To perform headspace analysis on a sample, its components must have a sufficient vapor pressure to populate the gas phase above the sample. For non-ideal solutions (which water:ethanol mixtures are well known to be), the vapor pressures of components are governed by Herny's law (shown below in equation 1.1), where 𝑝 𝑖 is the vapor pressure of a compound 𝑖 above a solution at equilibrium, with mole fraction 𝑥 𝑖 and activity coefficient 𝛾 𝑖 .