Purity Testing Explained

Purity is one of the first numbers researchers look at on a product listing or Certificate of Analysis — but a percentage on its own doesn’t tell the whole story. This article explains what purity actually measures, how it’s tested, and what to look for when comparing results.

What “purity” actually measures #

Purity refers to how much of a sample is genuinely the intended compound, as opposed to leftover synthesis byproducts, solvents, salts, or degradation products picked up during manufacturing or storage. It’s typically expressed as a percentage — for example, a peptide reported at 98% purity means 98% of the measured sample matches the target compound, with the remaining 2% made up of other substances.

Purity is a measure of chemical composition, not a measure of safety, dosing suitability, or biological activity. A high purity figure confirms what’s in the vial matches the label closely — it doesn’t say anything about how a compound should or shouldn’t be used.

How purity is tested #

Two methods dominate peptide purity testing:

• HPLC (High-Performance Liquid Chromatography) — separates a sample into its individual components as they pass through a column, allowing each component to be measured separately. The target compound typically shows up as the largest peak, with impurities appearing as smaller peaks. Purity is calculated as the proportion of the total peak area belonging to the target compound.
• Mass Spectrometry (MS) — measures the mass-to-charge ratio of molecules in a sample, confirming that the compound’s molecular weight matches what’s expected. MS is often used alongside HPLC to confirm identity, not just purity.

Reputable Certificates of Analysis will name which method was used. A purity figure without a stated test method is much harder to trust, since different methods can produce meaningfully different readings for the same sample.

Why the testing lab matters as much as the number #

Two purity certificates can both say “99%” and mean very different things depending on who ran the test:

• Self-reported / in-house testing — carried out by the manufacturer or seller themselves, with no independent verification
• Independent third-party testing — carried out by a separate laboratory with no financial interest in the result

Independent testing is the standard most serious researchers rely on, since it removes the incentive for a supplier to report an inflated figure. When comparing products or suppliers, it’s worth checking not just the purity percentage but whether it was independently verified and by whom.

Common misunderstandings about purity #

• “Higher purity always means a better product.” Purity matters, but it’s one factor among several — proper storage, handling, and sourcing all affect a product’s actual quality and consistency over time. A high purity figure on day one doesn’t guarantee the same result if a product is mishandled afterward.
• “99% purity means it’s 99% effective.” Purity is a chemical measurement, not a measure of biological activity or effect. The two are related but not interchangeable.
• “All purity tests are equivalent.” As covered above, the test method and the testing lab’s independence both materially affect how much weight a given number deserves.

What to check when comparing purity figures #

When you’re reviewing a purity claim, a few quick checks go a long way:

• Is a specific test method named (HPLC, MS, or both)?
• Is the testing lab named, and does it appear to be independent of the seller?
• Does the purity figure correspond to a specific batch number that matches your product?
• Is the Certificate of Analysis dated reasonably close to when the product was manufactured or sold?

A supplier who can answer these questions clearly, and who provides a proper Certificate of Analysis as standard rather than on request, is generally a good sign of a well-run operation. For more on reading the full document, see our guide on how to read a Certificate of Analysis.