using thermal desorption sampling and analysis by proton transfer reaction mass spectrometry (PTR-MS) to measurelong chain alkanes (C₁₂–C₁₈) and other larger organics associatedwith diesel engine exhaust emissions is described. Long chain alkanes undergodissociative proton transfer reactions forming a series of fragment ions withformula C_nH_2n+1. The PTR-MS is insensitive to n-alkanes less thanC₈ but displays an increasing sensitivity for larger alkanes. Fragmention distribution and sensitivity is a function of drift conditions. At 80 Tdthe most abundant ion fragments from C₁₀ to C₁₆ n-alkanes werem/z 57, 71 and 85. The mass spectrum of gasoline and diesel fuel at 80 Tddisplayed ion group patterns that can be related to known fuel constituents,such as alkanes, alkylbenzenes and cycloalkanes, and other compound groupsthat are inferred from molecular weight distributions such asdihydronapthalenes and naphthenic monoaromatics. It is shown that thermaldesorption sampling of gasoline and diesel engine exhausts at 80 Td allowsfor discrimination against volatile organic compounds, allowing forquantification of long chain alkanes from the abundance of C_nH_2n+1fragment ions. The total abundance of long chain alkanes in diesel engineexhaust was measured to be similar to the total abundance of C₁–C₄alkylbenzene compounds. The abundance patterns of compounds determined bythermal desorption sampling may allow for emission profiles to be developedto better quantify the relative contributions of diesel and gasoline exhaustemissions on organic compounds concentrations in urban air.