Measuring solute concentration with terahertz time-domain spectroscopy in single and multiphase systems

A versatile setup based on a microfluidics platform allows investigation of liquid samples at various temperatures with terahertz time-domain spectroscopy. The set up is applied to develop a novel method that performs temperature and concentration calibrations of liquid samples at terahertz frequencies. Other than measuring the concentration of pure liquid phase solutions, it enables the studies of local concentration of semi-crystalline systems. An equivalent solute concentration during crystallisation can be calculated from the extracted absorption at low frequencies. The MgSO4-water system is discussed as an example to illustrate the idea of this method.