What does 98% purity actually tell you?

The short answer

It says that 98% of the signal in one test came from the main peak. It does not say that the peak is the right molecule, what else is in the vial, or how much peptide you actually have.

The short read

Purity is usually peak area from HPLC, often read at a UV wavelength near 214 nm.
Identity needs a different test, usually mass spectrometry.
Water and salts are not counted, so peptide content can be much lower than purity.

High-performance liquid chromatography separates a mixture and reports each component as a peak. Purity is the main peak's share of total peak area. That one number says nothing about identity, which needs mass spectrometry; nothing about water and salt content; and nothing about sterility or endotoxin. A certificate that lists only purity answers one question out of several.

The full explanation

A certificate with the number 98% looks reassuring. Here is what the number is, and is not.

How purity is measured

The usual method is high-performance liquid chromatography, HPLC. A sample is pushed through a packed column and its components leave at different times. A detector, often set near 214 nanometres where peptide bonds absorb light, draws a peak for each one. Purity is the area of the main peak divided by the total area of all peaks.

What the number leaves out

Identity. The main peak is only a peak. Without a second method, you cannot be sure it is the intended molecule. Mass spectrometry measures molecular weight and is the standard way to confirm identity.
Look-alikes. By-products missing one link, or with a small chemical change, can leave the column at nearly the same time as the product and hide inside the main peak.
Peptide content. Freeze-dried powder contains water and salts from the purification step, commonly trifluoroacetate. A vial of 98% pure peptide can have a significant share of its weight made of something else.
Safety tests. Sterility and bacterial endotoxin are separate tests. A purity number says nothing about them.
Method. Different columns and settings can give different numbers from the same sample.

What a fuller certificate shows

A thorough certificate lists the method and conditions, the mass found against the mass expected, the peptide content, and the date and lab. It should come from a laboratory you can identify.

Rule of thumb

Treat a single number as a claim, not as proof. The more questions a document answers, the more it is worth.

Jargon? Say it simply

Education only. Not medical advice, no dosing, no product recommendations. Read this first.

Keep going

Related questions

Q18At the Bench

Does Bacteriostatic Water Need to Be Refrigerated and How Long Is an Opened Vial Good

The Hospira label says to store it at 20 to 25 C, so refrigeration is not required. After a vial is opened, CDC says to discard it within 28 days unless the maker states otherwise.

The labeled storage for the common product is room temperature, 20 to 25 C.
Cold slows growth but cannot sterilize a contaminated vial.
An opened multi-dose vial should be dated and discarded within 28 days, per CDC.

A bacteriostatic water carton states its own storage condition, and for one widely used product that is room temperature. Refrigeration slows microbial growth but does not clean a vial that was contaminated, and the preservative only reduces growth between uses. CDC advises dating a multi-dose vial when opened and discarding it within 28 days unless the maker says otherwise. The product contains benzyl alcohol and is not for newborns. Storage of a mixed medicine follows that medicine's label.

Q21At the Bench

Beginners Guide on How to Reconstitute Peptides and What a Kit Cannot Supply

For approved medicines, the maker's label gives the validated steps and diluent. For unregulated vials no validated method exists, and a reconstitution kit sells supplies, not a method.

A kit holds supplies such as a liquid, swabs and syringes, but no tested procedure.
USP 797 shows what real sterile mixing needs: controlled rooms, trained staff and set time limits.
Neutral solution math explains concentration but proves nothing about purity or safety.

Dissolving a peptide powder is a manufacturing step with a medical risk. In the United States, pharmacies follow USP 797, which ties allowed storage time to the category of preparation and the room where it was made, and requires documented staff testing. Approved powder products give the maker's own steps in the label. Unregulated vials come with none, and a kit of supplies cannot replace stability data, identity checks or sterility testing.

Q22At the Bench

Why There Is No Safe Guide on How to Reconstitute Retatrutide

There is no validated method, because retatrutide is not approved anywhere and no maker has written or tested one. Vials sold online have unknown contents, and counterfeits are a documented danger.

A reconstitution method belongs to an approved product label, and retatrutide has none.
FDA, Health Canada and the MHRA have acted against sellers of unapproved retatrutide in 2026.
Solution math cannot reveal what is really in a vial or whether it is sterile.

Approved powder drugs come with a manufacturer diluent, written steps and a tested time limit. Retatrutide is an investigational Lilly drug with no approval, so no such method exists, and registry entries describe trial supply only as subcutaneous. In 2026 FDA letters treated calculators and bacteriostatic water as evidence of intended human use, Health Canada warned about seized peptides, and the MHRA found thousands of fake pens. No recipe is given here.