The method for GC retention time measurements follows that of a previous study. Briefly, a 7890A/5975C GC/MS (Agilent Technologies) equipped with an MPS2 autosampler and CIS4 injector (Gerstel, Mülheim an der Ruhr, Germany) was used for all measurements. For each column, isothermal retention times were measured at four temperatures in the range of 30-120 °C to cover all target compounds. The temperature at the injector and transfer line was fixed at 120 °C. The carrier gas was He at a flow rate of 1.2 mL/min. Compounds were introduced into the GC by injecting either headspace (100-250 μL) above the pure compound or solution in acetone (1 μL). Split injections were performed with a split ratio of 20-250. Column dead time (t0) was determined by injecting air and measuring Ar (m/z 40). From the measured retention time (t), the retention factor (k') was calculated as k' = (t-t0)/t0. Measurements with t < t0 + 0.1 min were considered too short and discarded. For each column and temperature, k' values were obtained for 29-52 PFAS and 29-48 reference compounds. Only one injection was performed for each compound because repeated measurements showed that the k' variation was typically within 0.01 log unit. Because multiple isomer peaks were observed in the chromatograms for fluoroether (FE)-F4 and FE-F5, the log k′ value corresponding to the middle peak was used to represent all isomers, as previously done. Kow measurement Kow was measured for nine PFAS to supplement the data set for descriptor determination. The shared-headspace or batch partition method was used as described previously. Details of the methods are explained in SI-1.