How to compare two peptide vendors on testing

Compare evidence quality first, then price. A vendor whose certificates answer more of the basic questions — what the material is, how pure, how much of the labeled content is actually present — is offering a different kind of product than one whose certificates answer fewer, even at the same compound and size. Price comparison is only meaningful after the evidence comparison.

1. Assays present. Which of the standard assays appear across the vendor's certificates — HPLC purity, LC-MS/MS identity, net peptide content — and which are absent for the compound you are looking at. The missing-test guide states what each absent assay leaves open.

2. Whether the lab is named. Does each certificate name a testing laboratory? A certificate without a named lab cannot be checked against anything — see the lab-claim verification guide.

3. Whether identity is reported, and how. If an identity result appears, note the method used to establish it. Mass spectrometry methods confirm what the compound is; other techniques support it less directly. Vendor pages on this site describe identity testing in the same terms as the testing reference.

4. Whether net peptide content is reported. Purity is a proportion of the tested sample; net peptide content is how much of the vial is the peptide. A report with purity but no content figure does not answer the how-much question.

5. Whether acceptance criteria are stated. Does the certificate state the specification each result is judged against? Conformity statements only mean something relative to stated criteria — as covered in the missing-test guide.

6. Lot and fill size match. Does the certificate identify the lot, and does that lot correspond to the fill size listed for sale? A report for a different size than the one offered is weaker evidence for the product in the cart.

7. How recent the reports are. Note the report dates for the compound in question. Older reports are not wrong, but they describe past batches, not current ones.

8. How many reviewed reports exist. One report is one lot. Where a vendor page lists several reviewed reports for the same compound, that is evidence across more than one batch; a single report is evidence about one.

Suppose Vendor A reports purity and identity for the compound; Vendor B reports purity only. What does that difference tell you?

It tells you that for the tested samples, Vendor A's documents answer one additional question: whether the material is what it claims to be, not only how pure it is. Vendor B's documents leave that question open — purity alone does not rule out a different compound present at high concentration, which is the point of the purity-is-not-identity guide.

What it does not tell you: that Vendor A's product is better in any other respect. The documents say nothing about which vendor's lots you will receive, how either handles other compounds, or how the two compare on price, shipping, or service. It is one line of one comparison, not a verdict.

A lower price with a thinner testing panel is a different product decision, not a ranking error. If one vendor documents net peptide content and identity and another documents purity alone, the two offers are not the same product at different prices — they are two different levels of evidence at different prices. Deciding which level of evidence you need is a separate step from comparing prices within that level.

This guide takes no position on what that evidence is worth, and this site's coupon and pricing information is listed separately on vendor pages and the coupons page.

On the rankings page, use the Compare button to enter compare mode, then select up to four vendors. When you have selected at least two, a bar appears that opens the comparison page with your selection, where the vendors' details appear side by side.

Vendor pages list each vendor's certificates and testing summary individually; the side-by-side comparison itself runs from the rankings page and the comparison page.

One report is one lot. Everything in this checklist is evidence about the specific samples described in the reports — it is not a guarantee about the next batch. How a vendor's overall testing record is weighed is part of the ratings methodology, and what our review of a certificate involves is described in how PeptideScores uses a COA.