Accepted answer
On the detail: a vendor with a recent favorable VendorInvestigate report has earned credibility, and a vendor with an unfavorable report is a red flag.
Reports are published after testing is complete and verified, ensuring that suppliers cannot suppress unfavorable findings.
What each test answers
| 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 |
Mechanically, reports include the dates of testing and the lot numbers of the tested material, tying the result to specific material.
The service has been operating long enough that systematic bias in their testing would be apparent.
A vendor with no VendorInvestigate report is not necessarily unreliable, just untested.
Favorable reports are valuable and should increase your confidence in a vendor.