Application of through vial impedance spectroscopy (TVIS) for the determination of ice nucleation, solidification endpoint, and mannitol crystallization and melt-back temperature during freezing and annealing
Studies of the freezing behavior of aqueous solutions of mannitol are highly relevant to lyophilization process development given the prevalent use of this excipient. Here, through-vial impedance spectroscopy (TVIS) was used to study the behaviour of an aqueous solution of 5%w/v mannitol during a freezing and re-heating cycle. Temperature calibration of the TVIS parameter FPEAK enabled the determination of the ice nucleation temperature Tn at −13 oC while the ice solidification end point was observed based on the time profile of C' (0.2MHz), i.e. the real part capacitance at 0.2 MHz. A time difference of 20 min between the onset and end point then defines the ice solidification time ti. A later step in C^' (0.2MHz) indicated that mannitol crystallized at -32 oC and 20 minutes from end of ice solidification. Upon reheating at 0.2 oC/min, a large increase in C' (0.2MHz) was seen at −32 oC indicating the onset of melting of mannitol crystals which then lasted 20 min.