On-Line Analysis of Organic Compounds in Diesel Exhaust Using Proton-Transfer-Reaction Mass Spectrometry

In this study, diesel exhaust (DE) was measured in real time using a proton-transfer-reaction mass spectrometer (PTR-MS) to determine the effect of an after-treatment catalyst on gas phase volatile organic compounds (VOCs). DE after-treatment catalysts are being designed to reduce the pollutants in exhaust, which contains both particulate matter and gas phase constituents. The PTR-MS can make in-situ real time measurements of hydrocarbons in the air, from concentrations in the parts per million by volume (ppmV) down to the low part per trillion by volume (pptV) range. Spectrum scans were performed at varied engine loads from mass range m/z (mass to charge ratio) = 20 to 200. This showed the relative abundance of gas phase VOCs produced as the engine ran between idle mode and 80% of its maximum load. The mass spectrum was complex and appeared to be composed of aromatic species ionized by PTR (M+1) through the anticipated proton transfer reactions as well as unexpected alkane fragments, evidenced by a strong 14n+1 ion pattern showing intense peaks at m/z = 43, 57, and 71. A number of protonated M+1 masses could be identified. These compounds displayed M+2 peaks consistent with known 13C isotopic abundance. As the engine load increased, the concentrations of over 90% of the species decreased. An attached smoke meter showed that soot concentrations increased over the same conditions. In addition, the decrease in the concentration of compounds with a larger molecular weight (m/z>100) was greater than the rate that the smaller compounds experienced. This appears to be due to the affinity of VOCs, larger masses in particular, to adhere to soot particles. Further PTR-MS measurements of VOCs on soot confirmed this by producing a mass spectrum comprised of masses predominantly over 100 amu. On-line analysis of diesel exhaust by PTR-MS is a practical tool for quantifying selected organic species in diesel exhaust and should prove useful for developing better diesel exhaust after-treatment system