Reading a certificate of analysis
A purity percentage on its own is close to meaningless. Here is what actually makes a certificate informative, and what to check before you rely on one.
Identity comes before purity
The first thing to look for is not the purity figure. It is confirmation of identity.
A purity result tells you what proportion of the material is a single dominant species. It does not tell you that species is the compound you ordered. A sample can be 99% pure and 99% something else entirely.
Identity is normally confirmed by mass spectrometry: the observed mass is compared against the theoretical mass for the declared sequence. A certificate without identity confirmation is not a certificate of analysis — it is a purity result attached to an assumption.
What the purity figure actually measures
Peptide purity by RP-HPLC is normally reported as the area of the main chromatographic peak as a percentage of the total area of all peaks, at a specified detection wavelength.
Three things follow from that definition, and they are the reason context matters:
- It is relative, not absolute. It describes the proportion of detected material, not the proportion of the vial’s mass.
- It is wavelength dependent. Peptide bonds absorb strongly around 214 nm. A figure quoted at a different wavelength is not directly comparable.
- It only counts what elutes and absorbs. Anything that does not come off the column, or does not absorb at that wavelength, is invisible to the measurement — including water and salts.
A “99% pure” vial is not 99% peptide by mass. Water and counter-ion are part of the mass and are not counted by the HPLC figure at all. This is not a trick — it is simply what the method measures.
The three numbers most certificates omit
Water content
Lyophilised peptide is hygroscopic and retains residual moisture. Measured by Karl Fischer titration, it is typically a few percent of vial mass. Without it you cannot know how much of the vial is peptide.
Acetate content
Most synthetic peptides are supplied as acetate salts, because acetate is used in purification. The counter-ion can be a substantial fraction of total mass — often high single digits — and it is peptide-dependent.
Bacterial endotoxin
Relevant to any cell-based work, because endotoxin contamination confounds results in culture systems at very low concentrations. Measured by LAL assay and expressed in EU per milligram.
A checklist
- Is the testing laboratory named, and is it independent of the manufacturer?
- Is there a batch number, and does it match the vial in your hand?
- Is identity confirmed by MS, with the observed mass stated?
- Is the HPLC method stated, including detection wavelength?
- Are water and acetate content reported?
- Is there a date of issue, and is it plausibly recent?
- Is the full report available, not just a summary figure?
If the same certificate image appears against multiple batch numbers, or the batch number on the document does not match the vial, treat the document as unverified. A certificate that is not batch-specific is decoration.
How ours are structured
Every batch we sell has its own certificate page on this site, published before the product goes live, and it stays online after the batch is superseded. Each one carries identity confirmation, the HPLC method and wavelength, water and acetate content, and endotoxin. You can see the full set in the certificate library.
Frequently asked questions
Does 99% pure mean 99% peptide by mass?
No. HPLC purity is the main peak as a proportion of total detected peak area. Water and counter-ion are part of vial mass but are not counted in that figure.
Why does detection wavelength matter?
Because absorbance varies with wavelength. 214 nm is standard for peptides because the peptide bond absorbs strongly there. A purity figure at a different wavelength is not directly comparable.
What endotoxin level matters for cell work?
It depends entirely on the assay system’s sensitivity, so no universal threshold applies. What matters is that the figure is measured and published so you can judge it against your own requirements.
Can I see the raw chromatogram?
Yes. Email research@verolynlabs.com with the batch number and we will send the full report as issued by the laboratory.