As shown in Figure , pure POPC SLBs containing 2 H31-POPC and 13 C18-POPC did not show any clear correlation in concentration between the differently labeled lipids. Since the 2 H31-POPC, C18-POPC and natural abundance POPC within the pure POPC mixture only differ from each other in terms of isotopic labeling, when the concentration of one labeled component is higher in a GUV, whether the concentration of a labeled or natural abundance POPC is lower is essentially random.