2.2.6. Quality Control

The quartz filters were baked at 450 ◦C for 5 h in a muffle furnace before sampling to identify the possible presence of organics. All filters were placed in a clean room (temperature of 25 ◦C ± 5; relative humidity of 50 ± 5%) for 48 h and weighed by a high-precision electronic balance (EX125ZH) with an accuracy of 10 mg before and after sampling. Each filter was weighed twice, with the difference between the two results not exceeding 0.2 mg for quartz filters and 0.02 mg for Teflon filters, to guarantee the precision of the weighting results. All filters were stored in a freezer at −20 ◦C before analysis to prevent the loss of volatile components.

In the sample analysis process, the instrument was calibrated with standard gas before and after the sample analysis. Then, one sample was randomly selected from every 10 samples for parallel analysis, and the standard sample was measured twice a week. The recovery rate of the standard sample was 98%~102%. Finally, the system blank of the instrument and the blank of the laboratory system were measured every week. The results

showed no contamination during sample handling and collection, as assured by the quality assurance and control (QC/QA) procedures.
