Lab Techniques · Jul 26, 2026

Why Two Suppliers Quote Different Purity for the Same Peptide

For research reference only. Not guidance for use in humans or animals.

Two suppliers list the same compound. One says 98%, the other says 99.9%. Both may be telling the truth, and the second is not necessarily the better material. The reasons are worth understanding, because purity is the number the whole market is sold on.

Purity is a ratio, not a quantity

HPLC purity is the area of the target peak divided by the total area of all peaks, expressed as a percentage. It answers one question: of the material the detector saw, what fraction was the compound of interest?

It does not tell you how much peptide is in the vial. That is a different measurement, and the gap between the two is where most of the confusion lives.

Where the impurities come from

Most research peptides are made by solid-phase peptide synthesis: the chain is built one residue at a time on a resin support, with each residue coupled, washed, deprotected and coupled again.

Each coupling step is efficient but not perfect. Two failure modes dominate:

Deletion sequences — a residue fails to couple and the chain continues
without it, producing a peptide one residue short.
Truncated sequences — the chain stops growing entirely and is capped.

Both produce molecules chemically very similar to the target, which is exactly what makes them hard to separate. And the longer the peptide, the more coupling steps, so the more of this accumulates. A 5-residue peptide and a 39-residue peptide are not comparable manufacturing problems, which is why purity expectations should differ by compound rather than being a flat catalogue-wide number.

The counterion nobody mentions

Synthetic peptides are purified and isolated as salts. The counterion is usually trifluoroacetate (TFA), left over from the purification mobile phase, or acetate where an exchange step has been performed.

That counterion is real mass in the vial. Depending on the peptide and the number of basic residues, salt and residual water can account for a substantial share of the total solid — for some peptides, well into double digits as a percentage.

Here is the consequence: a vial can be 99.9% pure and still contain appreciably less than the stated mass of peptide. The purity figure is about the identity of what is there; the counterion is about how much of it is peptide at all.

Net peptide content

This is the measurement that closes the gap. Net peptide content — determined by amino acid analysis or nitrogen determination — reports what fraction of the total solid is actually peptide, counterion and water excluded.

Most suppliers in this market do not report it. It is a separate, more expensive analysis, and quoting it makes your product look worse next to a competitor quoting HPLC purity alone. That is precisely why a supplier who does report it is telling you something.

If you are comparing quotes, compare like for like: HPLC purity against HPLC purity, and treat a net-peptide-content figure as extra information rather than a lower purity.

Why the method matters

The same sample can return different purity figures on different methods. A shallow gradient resolves closely-related impurities that a steep one merges into the main peak. Detection wavelength matters too — 214 nm sees the peptide bond itself and therefore nearly everything, while 280 nm sees only aromatic residues and will miss impurities lacking them.

A purity figure without a method attached is not comparable to anything. This is one of the checks in how to spot a forged certificate, and it applies just as much to honest reports.

What to actually compare

1. Is there a certificate for the specific lot, not the product line? 2. Does it state the method, or only the number? 3. Is there a chromatogram, or only a claim? 4. Is identity confirmed by mass spectrometry, or is purity all you get? 5. Is net peptide content reported at all?

A supplier scoring well on one to four is giving you something checkable. A higher headline number from a supplier scoring badly on all of them is not evidence of better material.