Ask a researcher what they check on a certificate of analysis and almost all of them will say purity. Ask what they check second and the answers thin out fast. Fill weight — the actual mass of material in the vial against what the label claims — barely registers as a category, and it is the specification most likely to be silently wrong.
The arithmetic problem
Suppose a vial is labelled 10 mg and contains 8.2 mg. Reconstitute with 2 mL and you believe you have 5 mg/mL. You actually have 4.1 mg/mL. Every concentration downstream of that vial is out by 18%, and nothing in the experiment will tell you so. Purity was excellent. Identity was confirmed. The number that broke the work was never on the report.
Why it happens
Filling lyophilised powder accurately at scale is genuinely harder than synthesising it cleanly. Static, humidity, and the physical behaviour of a light amorphous solid all conspire against a consistent fill. A supplier under cost pressure has an obvious lever here, and it is one that no customer will ever notice without a balance and a reason to be suspicious.
What we do about it
We ask the lab to weigh net contents against label claim on every production lot, and we publish the figure alongside purity and identity. Our target is greater than 90% of label. When a lot comes in under, that is our problem to solve before it is offered for sale — not a number to quietly leave off the report.
How to check it yourself
An analytical balance and an empty vial from the same lot will get you close: tare against the empty, weigh the filled one, and compare to label. It is a five-minute check and it tells you more about a supplier’s process control than any marketing page will.
You can see the published figures for every lot we sell on the certificates of analysis page.