The retention volumes, the plate heights, and the volume of the adsorbed water layer were measured for the ACQUITYTM UPLCTM BEHTM 130 Å HILIC Column (unbonded BEH), ACQUITY UPLC BEH 130 Å Amide Column (amide group attached), and AtlantisTM Premier BEH 95 Å Z-HILIC (zwitterionic group attached) Column. 
ACQUITYTM UPLCTM
The method of Guo (toluene retention volumes in pure acetonitrile and in the HILIC eluent) was validated from the UNIFAC group-contribution method and applied to measure accurately the water layer volumes in these columns. 
UNIFAC