Since the exterior orientation of the PEG group is well established in the literature for LNPs containing DMG-PEG2k, we first examine the secondary ions from this molecule. Both DMG fragments and sodiated PEG2k fragment ions defined in the standards (Figure ) were detected in the SIMS spectra and are plotted versus etch time and estimated depth in Figure (replicates presented in Figure ). The PEG ion fragments in the LNPs are all sodiated and of the form CxH2xOx/2+1Na + (e.g. C23H46O12Na + , C24H48O13Na + ) or CxH2xOx/2+2Na + (e.g. C25H50O13Na + ). The DMG anchor is detected as the whole molecular fragment: C31H59O4 + , and by ions representing its FA14:0 moiety: C14H27O2 + , C14H27O2Na + . A complete list is provided in Table .