Through Vial Impedance Spectroscopy. A process analytical technology based on the measurement of the electrical impedance spectra of a solution contained within a freeze drying vial during the process of freeze drying. In this method the electrodes are external to the vial, hence the term "through-vial" impedance spectroscopy. The methodology for monitoring product temperatures and drying rates was first developed by researchers at De Montfort University, Leicester, United Kingdom.