Accepted answer
At minus 80 °C it can show up as either, and which one depends entirely on whether the product still elutes under the main peak. Purity is a ratio of areas, so a degradant only costs purity if the method resolves it. Content is a mass against a standard, so a degradant costs content whenever the parent is consumed — resolved or not. Met and Trp take up oxygen sixteen daltons at a time, and the oxidised species is more polar, so on a reversed-phase column it elutes ahead of the parent rather than behind it. That is why the two measurements are not interchangeable and why an unchanged purity figure after an excursion to minus 80 °C is weak evidence: the method may simply be integrating the degradant along with the parent and reporting the sum as one peak.
To be exact about it, this is answerable from the chemistry rather than from anecdote, which is unusual and welcome.
Freeze-thaw cycling drives aggregation through concentration at the ice interface and pH shifts as buffer components crystallise out at different rates. Each cycle costs something.
A mass spectrum resolves most of this: minus eighteen is dehydration or succinimide, plus one is deamidation, plus sixteen is oxidation, and an unchanged mass with a shifted retention time is an isomer.
Adsorption losses at low concentrations are quantified in formulation studies and are the reason carrier proteins are used in dilute preparations.
Nothing here is medical advice, and research-use compounds are not approved for human use.
A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.
5I would add a sentence about light, since tryptophan-containing sequences care. – b_delacroix 40 days ago 6Small correction: it is the number of cycles rather than the freezer temperature that does the damage. – amara_nwachukwu 3 months ago add a comment