How to Compare COAs From Two Peptide Suppliers Side by Side
Published 2026-10-13 · Last updated: 2026-10-13 · By: Index Peptides technical team
To compare certificates of analysis (COAs) from two peptide suppliers, first confirm that both documents describe the same compound in the same form, then check that the purity figures came from comparable HPLC methods, then line up the remaining rows one by one and record every empty row as a finding. Only after those steps does the headline purity percentage mean anything. This page sets out that side-by-side workflow for UK research buyers holding two candidate documents for the same product. It does not rank suppliers, and no figure on any COA implies suitability for any use.
Research use only. Not for human or animal consumption.
Step 1: confirm both documents describe the same material
Identity is the gate. Before comparing any numbers, check that both COAs name the same compound and give the same identity reference: compound name, CAS number where stated, molecular formula and expected molecular weight. If those do not match, the documents are not comparable, however similar the product names look.
Form matters as well. Research peptides are often supplied as a salt, commonly acetate or TFA, rather than the free form, and the salt form changes the as-weighed mass and the reference figures a certificate quotes. One supplier quoting free-form values and another quoting a salt is not a discrepancy in itself, but it is a difference to write down. In every case, the batch COA is the authoritative record of the form that was actually tested.
Step 2: match the method before comparing purity
HPLC purity is method-relative. A purity percentage is the area of the main peak as a share of all detected peak area under one specific method, so two figures are only directly comparable when the methods behind them are comparable. Before setting 98.9% against 99.4%, check each document for:
- Technique: reversed-phase HPLC on both, or something different on one of them.
- Detection wavelength: a figure read at 214 nm and one read at 280 nm measure the same kind of sample with different yardsticks.
- Purity basis: area percent and weight percent (assay) answer different questions and should never be compared as if they were the same number.
- Column and gradient: where stated, these decide which impurities are separated from the main peak.
If the method lines differ, or one document has no method line at all, the two percentages are not a like-for-like comparison. The reasons figures move between labs and methods are covered in why purity percentages differ between labs; here that check is one step in a wider comparison, not the whole of it.
Step 3: line up the rows
With identity confirmed and methods noted, work through the two documents row by row. A simple worksheet keeps the comparison honest:
| Row | What to record for each supplier | What an empty row tells you |
|---|---|---|
| Lot or batch number | The exact identifier on the certificate | The document cannot be tied to a vial |
| Identity | Expected vs detected mass, and the method (MS or LC-MS) | Identity is assumed from the name, not measured |
| HPLC purity | Figure, basis and full method line | The number cannot be interpreted |
| Water content | Figure and method | The mass balance is incomplete |
| Residual solvents | Solvents screened and results | No record of process solvents |
| Heavy metals | Elements screened, specification vs result | No elemental screen on record |
| Endotoxin or sterility | Method, units and result, where shown | Not tested, or not reported |
| Test date | Date of analysis | The result cannot be placed in time |
| Issuing lab | Name of the laboratory | The source of the figure is unknown |
Not every COA will carry every row, and not every row applies to every product. The point of the worksheet is that empty rows are findings, not just omissions. A document that leaves identity blank has told you something different from a document that reports it, even when both show the same purity figure.
Step 4: score transparency, not just the headline figure
Once the rows are filled in, compare how much each document lets you check. A COA that names the issuing laboratory, states the lot, gives the full method line and shows the impurity picture (a chromatogram, or at least the main impurity peaks) is more informative than a document carrying a single bare percentage, even when the bare percentage is higher. A higher figure with no method behind it is simply a less interpretable figure.
Flags worth noting in a side-by-side comparison:
- a purity figure with no stated basis or wavelength;
- a missing method line;
- vague impurity wording such as "conforms" with no figure;
- a template that looks identical across different lots.
Whether the issuing lab is independent of the seller is a separate question, explained in third-party peptide testing labs.
Step 5: compare spread within each supplier first
A single COA is one snapshot of one lot. Where you can, ask each supplier for documents from more than one batch of the same product and look at the spread within each supplier before comparing across them. Two suppliers whose current lots read similarly can look quite different over three or four lots, and a method line that stays the same from lot to lot is itself a useful signal. How lot-to-lot records are kept and compared is covered in batch-to-batch consistency documentation.
Step 6: verify the document against the vial
The comparison only describes the material you receive if the document matches the lot printed on the vial. Where a supplier offers a public batch lookup, run the lot number through it and keep the record that matches the label. Index Peptides publishes its batch records and certificates, and its lookup at /verify returns the record for a lot number. For a lot you have not yet received, ask for the certificate for that specific lot, as described in request a batch COA by lot number.
What this comparison does not do
This workflow compares documents. It does not produce a "best supplier" verdict, and it is not a ranking of anyone. No purity figure is a safety or suitability threshold: each figure is documentation against a stated method, for one lot. For the wider checklist of per-batch records, dispatch and research-use positioning, see the guide to choosing a UK research peptide supplier.
FAQ
Can I compare purity figures from two different suppliers directly?
Only when the methods match. Check that both figures come from reversed-phase HPLC, at the same detection wavelength, on the same purity basis (area percent or weight percent). If any of those differ, or one certificate has no method line, the two percentages are not a like-for-like comparison.
What should I check first when comparing two COAs?
Identity. Confirm both documents name the same compound with the same CAS number, molecular formula and expected molecular weight, and note whether each tested the free form or a salt. If the identity references differ, the documents describe different materials and cannot be compared.
Is the COA with the higher purity figure the better document?
Not necessarily. A document that names the lab, the lot, the full method and the impurity picture is more informative than a bare percentage, even if the bare percentage is higher. A higher figure without a method line is a less interpretable figure, not stronger evidence.
Why compare more than one batch from each supplier?
A single certificate describes one lot. Looking at two or more lots of the same product from each supplier shows how consistent their results and method lines are, which one snapshot cannot. Compare the spread within each supplier before comparing across suppliers.
How do I check an Index Peptides COA against my vial?
Enter the lot number printed on the vial label at /verify, or find the batch record and certificate at /coa. The lot on the certificate should match the label character for character. Research use only. Not for human or animal consumption.
Research use only. Not for human or animal consumption.