Examining the cholesterol ion profiles showed that the positively charged molecular ion after the loss of its hydroxyl group [M-OH] + was seen to be most intense at the outer surface and decreased within the first 3 nm before reaching a relatively constant value in the interior of the particles (Figure ). This is consistent with cholesterol molecules sitting at the lipid surface with their hydroxyl groups oriented into the hydrophilic region occupied by the PEG, and would be consistent with there being areas with only a thin PEG coverage that allow this [M-OH] + ion to be present in the earliest analyses.