Reading a certificate of analysis

Mass spectrometry weighs molecules very precisely. If the measured mass matches the expected mass of the named peptide, its identity is strongly supported. A vial can be very pure and still be the wrong molecule, so identity comes first.

HPLC (high-performance liquid chromatography) separates everything in a sample. The result is a chart called a chromatogram, with one tall peak for the target peptide and small peaks for impurities. Purity is the target peak's share of the total, such as 99.7%.

Content is different. It reports how many milligrams of peptide are actually in the vial, measured against a reference standard, a sample with a precisely known amount.

Endotoxin testing checks for a toxin from the outer membrane of certain bacteria, which can remain even after the bacteria are gone. Some lots are also screened for heavy metals and sterility. The supplier's certificates show these only for lots where they were tested.

Match the lot number on the vial to the certificate, character by character. Check the testing lab's name and verification code. Note whether a product page's purity chart is labeled "illustrative"; the real chart is on the lot's certificate. Never round up results or apply one lot's numbers to another lot.