Initially, the PP-LFER system parameters were calibrated using only the data for nonfluorinated reference compounds and used to determine the solute descriptors for PFAS, as described above. However, this straightforward approach resulted in inconsistent descriptor values for some PFAS, as indicated by relatively large standard errors of descriptor values or unexpected negative A or B values. The reason for this is likely the problem of extrapolation; the system parameters calibrated with only nonfluorinated compounds are not applicable to PFAS. To circumvent this problem, the system parameters of eq 1 and the solute descriptors for PFAS were determined iteratively in this work. That is, relatively data-rich PFAS (i.e., FTOHs, PFASAs, N-alkylperfluoroalkanesulfonamides (FASAs), Nalkylperfluoroalkanesulfonamidoethanols (FASEs), PFAIs, FTIs, FTOs, FTACs, FTMACs; see Table ) were included in the calibration sets and used to tentatively calibrate the system parameters. The descriptor values predicted by the IFS-QSAR of EAS-E Suite were used as initial values for PFAS. The obtained system parameters were then used to update the PFAS descriptors, and using these updated PFAS descriptors, the system parameters were recalibrated, and so on. This cycle was repeated 20 times, which was sufficient to obtain stable results. L was also updated in this iterative calculation using the literature data for nonpolar GC columns (squalane, SPB-Octyl, and Apolane) because small influences of polar interactions on log k', which were not considered in the previous studies, could be taken into account with the values of S and A for PFAS. The differences from the previous values, however, were small. The complete scheme of the iterative calculation is shown in Figure .