Start with the page, not the percentage

How to read a peptide COA

A Certificate of Analysis records tests on one submitted sample. Each line answers a narrow question. None is a blanket guarantee.

First: what is the sample supposed to be?

The product name, sequence, molecular formula, or expected molecular mass states the claim. Identity evidence should support it independently. Mass spectrometry (MS) measures mass-to-charge ratios; for example, semaglutide has a published molecular weight of about 4,113 Da. A matching observed mass supports identity. It does not establish purity or the amount in a vial.

Batch or lot

The lot number connects the report to a production batch. It should match the vial or packaging exactly. A valid report for a different lot does not document the material in hand.

Test date

The test date says when the laboratory analyzed the sample. Distinguish it from a report-issue date, manufacturing date, or vendor upload date. A complete report identifies the sample and when testing occurred.

Method: HPLC and MS

High-performance liquid chromatography (HPLC) separates components as they move through a column. A chromatogram shows detector response over retention time; integrated peaks are commonly used to estimate relative purity. Useful method detail includes column, mobile phases, gradient, flow rate, detector wavelength, injection amount, and run time. MS answers an identity-related question; HPLC commonly answers a relative-composition question. Strong documentation often includes both.

Area-% is not mass-%

Area-% is the target peak's integrated detector area divided by total integrated peak area under the stated method. That is relative detector response. Mass-% is the target compound's mass divided by total sample mass. These are not interchangeable: substances respond differently, while water and some salts may not appear under the method at all. “99% HPLC purity” does not mean 99% of the vial's total mass is target peptide.

Mass or content

A labeled vial amount needs a quantitative assay designed to measure content, not merely an HPLC area percentage. Depending on the validated procedure, reports may use a calibrated reference standard, quantitative NMR, amino-acid analysis, or another suitable quantitative method. Check the units, preparation, calibration, result, and uncertainty or acceptance range if provided. “Mass” can also mean molecular mass in an MS section; that is not vial content.

Impurities

A chromatogram may list individual impurity peaks and their relative areas. Named or identified impurities provide more information than an unlabeled total. Also look for tests addressing relevant non-peptide components when claimed, such as water, residual solvents, counterions, or inorganic residue. No single assay detects every possible impurity.

LOD and LOQ

The limit of detection (LOD) is the lowest level at which a method can reliably distinguish that an analyte is present. The limit of quantitation (LOQ) is the lowest level at which it can be measured with acceptable performance under the method. “Not detected” means below the method's detection capability; it does not prove absolute absence. A number below the LOQ may be detectable but not reliably quantifiable.

Five questions beat one headline number

What sample was tested? By whom, and when? Which method did the lab use? What claim does each result support? Identity, relative purity, quantity, and specific impurity tests remain separate lines of evidence.