Fig. 1. Profile of PEG oligomers after flux across a Caco-2 monolayer reveals a sharp size discrimination. A mixture of continuous PEG oligomers (dimer to 28mer) was added to the apical compartment of monolayers cultured on semipermeable filters. Aliquots were removed from the donor (A) and acceptor (B) compartments after 90 minutes, derivatized with 1-NIC and subjected to HPLC and detected by fluorescence emission. Each peak represents a single ethylene glycol adduct size, and retention times are proportional to molecular mass. The peak visible at 31 minutes is pure PEG₂₈ added as an internal recovery standard after the flux protocol.