Development and test of an on-line coupling of thermodesorption with fast gas chromatography for studies of N-alkanes in atmospheric particulate matter.

摘要

A thermal desorption instrument (TDI) based on the method of evolved gas analysis was developed and tested using different materials. Two different methods for detection were used. One detection method was based on conventional principles of detecting all carbonaceous gases evolved. For this purpose the evolved carbonaceous substances were oxidized to CO2 and/or CO, and CO2/CO was reduced to CH4 which is measured by a flame ionization detector (FID). This was used to analyze atmospheric PM samples for total carbon (TC), organic carbon (OC) and elemental (or black) carbon (EC). A variety of tests demonstrated that this method is suitable to analyze carbonaceous components in PM in the concentration range of 1 mug m -3 to 30 mug m-3 with a reproducibility of better than 10%. An exception is the analysis of EC. In this case variable extent of charring of OC resulted in a larger variability of the EC measurements. Due to the typically low EC content of atmospheric PM, the relative variability of the EC measurements significantly exceeded the good reproducibility achieved for TC and OC. Moreover, similar to almost all existing methods for EC measurement in atmospheric PM, the results obtained for EC significantly depend on details of the measurement procedure.;The organic carbon thermograms of different particulate matter samples often show significant differences in their shapes. However, due to the numerous different organic compounds contributing to organic carbon in atmospheric particulate matter and the complex processes responsible for releasing carbon from the filter it is not possible to identify individual compounds from the thermograms alone. A medium fast (analysis duration in the range of 100s to 200 s) on-line GC-FID was added as "second dimension" to the thermal desorption procedure. This allows compound specific analysis of the evolved organic material. The TDI was interfaced to the GC using a cryogenic trap which collects the evolved organic compounds from the carrier gas passing through the TDI for periods between 100s and 200s and then rapidly releases the trapped substances for analysis on a short capillary column. The system operates automatically and allows programmed changes of the GC temperature independent of the TDI temperature program.;The on-line TDI/GC coupling was developed and tested using n-alkanes in the range of C18-C34. Different pure n-alkanes as well as mixtures spiked on different surfaces and n-alkanes in particulate matter sample were investigated. The n-alkane concentrations in a small set of particulate matter samples were determined.;Small modifications of the TDI/GC instrument allowed measurements of equilibrium vapor pressures and binary diffusion coefficients. A simple theoretical equation was derived to describe the relationship between measured vapor pressure and equilibrium vapor pressure and mass transfer resistance. This equation was used to interpret the dependence of the measured vapor pressures of n-alkanes on experimental parameters such as carrier gas flow, saturator dimension, surface area and diffusion coefficient. Based on the results of these studies the different conditions which allow unbiased measurements of equilibrium vapor pressures or binary diffusion coefficients were identified.;The binary diffusion coefficient for n-alkane C20 and C 28 in helium were determined for a range of temperatures. Diffusion coefficient data for heavy n-alkanes in literature are scarce and experimentally determined diffusion coefficients for n-alkanes heavier than C18 are not available. The measured diffusion coefficients show good agreement with extrapolation of published diffusion coefficients for n-alkanes to higher carbon numbers.;The method used here is in principle a variant of the established evaporation tube method. The advantage of the newly developed variant is that it provides a relatively simple, reliable procedure to measure binary diffusion coefficients for heavy organic compounds with low vapor pressures, which is a significant experimental challenge for many of the established techniques. (Abstract shortened by UMI.)