Electrical impedance and measurement: The theory behind the technique of through-vial impedance spectroscopy (TVIS)

Through-vial impedanace spectroscopy is a relatively new technique for characterising the electrical properties of glass vials and ampoules, containing solutions of drug substances, as they are processed through a freeze-drying cycle. 

Understanding the theoretical aspects under-pinning this technique is a pre-requisite for the development of applications across the various stages of the freeze-drying cycle. 

This text aims to address that requirement, by providing a basic introduction to the passive electrical properties of the various materials used in pharmaceutical freeze-drying, including :

(i) aqueous solutions of drugs and excpients (such as salts, sugars, polymers), and 

(ii) the glass from which vials and ampoules are formed.