How to read a certificate of analysis

A certificate of analysis, or COA, is a laboratory’s report on one sample. It says what was measured, how, when, and what the lab found. Read carefully, it tells you a lot. Read quickly, it can seem to say more than it does. This page walks through an invented example, line by line.

Everything in the example is made up for this page: the lab, the sample, the report number, the lot number and the dates. It matches no real lab, lot or material. The numbered markers on the example match the numbered sections below.

1. Who issued it: the lab and the report number

Start at the top. A certificate should name the lab that did the work and give a way to reach it. A report that names no lab cannot be checked, however good its numbers look.

The report number is the lab’s own identifier for this piece of work. It is what lets you check the document with the lab, and checking is worth the few minutes it takes:

• Find the lab’s contact details yourself, from its own website, rather than from the certificate. An edited document can carry edited contact details.
• Send the lab the report number and ask whether the report exists, and whether the sample description, lot, dates and values match what you have.
• Some labs offer an online lookup by report number. If you use one, make sure it sits on the lab’s own web address.

While you are at it, look for signs of editing: fonts that change mid-line, figures that do not line up, or a date written in two different styles.

2. When: the dates

A certificate usually carries up to three dates: when the lab received the sample, when it ran the analysis, and when it issued the report. All three should make sense together, and the report should postdate the analysis.

The dates also set a limit on what the certificate can tell you. It describes the sample as it was on the day it was measured. It says nothing about how the material has been stored since, or how it has changed. Dry peptide material changes slowly, and heat, light and moisture set the pace; see Storage science for lyophilized material.

3. Which material: the lot, and why it must match the vial

A lot, also called a batch, is a quantity of material made in one production run and given one identifying number. Material within a lot is expected to be uniform. Material from different lots can differ, because each run is a separate synthesis and purification.

So a certificate describes the lot its sample came from, and nothing else. The lot number printed on the certificate must match the lot number on the label of the vial in front of you. If the numbers differ, or the label shows no lot number at all, the certificate does not describe that vial. A certificate for another lot of the same material describes a different production run.

Check the sample description too. It should describe the same material, in the same form, as the label: here, a white freeze-dried powder.

4. The method beside each value

Every value on a certificate should sit beside the method that produced it. “Purity 99.1%” on its own says little. “99.1% by HPLC-UV at 214 nm, area percent” tells you which instrument measured what, and how the number was calculated. That matters, because different methods measure different things: area percent from a chromatogram is not the same as percent by mass, as HPLC and how to read a chromatogram explains.

Look at the units, and at the specification column. A specification such as “not less than 98.0%” is a limit set before the test; “conforms” means the value met it. Limits can be loose or tight, so read the value itself and not only the verdict. If the certificate lists attachments, such as a chromatogram or a mass spectrum, the numbers in the table should agree with them.

5. Purity and identity answer different questions

These two lines are the core of most peptide certificates, and they are easy to run together.

• Identity asks whether the main substance is the intended molecule. It is usually tested by mass spectrometry: the lab compares the mass it measures with the mass calculated from the intended sequence. In the example, the expected mass is 1,800.00 Da and the lab found 1,800.02 Da, inside its stated acceptance window. See Mass spectrometry: identity, not purity.
• Purity asks how much of what the method can see is the main substance. It is usually measured by HPLC with UV detection and reported as area percent.

Neither answers the other’s question. A high purity figure with no identity test could describe a very clean sample of the wrong molecule. A correct mass with no purity figure says the molecule is present, not how much else is there. A useful certificate reports both, each with its method.

6. What is absent, and what absence means

Read a certificate for what it leaves out as well as for what it says. The example reports appearance, identity and purity, and nothing else. It does not report:

• how much peptide the vial holds, either as net peptide content or as an amount per vial (see Purity, net content, net peptide content and salt forms);
• water content;
• counterion content, the salt part of the powder;
• residual solvents from synthesis and purification;
• elemental impurities, such as metals;
• microbiological tests, and the bacterial endotoxin test.

Absence means “not reported.” It does not mean “passed.” A test that was never run cannot be passed or failed; on that question the certificate is silent. If a missing value matters to your work, ask whether it was measured. If it was not, the honest answer is “unknown.”

Check for missing paperwork as well. The example lists a chromatogram and a spectrum as attachments and says “page 1 of 3,” so there should be two more pages. It also makes no statement about accreditation. Whether a lab is accredited, and for which tests, is something you can check yourself; see Accreditation in the glossary.

A short checklist

• The lab is named, and the report is confirmed with the lab using contact details you found yourself.
• The lot number on the certificate matches the lot number on the vial.
• The dates make sense, and you know how old the analysis is.
• Every value has its method, units and specification beside it.
• Identity and purity are both reported, each by a stated method.
• Anything missing is noted as unknown, not assumed.