Accepted answer
At minus 80 °C the question is which route is fastest, not whether oxidation happens — and the routes do not share an activation energy, so their ranking changes with temperature. minus 80 °C is 85 kelvin below a refrigerator, and below the glass transition of a lyophilised cake the ten-degree rule of thumb stops applying at all — solid-state chemistry is not slow liquid chemistry, it is a different regime, and the failure modes that survive it are mechanical rather than chemical. That multiplier is an average over every route at once, which is exactly why it cannot tell you which one wins. 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. So the way to answer it for your vial is to pick the method that sees oxidation specifically and run it against a control held cold, rather than to infer a mechanism from a purity number that averages all of them.
The relevant point is that a mass shift of plus one dalton is deamidation and plus sixteen is oxidation, so degradation is often visible in a mass spectrum if anyone looks.
Adsorption onto glass and plastic is significant at low concentrations — micrograms per millilitre — and negligible at milligrams per millilitre. It is the usual explanation for an apparent loss in a dilute preparation.
Hydrolysis cleaves the backbone, most readily at aspartate-proline and aspartate-glycine sequences, and is acid-catalysed. In a dry solid it barely proceeds at all.
Adsorption losses at low concentrations are quantified in formulation studies and are the reason carrier proteins are used in dilute preparations.
A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.