The profile of the ionisable lipid Dlin-MC3-DMA (Figure ) indicated that it was strongly represented at depths less than 3 nm where PEG is also present (< 8 nm), thereafter decreasing in intensity and recovering at depths greater than 6 nm. This may represent a propensity for surface segregation, or an association of Dlin-MC3-DMA with the PEG at the exterior, similar to the sodium and chloride ions from the PBS.