The case in front of me: an HPLC purity test · orforglipron.
I would rather over-plan the first cycle and simplify later.
I am prepared to do the work if someone can tell me which work matters.
What is the minimum version of this that is still defensible?
Twelve months is 52 weeks, so the budget is set by lot turnover, not by the price of an HPLC purity test. Take one lot a quarter as the low case: 4 lots a year, so a test-every-lot policy is 4 assays and a test-every-third-lot policy is 2 once you round up. Take one lot a month as the high case: 12 lots, and the same two policies are 12 assays and 4. The spread between the cheapest and the dearest defensible policy is therefore about a factor of six across the same 52 weeks. Choose the policy before the first result. One chosen after a disappointing figure is a reaction to that figure, and it will not survive the second one. Then spend it where it changes a decision: over a year, one content assay on each new lot tells you more than four purity figures on the same lot, because purity and content are independent and only one of them changes your arithmetic.
The practical consequence is that spot-testing one vial from a new supplier is better than assuming they are all the same.
The statistical foundation here is well-established, which is why sampling plans from decades ago are still valid.
The sample size determination requires choosing a confidence level and an acceptable error rate, and the smaller the error rate you want, the larger your sample must be.
The practical summary: a lot number without a sampling statement is a lot number without meaning.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsStart from the question: how many vials from this lot do I need to test to claim that the lot meets specification, and the answer depends on both the lot size and the acceptable risk.
If you have reason to suspect inhomogeneity — different appearance in different vials, or a long or warm shipment — testing more vials is the diagnostic move.
Concretely, if the entire lot failed qualification, a retest on a different vial is sometimes done, but reporting a retest result under the same lot number is misleading.
The caveat is that sampling is a trade-off between cost and confidence, and neither test nor assumption is cost-free.
Assume segregation is possible, and design your sampling to catch it if it exists.
Sampling plans exist precisely because testing everything is expensive, and they define the statistical relationship between sample size and lot-wide inference.
A statement that "lot 20260412 complies with specifications" is meaningless without stating which vials from the lot were tested and how many there were.
For a quantitative result like content, the acceptable range determines how many vials you need to test to establish the lot complies.
Lyophilised peptide homogeneity studies show that vial-to-vial variation is usually small but occasionally large, depending on the distribution in the freeze-dryer.
I would treat a "complies with" statement without sampling details as a claim rather than as evidence.
If testing multiple vials, state how many you tested and why you chose those vials.
The part that matters: two vials tested from a lot of ten is very different from two vials tested from a lot of ten thousand, and most certificates do not state the lot size.
Acceptance Sampling by Attributes defines the number of samples you need from a lot to claim a specified quality level at a specified risk — it is in ANSI standard Z1.4.
If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.
edited 2 Aug 2024 by a_lindgren — fixed an arithmetic slip in the third paragraph
The honest statement is that unless you have tested multiple vials or have segregation data, you are making an assumption about lot homogeneity that may not hold.
Under AQL sampling plans, testing two vials from a fifty-vial lot gives you an operating characteristic curve that tells you what risks you are accepting.
Published data on lot homogeneity from manufacturers who sample multiple vials consistently find variation below the published specifications, suggesting the sampling plans work.
In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.