Development of graphitization method for low carbon aerosol filter samples with Automated Graphitization System AGE-3

The wide applications of the radiocarbon (¹⁴C) approach in environmental, archeological, and geological research often necessitates the analysis of microgram-sized samples. The ability to measure low carbon samples is particularly relevant for aerosol particle filters, especially for samples from pristine environments. For this purpose, we investigated the sample dilution method for graphitization of low-carbon samples (20-200 μg C) with an Automated Graphitization System (AGE-3), and applied a mass balance equation for the calculation of ¹⁴C values. Materials with known ¹⁴C values (standards NIST-OXII and IAEA-C7) were diluted with ¹⁴C-free phthalic anhydride (PhA) until sufficient mass (500 μg C) for graphitization with the AGE-3 system was acquired. Reliable ¹⁴C values were obtained for samples with carbon amount in the range of 40-200 μg. Next, we adapted the dilution method for estimation of aerosol sample ¹⁴C values. Using it, we attained a precision of 0.71 ± 0.83 pMC for ¹⁴C measurements of aerosol samples containing 40-200 μg C. A shift of radiocarbon values to 5.07 pMC (average 3.08 ± 1.7 pMC) was observed for samples with low carbon content (<20 μg C). We determined that a precision of 2-3 pMC is acceptable for aerosol particle source apportionment studies. Using the sample dilution method, graphitization with AGE-3 of aerosol samples with carbon content >40 μg becomes a viable and efficient way of sample preparation for ¹⁴C analysis.