The electronic cigarette (e-cigarette) industry is a fast-growing industry, already representing a multi-billion-dollar market. E-cigarettes deliver nicotine to the user through droplets generated from an e-liquid. E-cigarettes hence represent an alternative to conventional tobacco products. The health impact of e-cigarettes is still debated among scientists. Despite numerous studies on e-cigarette droplets, certain aspects remain largely unexplored. In this study, we addressed two key aspects of e-cigarette droplets: the time evolution of their chemical composition and the partitioning of their main constituents between the droplet and gas phase. For this, in situ Raman scattering measurements were performed on single e-cigarette droplets isolated in air by using an optical trap. Thereby, we were able to measure the time evolution of the concentrations of the main compounds in the e-cigarette droplet phase separate from those in the gas phase. The results demonstrated that the chemical composition of the e-cigarette droplets undergoes major changes on a time scale of a few to some 10 s. More than 50% of the total mass of the ecigarette droplets evaporates within 20 s. Moreover, the pH of the e-liquid dictates the time evolution of the nicotine concentration inside the generated e-cigarette droplets. When an e-liquid with acidic pH is used, nicotine remains in the generated e-cigarette droplets, while, under basic pH, nicotine completely evaporates from the droplets within ~20 s. Such destruction-free in situ measurements of single particles are opening up new perspectives for further research on e-cigarette droplets and e-liquid manufacturing. The measured partitioning of the main e-cigarette compounds between the droplet and gas phase as a function of time will improve our understanding of their deposition in the respiratory tracts and hence of their impact on health.