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What does 99% purity mean?

Written by [[CONTENT AUTHOR NAME]]Reviewed by [[SCIENTIFIC REVIEWER NAME]], [[REVIEWER CREDENTIALS]]Updated July 28, 2026Editorial policy

That the target accounted for 99% of the total peak area the detector recorded at the stated wavelength. It is a proportion of what was detected, not a proportion of the vial's mass — the vial also contains water and counter-ions the measurement cannot see.

A 99% specification is normal for a short synthetic peptide and says the peptide fraction is clean: minimal deletion sequences, truncations or oxidation products.

It is entirely consistent for a lot to be 99% pure and contain appreciably less peptide than its label weight. The two statements describe different things — one the composition of the peptide fraction, the other how much of the vial that fraction is. The second is the net peptide content.

A purity figure that is not on a certificate, with a method and a lot number attached, is a marketing claim rather than a result. We do not publish one.

Why higher advertised purity is not automatically better material

Above roughly 98%, differences between reported figures are often smaller than the variation introduced by the method itself. Integration of a chromatogram involves choices — where the baseline sits, whether a shoulder is resolved as a separate peak — and two competent analysts can produce results a few tenths of a percent apart from the same sample.

This has a practical consequence. A supplier advertising 99.9% against another's 99.1% is not necessarily selling better material; they may be measuring at a less demanding wavelength, integrating more generously, or quoting a number that was never on a certificate at all. The comparison only means something if both figures came from a lot-specific certificate using a stated method.

The parameter that more often distinguishes two lots in a way that matters is net peptide content, because it varies over a much wider range and is far less commonly reported. A certificate showing 98% purity with net content stated tells you more than one showing 99.9% with net content omitted.