How to Read a Peptide COA (and Look Up a Lot Number)

A peptide certificate of analysis (COA) is read in a fixed order: confirm the lot number, then the identity result (mass spectrometry), the purity result (HPLC), the net peptide content, and finally who tested the sample and when. A lot number lookup is the first step because every other number on the page only describes the one lot that report names.

This post is a walkthrough of that order. For the definition of a COA, see what a certificate of analysis is; for the lot number itself, see what a lot number is.

Step 1: Match the lot number

Before any result matters, the lot number on the report has to be the lot number on the vial. U.S. drug regulations say that laboratory records must identify the sample by its lot or code identifier, along with where it came from, how much there was, and the dates it was collected and received (21 CFR 211.194). Raw data such as graphs and spectra must be labeled with the material and lot tested. A report with no lot number, or a number that differs from the container, describes some other batch.

Lot numbers are also easy to mistype or to read differently from a label, so a lookup that returns "not found" is worth repeating once with the characters checked, one at a time. On this site the lot number lookup checks a number against the lab reports that have been published here. If a lot has no published report, the lookup says so; it does not fill the gap with a purity figure.

Step 2: Read the identity result

Identity answers the question "is this the molecule it claims to be?" For peptides it is usually measured by mass spectrometry (MS), which reports molecular weight and compares it with the expected value (Bachem). In pharmaceutical practice the observed monoisotopic mass is typically expected to fall within about 1.0 mass unit of the theoretical value, and larger peptides may need high-resolution instruments or average mass instead (BioPharm International).

What a matching mass does not prove is the sequence. MS alone cannot distinguish isomers such as isoleucine and leucine, or D- from L-amino acid substitutions, which is why two identification methods are generally expected for drug-grade material. A COA that lists only a mass has confirmed the mass. See the guide on mass spectrometry and peptide identity for more.

Step 3: Read the purity result

Purity is almost always reported from high-performance liquid chromatography (HPLC). The detector watches ultraviolet absorbance, commonly at 210 to 220 nm, and purity is the share of the total signal that belongs to the main peak (Bachem). Two details change how a figure like "99%" should be read:

• It is a relative figure. It compares the main peak with the other peaks the method detected; it says nothing about how much of the powder in the vial is peptide.
• It depends on the method. A different column, gradient or wavelength can separate different impurities, which is why orthogonal methods are recommended and why the method name belongs on the report (BioPharm International).

Many COAs include the chromatogram itself. A single dominant peak with a flat baseline is what a high-purity result looks like; the report's number should agree with the trace beside it. The guide on HPLC peptide purity covers the chromatogram in more detail.

Step 4: Check net content, not just purity

Purity and net peptide content are different measurements. Net peptide content is the percentage of the material that is peptide, as opposed to counterions and moisture. A peptide can be 99% pure by HPLC and still be well under 100% peptide by weight, because lyophilized peptides carry water and counterions such as trifluoroacetate from synthesis and purification. In a well-characterized batch, the assay, counterion and water results should add up to close to 100%; BioPharm International's wording is that the mass balance "should approach 100%."

This is why a label's stated amount and a COA's content result can differ, and why a report that gives purity but no content or water figure is only partly answering the question. The guide on peptide net content and the earlier post on TFA salt in peptides explain the gap.

Step 5: Look at who tested it, and when

The remaining fields are the ones that make a report accountable. U.S. laboratory-record rules call for the name or initials of the person who ran each test, the test dates, and a second person's sign-off confirming that the records were checked for accuracy and completeness (21 CFR 211.194). A research-peptide COA is not bound by that regulation, but the same fields are a reasonable checklist: the laboratory's name, a report number, the test dates, the methods used and the signature or verification link of the reviewer. A report with a missing lab name, an undated result or no way to verify it offers little to check.

What is known and what is not

COAs are a convention rather than a standard form. There is no single registry of peptide lot numbers, and a PDF cannot prove by itself that the sample tested is the sample in a given vial; that link depends on the lot number and on the seller's records. A COA also describes one sample on one day. These are limits of the document, not reasons to ignore it: it is the best available record of what a laboratory measured.

What this means for researchers

Read the five checks in order and stop at the first one that fails: lot number, identity, purity, net content, then lab and date. For a quicker version of the same list, the ten-point lab report guide and the COA checker tool cover the common problems, including how to spot a COA that is not real. Anything that cannot be matched to a lot number is not yet evidence about that lot.