To test the capability of Cryo-OrbiSIMS depth profiling for the characterisation of the surface and subsurface chemistry of the LNPs, we analysed a model formulation comprising mRNA and 4 lipids; DMG-PEG2k, cholesterol, Dlin-MC3-DMA as the ionisable lipid and DSPC as the helper lipid (Table ) formed using microfluids. The nanoparticle suspension in phosphate buffered saline (PBS) was deposited onto a plasma etched 10 nm thick amorphous carbon film with regularly spaced 2.4 µm diameter holes supported by copper TEM grids. These were blotted to modulate thickness before plunging into liquid ethane, after which they were transferred under liquid nitrogen for Cryo-TEM imaging (-172°C). The particles were found by Cryo-TEM to be relatively monodisperse, with their diameters distributed around a mean of 60-80 nm (Figure ) while with DLS they were estimated to be slightly larger with a mean diameter of 91 nm and a PDI of 0.03 (Figure ).