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 as a function of etch time and estimated depth in Figure (2 further replicates are presented in Figure ). The PEG ion fragments in the LNPs are all sodiated and of the form Na