98% or 99+% purity: what actually changes
Read as impurity fraction the gap is a halving. What sits in that fraction, why 99+% costs more to reach, and when it matters enough to pay for.
Most UK research peptide suppliers specify 98% purity. Alphex specifies 99+%. One percentage point looks like marketing. Expressed as impurity it is a halving, and the impurities in a synthetic peptide are not inert filler: they are near-identical peptides that can interact with the same assay. This note explains what changes between 98% and 99+%, why it costs more to reach, and when it matters enough to pay for. All Alphex products are for laboratory and in-vitro research only.
This note is the short answer to one question. The full explanation of what an HPLC purity figure measures, and what it leaves out, is in what 99+% HPLC-verified purity actually tells you.
Purity is a ratio; impurity is what you should read
HPLC purity is the main peak's share of total peak area at 220 nm. 98% purity means the other 2% of peptide-like material is something else; 99+% means under 1%. Read as impurity fraction, the gap is 2% versus under 1%, a factor of two or more. If your assay is sensitive to a competing peptide at a tenth of the target's concentration, the difference between 2% and 0.8% is the difference between an artefact and a clean result.
What is in the impurity fraction
For a solid-phase-synthesised peptide the impurities are predictable, and they are all peptides:
- Deletion sequences, missing one residue where a coupling failed. Same length minus one, similar retention time, similar chemistry.
- Truncated sequences, where chain growth stopped early or capping failed to remove them.
- Protecting-group remnants, where a side-chain protecting group was incompletely removed at cleavage.
- Oxidised or deamidated forms of the target, generated during synthesis, cleavage or work-up.
- Diastereomers from racemisation during coupling, especially at histidine and cysteine.
Every one of these absorbs at 220 nm, every one is structurally close to the target, and several of them will bind the same receptor or enzyme, sometimes as agonists, sometimes as antagonists. That is why halving the impurity fraction is a real reduction in noise. See common peptide impurities.
Why 99+% costs more
Synthesis yield falls with every coupling. A 15-residue peptide with 99% efficiency per step gives about 86% crude target; the rest is the impurity family above. Reaching 98% purity from that crude needs one preparative HPLC pass. Reaching 99+% typically needs a second pass, or a much slower gradient on the first, and each pass discards material in the shoulders of the main peak. The extra purification is a real cost per milligram, and it rises steeply with peptide length: for a 39-residue lipidated peptide such as GLP-3 (RT), 99+% is genuinely difficult. A supplier that specifies 98% has made a legitimate commercial choice; the buyer should know what it means.
When it matters
- Dose-response and potency work. A 2% contaminant that is itself active shifts curves.
- Receptor and enzyme assays where deletion sequences can bind.
- Any work comparing batches or suppliers. Purity differences confound the comparison.
- Reproducibility across a project. Tighter specification means less batch-to-batch variation in the impurity profile.
Where it matters less: purely qualitative or screening work, and preliminary experiments that will be repeated with characterised material. Even then, knowing the number is better than not.
Reading the number correctly
A stated purity is only meaningful with the chromatogram, the detection wavelength, and the batch number. "99%" typed into a table is a claim; a trace with a labelled main peak at 99.2% area is a measurement. Alphex COAs show the trace, the wavelength (220 nm), the reference standard, the batch number and the issuing laboratory. How to read one is in what 99+% HPLC purity actually tells you and how to read a research peptide COA.
Sources and further reading
- ICH Quality Guidelines (Q1A stability, Q3A/Q3B impurities, Q6 specifications): the reference framework for purity, impurity and stability testing
Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.
Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.
Published 29 June 2026, last reviewed 16 August 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.