The intact sodiated molecular ion [M+Na] + appears to follow a profile which would be expected from an ion generated from the sum of the profiles containing sodium chloride (Na2Cl + ) and cholesterol [M-OH] + , at a low intensity at the outermost surface and increases to a plateau at approximately the same depth of 3 nm. Therefore, we interpret this as an analytical artefact rather than indicating the distribution of a molecular component in the LNP formulation. The cholesterol-Na adduct secondary ion may have formed at the surface during the collision cascade generating secondary ions from both of these species. This is in contrast to the PEG sodium adducts that we did not consider artefactual since their profile that could result from a combination of the profiles of the separate components (the sodium and the PEG).