GUVs are a widely used model system for probing lipidprotein and lipid-lipid interactions. Despite the widespread use of GUVs, little work has been done to probe GUV-to-GUV compositional variation. This is likely due to the lack of methods to accurately assess the concentration of a given lipid within a single GUV. The Cameca NanoSIMS 50L allows for high-precision determination of the concentration of individual lipid species via non-perturbative stable isotope labeling.