Peptide COA glossary: every term on a certificate of analysis

By Jordan Sanders · Updated 2026-10-26 · How this site is paid

Short answer: a certificate of analysis (COA) is a lab report on one tested sample of one lot. Most peptide COAs combine three kinds of test: identity (is it the right molecule, usually by mass spectrometry), purity (how much of the detected material is that molecule, usually by HPLC) and content (how much peptide is in the vial). The terms below are the ones you'll meet on the page, in plain English.

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The report itself

Certificate of analysis (COA)

A lab report stating what was measured on a specific sample, by which methods, with what values. It describes the sample tested and the lot it came from, nothing more. See how to read a COA.

Lot number (batch number)

The identifier for one production run. The lot on the COA must match the lot on your vial, or the report isn't about your vial. See why "same batch" isn't proof.

Client

The company that ordered the test. It should be the supplier you bought from, or a company the supplier can explain.

Report number / verification code

The ID you use to look the report up with the lab that issued it. It's the strongest check available. See how to check a COA is real.

Specification (spec)

The limit or range a value is judged against, such as "purity ≥ 98%". A value shown next to its spec, marked pass or fail, tells you what the lab or supplier considers acceptable.

Representative COA

A real report for one lot, used to illustrate a product. It says nothing about a different lot, including the one you received.

Research use only (RUO)

A label meaning the material is sold for laboratory research only. See what research use only means.

Identity: is it the right molecule?

Mass spectrometry (MS)

A method that measures the mass of molecules. On a peptide COA it confirms identity by checking the measured mass against the mass the sequence should have.

LC-MS / LC-MS/MS

Liquid chromatography coupled to mass spectrometry: the sample is separated first, then each part's mass is measured. LC-MS/MS adds a second stage that breaks molecules into fragments for more detail.

Observed mass vs. expected (theoretical) mass

The mass the instrument measured, and the mass calculated from the peptide's sequence. They should agree within the lab's stated tolerance. A mismatch means the sample may not be the labeled compound.

Dalton (Da)

The unit of molecular mass used on mass spectrometry reports.

Purity: how much of the sample is that molecule?

HPLC (high-performance liquid chromatography)

A method that separates a sample into its components so each can be measured. It's the standard way peptide purity is reported. See what a purity percentage means.

Chromatogram

The trace HPLC produces: a line with peaks, one for each component the detector picked up. A purity figure is only checkable if the chromatogram is shown.

Retention time

How long a component takes to pass through the HPLC column. It's the position of a peak along the chromatogram's horizontal axis.

Peak area / area %

The size of a peak. HPLC purity is usually the main peak's area as a percentage of all the peaks counted.

Detection wavelength

The UV wavelength the detector reads. Peptides are commonly detected around 214–220 nm, where the peptide bond absorbs. Material that doesn't absorb there, such as water and many salts, doesn't appear as a peak.

Purity

The share of the detected material that is the target peptide. It doesn't tell you identity, content, or anything about other lots.

Content: how much peptide is in the vial?

Net peptide content

The share of the powder's total mass that is peptide. It's normally lower than purity because the powder also holds water and counter-ions. See net peptide content.

Counter-ion

A salt partner the peptide is isolated with, such as acetate or trifluoroacetate (TFA). It adds mass that isn't peptide.

Water content (Karl Fischer)

The amount of water in the powder. Karl Fischer titration is a common method for measuring it.

Amino acid analysis (AAA)

A method that breaks the peptide into its amino acids and measures them. It's one way labs determine peptide content.

Lyophilized

Freeze-dried. Most research peptides ship as lyophilized powder. See lyophilized peptides.

Appearance

A visual description of the sample, for example "white lyophilized powder".

Contaminants

Endotoxin

Material from the outer membrane of Gram-negative bacteria. It needs its own test. Purity and identity tests don't detect it. See how to read the endotoxin line.

EU (endotoxin unit)

The unit endotoxin is reported in, usually per milligram or per vial. Under USP, one EU equals one International Unit (IU).

LAL / rFC

Endotoxin test methods: LAL uses horseshoe crab lysate (USP chapter 85), and recombinant factor C (rFC) uses lab-made reagents (USP chapter 86).

The lab

ISO/IEC 17025

The international standard for the competence of testing and calibration labs. Accreditation covers specific methods, listed in the lab's scope.

Accreditation scope

The list of tests a lab is accredited for. Check that the method on your COA is on it, with the accreditation body that issued it.

Third-party (independent) lab

A lab with no stake in the sale. See how to choose a testing lab.

Have a COA in front of you? The free COA checker walks through each of these fields for your report.