What lyophilisation critical temperatures do you need to measure before cycle design?

What are Tc, Tg’, and Te?

Answer

Tc is the collapse temperature – the temperature above which the frozen amorphous matrix loses rigidity during primary drying, causing the pore structure of the cake to deform irreversibly. It is the upper limit for product temperature throughout primary drying.

Tg’ is the glass transition temperature of the maximally freeze-concentrated amorphous phase – the temperature at which the amorphous fraction of a frozen formulation transitions from a glassy, rigid state to a rubbery, viscous one. It correlates closely with Tc and is typically 2-5°C below it. A formulation held above Tg’ during primary drying will tend towards collapse as the amorphous matrix softens.

Te is the eutectic temperature – relevant only to formulations containing crystallising components such as mannitol, glycine, or inorganic salts. It is the temperature at which those components exist at the boundary between solid and liquid phases. Below Te they are fully crystalline; above Te they are partially molten. The freeze stage must bring the product below Te to ensure complete crystallisation before drying begins.

Why

The three temperatures characterise fundamentally different properties of the frozen formulation. Tc is an observation of structural failure under the conditions of freeze-drying. Tg’ is a thermodynamic property of the amorphous phase. Te is a phase-transition property of crystalline components. Which temperatures matter for a given formulation depends on its composition: a fully amorphous biologic formulation requires Tc and Tg’; a mixed crystalline-amorphous formulation requires all three.

How is Tc measured, and what does the result tell you?

Answer

Tc is measured by freeze-drying microscopy (FDM). The formulation is placed on a temperature-controlled microscope stage, frozen, and then warmed at a controlled ramp rate – typically 1-2°C per minute – while the frozen matrix is observed directly. Tc is recorded as the onset temperature at which structural collapse first becomes visible: the collapse front advances through the frozen cake as pore walls soften and flow. The result is used directly as the upper temperature limit for primary drying, with a safety margin of 2-3°C applied in practice.

Why

FDM is the only measurement method that directly observes collapse in the frozen matrix under conditions that approximate primary drying. It is specific to the formulation at the concentration tested and accounts for all component interactions. DTA and EIS measure Tg’, which predicts Tc, but FDM confirms it visually. Where a formulation’s Tc is unknown, FDM is the starting point for primary drying design.

When this advice may not apply

Where FDM is not available, Tg’ measured by DTA or EIS can serve as a conservative proxy for Tc – the true Tc is typically a few degrees above Tg’, so using Tg’ as the design limit provides an additional margin of safety. This approach is most reasonable for early-stage screening, where a precise Tc is not yet required. For clinical and commercial development, direct Tc measurement by FDM is the expected standard.

How is Tg’ measured, and how does it relate to Tc?

Answer

Tg’ is measured by differential thermal analysis (DTA), electrical impedance spectroscopy (EIS), or both simultaneously. In DTA, the frozen sample is warmed at a controlled rate; Tg’ appears as a step change in heat flow as the amorphous phase transitions from glassy to rubbery. In EIS, the same transition appears as a step change in electrical impedance. Simultaneous DTA/EIS measures both signals from a single sample in a single run, providing two independent determinations without additional material or time. Tg’ is typically 2-5°C below Tc for the same formulation.

Why

Tg’ and Tc are related but not identical. Tg’ is a thermodynamic property of the amorphous phase; Tc is an observation of structural failure under the specific conditions of freeze-drying microscopy. The offset between them is broadly consistent, which makes Tg’ a useful cross-check and a conservative design boundary when Tc has not been measured directly.

DTA alone gives a weak or ambiguous signal in dilute biologic formulations, where the amorphous fraction is small relative to the ice fraction. EIS detects the same transition through a different physical mechanism – the change in ionic mobility as the amorphous phase softens – and tends to give a clearer signal in these cases. Running both from the same sample removes the ambiguity without requiring a repeat measurement.

What is Te, and when does it matter?

Answer

Te is the eutectic temperature of a crystallising component – the temperature at which that component transitions between fully crystalline and partly molten states. Formulations containing mannitol, glycine, sodium chloride, or potassium dihydrogen phosphate all have a eutectic transition. The freeze stage must take the product below Te to ensure those components fully crystallise. The annealing step – used to promote complete crystallisation – is conducted above Tg’ (to allow molecular mobility) but below Te (to keep crystalline components solid). Te is measured by DTA or EIS as a distinct thermal event on cooling and warming.

