Routinely, accelerator mass spectrometry (AMS) analyzes rare isotopes which usually are in attomole to zeptomole amounts and at small proportions relative to the abundant isotope (e.g. for modern carbon 14C/12C~10-12). Our group specializes in measuring 14C for apportionment of fossil from non-fossil sources for atmospheric aerosols based on the fact that 14C is completely extinct in fossil aerosol emissions. We use an aerosol analyzer that thermally desorbs molecular fractions with different volatilities from aerosol filters and transports the analytes into a combustion oven to create CO2 peaks (FWHM~4 min). A commercial gas handling system traps, desorbs, dilutes and injects the CO2 into the AMS. An elemental analyzer that separates nitrogen oxides, water and CO2 from the sample combustion has also been coupled offline with the AMS. Samples sizes of 10 µg C containing 0.017-1.1 amol 14C were analyzed obtaining an uncertainty of 4%. Standard substances and previously measured aerosol samples with conventional sample preparation were used to test the new offline Aerosol analyzer-AMS and elemental analyzer-AMS high-throughput couplings. Statistically, the nominal values and the previous results were in good agreement with new measurements. A second type of developed coupling consisted on connecting online the aerosol analyzer by means of a copper reactor. By oxidazing the copper, the high O2 flow is scrubbed away from the carrier mixture and the CO2 is injected with a small residual helium flow that can keep the ion source vacuum. This coupling bypasses the trapping step and concentrate each small sample fraction (2-10 µg C) in a peak thus the online coupling is more sensitive than the offline method. Also, the online coupling allows us to trace the 14C signal in real time while changing the desorption/separation temperature of the aerosol analyzer, similar to a GC-MS system. The results show a direct evidence of a trend between fossil carbon content and volatility of the fraction components for ambient samples collected under special conditions.

Developing AMS hyphenations with analytical instruments for atmospheric aerosol characterization

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.