Accepted answer
Nobody has published a cycle count, and at 2 mg/mL the concentration tells you what a wrong guess costs: a 0.1 mL aliquot holds 0.2 mg and every microlitre is 2 µg. Damage from freezing is not gradual attrition — it is concentrated at the phase transitions, where ice excludes solute and the unfrozen fraction climbs well above 2 mg/mL for as long as the transition lasts. Two slow cycles can therefore do more than four fast ones, which is why a cycle count is the wrong unit in the first place. The way to make the number one is to make it one: split at reconstitution into single-draw aliquots, label each with 2 mg/mL and the date, and never thaw a container you will refreeze.
Start with how many cycles are actually planned, because one or two are immaterial and ten are not.
Buffer salts crystallise at different points during freezing. Sodium phosphate is the classic example: the dibasic form crystallises first and the pH of the residual liquid falls by several units. That pH excursion is the real damage in many cases.
Reported and extrapolated stability by condition
| State | Condition | Usable window | Basis |
|---|
| Lyophilised solid | −20 °C, sealed, dry | 24–36 months | Supplier guidance |
| Lyophilised solid | 2–8 °C, sealed | 12–24 months | Supplier guidance |
| Lyophilised solid | 25 °C, sealed | 4–8 weeks | Extrapolated (Arrhenius) |
| Lyophilised solid | 40 °C, sealed | 1–2 weeks | Extrapolated |
| Solution, preserved | 2–8 °C | 28 days | USP microbiological convention |
| Solution, preserved | 25 °C | 3–7 days | Extrapolated |
| Solution, unpreserved | 2–8 °C | 24 hours | USP microbiological convention |
Windows for the solid state are chemical; windows for solution are microbiological and usually shorter than the chemical limit.
Count cycles rather than worrying about degrees. Minus twenty and minus eighty differ far less than one cycle and five do.
Selective crystallisation of sodium phosphate buffer components producing large pH shifts on freezing is a classical result in the lyophilisation literature.
Count cycles, not degrees. The cycle is the damaging event.