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 (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 artifact 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 secondary ion adducts that we did not considered artefactual since it did not display a profile that could result from a combination of the profiles of other separate components (the sodium and the PEG).