Purity 98 or 99 percent: what the number on a peptide certificate does and does not tell you

Written 23 September 2026 by the trade desk. About 1 minute to read.

Purity is the figure everyone quotes and few explain. It is a real measurement with a specific meaning, and it is narrower than it sounds.

What is being measured

The sample is run through a reverse-phase HPLC column and the peaks are recorded at 220 nm. Purity is the area of the main peak as a share of the area of all the peaks. Our release limit is not less than 98.0 percent, and any single other peak must be below 1.0 percent.

What the rest is

Related substances: sequences missing a residue, sequences with an extra one, oxidised or deamidated forms. They are structurally close to the target, which is why they come off the column near it. A 99 percent result means those together are one percent of the peak area.

What the number is not

It is not the weight of peptide in the vial. The dry cake also contains water, up to 8 percent by our limit, and the acetate counter-ion. If you need to know how much peptide by weight is in a vial, that is net peptide content, and it is a separate figure to ask for.

Why identity comes first

A pure peak of the wrong molecule is still the wrong molecule. Identity is confirmed by retention time against a reference standard and by mass spectrometry, which checks the measured mass against the calculated one. Purity without identity is a number without a subject.

98 against 99 in practice

Both are inside our limit. The difference is one percentage point of related substances, which for most research use is immaterial next to whether the identity, water and content lines pass. Treat a certificate as a whole rather than as one number.

Applies to

Semaglutide, BPC-157, among others. Every compound.

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