Isothermal GC retention behavior of neutral PFAS A total of 529 and 642 log k' data were measured for PFAS and nonfluorinated reference compounds, respectively (Tables S6, S7, Figure ). For each column and compound, log k' was linear against 1/T (Figure ), confirming the high measurement consistency over temperature. Log k' data for a given class of PFAS (e.g., X:2 FTOHs) for the HP-5ms and DB-200 columns were perfectly linear against the number of CF2 units (Figure ). However, the relationship for DB-225ms was very slightly concave upward, and it was more so for SolGel-WAX. The reason for the nonlinearity is unknown. Since linearity is a prerequisite for PP-LFER model fitting, the log k' data for SolGel-WAX was not used for solute descriptor determination. The log k' values for the low-polar HP-5ms column show a high correlation with L (Figure ). Clearly, the nonpolar van der Waals interactions are prevalent with HP-5ms, and additional polar interactions have only minor influences on log k'. The correlation between log k' for the DB-200 column and L is less good, and high scatter is evident for the highly polar DB-225ms and SolGel-Wax columns. For all columns, log k' for nonpolar PFAS (i.e., without a polar functional group, e.g., PFAs, PFAIs, FTOs) shows a linear relationship with L and forms a baseline in the log k'-L plot. Other PFAS deviate upward from the PFAS baseline, depending on their polarities. This is most evident in the DB-225ms and SolGel-WAX data; PFASAs and their Nsubstituted compounds are farthest from the nonpolar PFAS in the plot, reflecting their strong polarity, followed by the moderately polar FTOHs. The other fluorotelomer compounds (i.e., FTACs, FTMACs, FTIs) deviate slightly from the baseline, reflecting their weak polarity. The GC retention measurement is therefore useful to roughly assess the polarity of PFAS. The relevant types of polar interactions are discussed quantitatively in the following section.