This is my second independent submission on material from the same supplier.
This is a planning question. I know what my options are; I do not know how to weigh them.
What I want is the minimum viable version, which I suspect is smaller than what I would design.
How do I make this decision on evidence rather than on feel?
The underlying point is that if a lot has visibly segregated — some vials showing different appearance — then sampling the top and bottom of the shipment is worth doing.
Stratified sampling — testing one vial from the top, one from the middle, and one from the bottom of a shipment — is cheap insurance against segregation.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
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.
Sampling plans for pharmaceutical manufacturing are defined in ISO 2859 and ANSI Z1.4, and they are based on statistical sampling theory.
The caveat is that sampling is a trade-off between cost and confidence, and neither test nor assumption is cost-free.
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 resultsThe 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.
Concretely, 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.
If testing multiple vials, state how many you tested and why you chose those vials.
edited 21 Feb 2025 by b_delacroix — tightened the wording; no substantive change
Worth being precise here: the single most misleading statement on a research-grade certificate is a lot number with no statement of how many vials from that lot were tested.
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 the lot was manufactured in multiple batches, testing vials from each batch separately establishes whether batch-to-batch variation is acceptable.
The limitation is that you cannot know for certain without testing every vial, and you almost never can afford to do that.
Assume segregation is possible, and design your sampling to catch it if it exists.
The relevant detail is that the failure mode here is publishing a result that applies to the tested vial alone while implying it applies to the entire lot.
A statement that "lot 20260412 complies with specifications" is meaningless without stating which vials from the lot were tested and how many there were.
Worth noting that thermal excursions during shipping affect different vials differently, so the lot may not be homogeneous even if it left the factory that way.
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.
Start 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 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.
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.