摘要:

A method to measure C-13/ C-12 ratios of individual carbon fractions of airborne particular matter (PM) from filter samples using a stepwise thermal desorption/combustion OC/EC analyzer (via thermal optical transmission, (TOT) coupled with gas chromatography separation, followed by isotopic ratio mass spectrometer (GC-IRMS) analysis has been developed. In the TOT instrument, carbon fractions are released at different temperature ranges and different redox conditions. Organic carbon fraction (OC) was released at a relatively low temperature (T = 550 degrees C), whereas, elemental carbon or black carbon fraction (EC or BC was released at a high temperature (T > 800 degrees C) via combustion. A temperature step of 870 degrees C without oxygen was chosen to remove the impact of carbonate carbon (CC) and possible crossimpact from OC and EC. All the fractions were collected cryogenically and subject to carbon isotope measurements via GC-IRMS. To evaluate the precision, accuracy and linearity range of the measurements, the different types of blanks and standards were investigated, including OC (i.e. glucose, sucrose, n-Alkanes and polycyclic aromatic hydrocarbons (PAHs), CC (i.e. carbonates) and EC (i.e. carbon black and graphite). The overall precision and the accuracy of the method is similar to 0.3 parts per thousand. The method was applied to Pacific2001 aerosol samples from the Greater Vancouver area in Canada. The results show that good baseline separations in thermographs can be achieved for individual carbon fractions (i.e. OC and EC) using optimized temperature plateau and retention times; relative small difference in carbon isotopic composition between OC and EC (Delta(13) C-OC-(EC) = delta C-13(OC)delta C-13(EC)) were found in tunnel samples, whereas, the largest Delta C-13(OC-EC) were obtained in forest air samples; the Delta C-13(OC-EC) in ambient PM is likely dependant upon the dominant sources present in the vicinity of the sampling sites; the distribution of 13C/ 12 C ratios of OC/EC can provide useful information for source characterization and apportionment of ambient particulate matter. (c) 2006 Elsevier Ltd. All rights reserved.