Why

Crystallising excipients that have not fully crystallised by the end of the freeze stage remain amorphous. Amorphous mannitol and glycine depress Tg’ and therefore Tc of the formulation as a whole. A cycle designed on the assumption that mannitol is crystalline will produce collapse if annealing is incomplete, because the effective Tc is lower than measured. Knowing Te confirms whether the freeze stage – including temperature, hold time, and cooling rate – was adequate to drive crystallisation to completion.

Te is not the same as Tg’. They can be confused because both are measured by the same instruments and appear as thermal events during warming. Te is a first-order phase transition (latent heat, sharp peak in DTA); Tg’ is a second-order transition (step change in heat capacity, broader signal). In a mixed formulation, both events may appear in the same scan.

Which measurements do you need for your formulation?

Answer

For most biologic formulations, measure Tc by FDM and Tg’ by DTA/EIS. If the DTA signal is weak, add EIS or run both simultaneously. If the formulation contains mannitol, glycine, or inorganic salts, measure Te as well, and confirm annealing conditions are adequate to drive crystallisation below Te before drying begins. If concentration changes are planned during development, re-measure: Tc and Tg’ both shift with concentration, and a measurement at 5 mg/mL does not apply at 50 mg/mL.

Why

Each measurement provides information the others cannot reliably substitute for. FDM shows collapse directly; DTA/EIS measures the thermodynamic transition that precedes it. Neither can confirm whether crystallising excipients have fully crystallised – that requires Te. Doing all three relevant measurements in a single characterisation phase is more efficient than diagnosing failures that trace back to missing data later in development.

When this advice may not apply

For predominantly crystalline small-molecule formulations where no amorphous stabiliser is present, Tc and Tg’ may not be relevant – primary drying is governed by Te rather than collapse temperature. In these cases, confirm that the formulation is fully crystalline by X-ray diffraction or DSC before omitting Tc measurement, and ensure Te has been measured rather than assumed.

How do these measurements feed into cycle design?

Answer

Tc sets the upper limit for product temperature during primary drying. Set shelf temperature and chamber pressure so that product temperature – at the warmest vial position, accounting for radiation from chamber walls – stays at least 2-3°C below Tc throughout the drying stage. Tg’ provides a cross-check: if Tg’ and Tc diverge by more than 5°C, re-examine the measurements before committing to a cycle. Te confirms the freeze stage is complete and that annealing has driven crystallisation to the expected end point before primary drying begins.

Why

These temperatures are design inputs, not safety checks after the fact. A primary drying cycle set without Tc data is either unnecessarily conservative – which lengthens the cycle and reduces batch throughput – or unknowingly aggressive, which risks collapse. The margin between measured Tc and actual product temperature determines both the safety of the cycle and the efficiency it can achieve. More headroom below Tc means the cycle can run at a higher shelf temperature and shorter hold time without risk.

For regulatory submissions, cycle parameters must be justified against formulation characterisation data. Under ICH Q8, the design space for primary drying – the range of shelf temperature and chamber pressure that maintains product temperature below Tc – is built directly from these measurements. Without them, there is no documented scientific basis for the design space.

Frequently asked questions

Do I need to measure Tc and Tg’ separately, or will one do?

Both are useful but they answer different questions. Tc from FDM is a direct observation of collapse during freezing and is the primary input to primary drying cycle design. Tg’ from DTA or EIS confirms the thermal event underlying Tc and is expected in regulatory submissions. If FDM is unavailable, Tg’ provides a conservative proxy – typically 2 to 5 degrees C higher than Tc – but it is not a direct substitute for cycle optimisation.

How long does an FDM measurement take?

Typically a few hours per sample, including the freezing ramp, controlled drying under reduced pressure, and image capture across the temperature range. Multiple formulation variants can be screened in a single day, which makes FDM practical for early-stage candidate selection as well as development characterisation.

My DTA signal is too weak to read Tg’ clearly – what should I do?

Add electrical impedance spectroscopy. EIS is significantly more sensitive than DTA alone for detecting the glass transition in dilute or low-solids systems where the thermal signal is small. Running DTA and EIS simultaneously on the same sample gives two independent detection methods and improves confidence in the assigned Tg’ value.

Do I need to re-measure Tc if I change the protein concentration?

Yes. Tc shifts with formulation composition, and protein concentration is one of the more significant variables. A higher protein load typically depresses Tc, narrowing the margin available for primary drying. Assuming the original Tc still applies after a concentration change is a common source of unexpected collapse at scale.