A Novel Approach to Collection, Speciation, and Quantification of Semi-Volatile Hydrocarbons on Diesel Engine Emissions, 08-R9490Printer Friendly Version
Inclusive Dates: 07/01/04 12/31/05
Background - Unlike gasoline engine emissions, the gas-phase pollutants of diesel engine exhaust emissions have not been thoroughly studied. This lack of study is primarily a result of the extreme complexity of the diesel fuel, which constitutes a significant portion of the diesel emissions. The conventional sampling media for the gas-phase engine exhaust emission has been PUF/XAD-2/PUF (polyurethane foam/XAD-2 synthetic resin). Typically these materials are difficult to clean, particularly the PUF. As is the case, the PUF prevents low detection limits of target pollutants from being achieved because of the background signals they exhibit. The quantification of the target semi-volatile hydrocarbons (SVHCs) have been mostly performed using largely inaccurate external standard methods.
Approach - On sample collection, four different sampling methods were tested, namely Hexane Impinger, Cold Trap, XAD-2 cartridges, and Empore Membrane. Hexane impinger traps SVHCs in the chilled hexane while the emission gas bubbles through it. Cold Trap method uses the coldness of the 2-propanol/dry ice bath to trap the SVHCs. Empore membrane, a 3M® product, was used for the solid-phase extraction methodology. The XAD-2 method uses glass cartridges packed with XAD-2 resin to trap SVHCs. Teflon O-rings were, for the first time, designed and used in conjunction with the stainless steel screens to hold the XAD-2 resin beads inside the glass cartridge. After sampling, the entire glass cartridge was inserted into the Soxhlet extractor for extraction. On SVHC speciation, a 60-meter DB-5 column (0.32-mm id, 0.25-micrometer film thickness) was used to maximize the separation of the SVHCs. A gas chromatograph/mass spectrometer (GC/MS) was used to identify the components in the diesel emission samples in both electron impact/positive ion (EI/PI) mode and chemical ionization (CI/PI) mode using methane as chemical reagent gas. On quantification, as many as 26 deuterated SVHCs were used as internal standards (IS) and were spiked at the time of sample extraction for accurate quantification using GC/MS. A subset of 29 specific SVHCs were used to compare the sampling efficiency of the four sampling methods.
Accomplishments - A total of 107 SVHC types were identified. Among the four SVHC sampling methods, the XAD-2 glass cartridge with the Teflon O-rings had the highest quantity of SVHC pollutants followed by the Empore membrane and Cold Trap methods. The XAD-2 with Teflon O-ring method had much higher trapping efficiency for the oxygenated SVHCs, alkyl cyclohexanes, alkyl aromatics, and polycyclic aromatic hydrocarbons (PAHs). The Empore membrane method had the highest trapping efficiency for the normal alkanes. The Cold trap and Hexane Impinger methods suffered flow rate reduction problems, presumably caused by frost formation when dry ice/2-propanol was used as a cold bath (-78°C). However, when ice bath was used, the Hexane Impinger method showed the second highest SVHC trapping efficiency, or the highest if the oxygenates were not counted. Essentially no "breakthrough" was observed on the XAD-2-Teflon O-ring method. A differential IS spiking and analysis scheme was developed specifically for the PAH determination. This strategy renders a way of accurately quantifying the lighter PAHs (some at extremely high concentrations) and the heavier PAHs (usually at very low concentrations). Ultimately this internal research resulted in an analytical technique.