Accepted answer
sixteen weeks is 112 days and, on a weekly schedule, 16 stopper punctures out of one vial at 8 mg/mL. Set the chemical question aside for a moment, because the puncture count is the one with a convention attached: 112 days is 4 times the twenty-eight days conventionally allowed for a preserved multi-dose preparation once it has been entered. Chemically, 8 mg/mL is high enough that adsorption to the glass is a rounding error and low enough that it is not protecting you from anything. What 16 withdrawals do add is 16 opportunities to introduce air, 16 coring events on the same stopper, and a headspace that grows with every draw — none of which show up on a certificate and all of which are avoided by splitting into aliquots at reconstitution.
More usefully, this is answerable from the chemistry rather than from anecdote, which is unusual and welcome.
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 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.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Nothing here is medical advice, and research-use compounds are not approved for human use.
Swirl, never shake. Aggregation is a handling problem more than a time problem.
edited 10 May 2026 by laminar_bench — added a caveat about sampling
I have kept vials both ways for a year and this matches what I saw. – Dr_Ravi_Selvarajah 4 months ago Two lots stored differently, reassayed at a year — the difference was smaller than I expected. – Dr_Rosalind_Achebe 6 months ago add a comment