Inter-lab variability in peptide testing — July 2024

This one has a reasonably settled answer, so here it is. They are not contradictory, they measure different things, and this is the single most common misreading of a certificate. Purity is a relative area measurement: the main peak as a percentage of total peak area in the chromatogram. Content is an absolute mass measurement against a reference standard. A vial can be 99% pure and contain 6.1mg — pure material, overfilled — or 99% pure and contain 3mg, which is pure material and a short fill. Purity tells you what fraction of what is there is the right molecule; content tells you how much of it there is. You need both, and a certificate with only purity is telling you half the story.

RetaRick_CA said:

They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.

Agreed, and mass spec answers a question purity does not: identity. High purity tells you one thing dominates the chromatogram, not that it is the molecule you ordered. A mass within about a dalton of expected is the identity check.

JennaRN said:

Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.

Can confirm the pattern JennaRN describes. The provenance questions are more useful than the numbers. Does the report name the batch on your vial. Is the date consistent with when that batch existed. Is it addressed to the vendor or to the person who paid for the test. Was the sample taken from general stock or supplied by the vendor. A perfect certificate for a different batch is worth nothing, and vendor-supplied samples are the most common way a genuine report ends up misleading.

Adding the clinical framing, because it changes how the question reads.

My HPLC chromatogram analysis for certificates of analysis — I sent my batch to Janoshik and here's what the results showed:

Main peak retention time: 14.5 minutes (consistent with semaglutide reference standard)
Peak purity: 98.3%
Related substances: 0.8% (single impurity at 8.4 minutes — likely a synthesis intermediate)
Mass spec: m/z 4113.58 (matches expected molecular weight)

This is a clean result. The mass spec confirmation is particularly important — it proves the molecule is actually semaglutide and not a substitution.