Peptide Purity, Testing & Stability

Published August 21, 2026 · Artemis Labs

Research Buyer FAQ › Peptide Purity, Testing & Stability

Purity, net peptide content and shelf life are three different numbers, and vendors often print only the first. This page defines each one in plain words: what a 99% figure measures, why the powder weighs more than the peptide, and how long a sealed dry vial keeps. All material is supplied for laboratory research use only.

How this page was built. Every answer here is about material in a sealed, unopened vial. Nothing on this page covers what happens after a vial is opened. Sources are listed under References.

What does 99% purity actually mean?

It means that in one HPLC run, 99% of the total peak area came from one peak. That is the whole claim. HPLC pushes the sample through a column, and things come off at different times. The instrument draws a graph. Purity is the area under the main peak divided by the area under every peak.

Two limits follow from that. First, it is an area figure, not a weight figure — the number describes the peaks the method could see, not the whole contents of the vial. Second, it is method-dependent. Yoshida and colleagues showed in 2025 that two different column chemistries separate different impurities. A single method can therefore leave some impurities hidden under the main peak (PMID 39922152). A purity figure with no method printed beside it is half a number.

Is 98% purity good enough?

For most laboratory work, yes — and the honest answer is that it depends on the work, not on the number. The gap between 98% and 99% is one part in a hundred. Whether that matters depends on what the other one or two parts are, which is a question about the impurity profile rather than the headline figure.

Compare two vendors instead. One prints 98% with a method, a date, a lot number and a chromatogram. The other prints 99% with none of those. The first number is checkable. The second is a marketing claim wearing a decimal point.

What is HPLC purity testing?

HPLC — high-performance liquid chromatography — separates a mixture by pushing it through a packed column under pressure, so each component leaves at its own time and can be measured. For peptides it is the standard way of putting a number on purity. The full walk-through, including how to read the output, is our explainer on HPLC purity testing explained.

What is the difference between purity and net peptide content?

Purity is about the peptide. Net peptide content is about the powder. Purity asks: of the peptide-like material in this sample, how much is the target sequence? Net peptide content asks a blunter question: of the total weight in this vial, how much is peptide at all?

They can be far apart. A sample can be 99% pure and still carry weight that HPLC was never looking at. Two things account for most of it: counterion salt left over from purification, and water the dry powder pulled in. Those are not impurities in the chromatography sense. They are simply not peptide. The two figures come from two different tests, and a COA that reports one has not reported the other. Our per-compound documentation is linked from each product page, including BPC-157 purity and Certificate of Analysis.

What is net peptide content?

Net peptide content is the share of the powder’s weight that is actually peptide. It is measured separately from purity, usually by amino acid analysis or by a nitrogen determination. The remainder is counterion, bound water, and any residual salt. If a Certificate of Analysis does not name a net peptide content test, that test was not run — the purity percentage does not stand in for it.

Why does the measured mg differ from the mg on the label?

Because the label figure is a nominal fill and the measured figure is a measurement, and three separate things sit between them. The first is fill tolerance: filling equipment aims at a target and lands near it. The second is net peptide content — the powder weighs more than the peptide it contains. The third is moisture, because lyophilised material draws in water.

None of this is unique to peptides; it is ordinary for freeze-dried solids. What matters is whether a vendor tells you which figure is which. The milligram number on a vial label describes the contents of that sealed vial and nothing else. You can see how it is reported per compound on a product page such as BPC-157.

What is in the other 1%?

Mostly close relatives of the peptide itself. Solid-phase synthesis builds a chain one residue at a time, and a small share of chains do not finish correctly. The usual results are truncated sequences that stopped short, deletion sequences missing one residue, and side products from incomplete removal of protecting groups. Oxidised forms show up too, particularly where the sequence contains methionine or cysteine.

There is a second, quieter category: things the method did not resolve. Impurities that come off the column at nearly the same time as the target sit under the main peak and get counted as target. That is the finding behind the Yoshida 2025 work on running two different separation chemistries against the same sample (PMID 39922152).

Why does purity vary so much between suppliers?

Because purification costs money and measurement is optional. Crude synthetic peptide is not 99% anything; it gets there through purification passes, and each pass costs yield and time. A vendor selling at half the market price has usually bought fewer of those passes.

Measurement adds a second layer of variation. Two labs running different methods on the same batch can report different numbers, both honestly. That is why a bare percentage from an unnamed lab is not comparable to a percentage from a named lab with the method printed. The number is only as good as its provenance.

What does “research grade” mean?

It means the material is sold for laboratory research use, and that is all it means. “Research grade” is not a regulated grade, not a certification, and not a threshold anyone administers. No agency defines it. It is a statement about intended use, not about quality.

Because the phrase carries no fixed content, it should never be the reason you pick a supplier. What carries content is the COA: named lab, named methods, lot number, date, and the traces themselves.

What is the difference between RUO and GMP?

RUO describes what material is sold for. GMP describes how a facility is run. Research use only is an intended-use label. Good Manufacturing Practice is a formal quality system covering facilities, records, validation and audits, and it exists because regulators require it of drug manufacturers.

They are not two rungs on one ladder — they answer different questions. Artemis Labs supplies research-use-only material and does not describe it as a pharmaceutical product of any kind. The regulatory background is set out in research-use-only status and FDA regulation.

What is TFA in peptides, and does it matter?

TFA is trifluoroacetic acid, the acid most commonly used during peptide purification. Because it is present at that step, most synthetic peptides finish as TFA salts — the acid stays behind as a counterion paired with the peptide. That is standard practice across the industry, not a defect.

It matters in two ways. First, TFA adds weight that is not peptide, which is part of why net peptide content is lower than purity. Second, residual TFA can affect sensitive cell-based work, so some laboratories specify a different counterion such as acetate when the application calls for it. If a COA does not mention counterion, the material should be assumed to be a TFA salt.

What does lyophilised mean?

Lyophilisation is freeze-drying: the material is frozen and the water is drawn off under vacuum, leaving a dry solid. It is the standard form for shipping and storing peptides because a dry solid changes far more slowly than a solution. The full definition sits in our glossary entry for lyophilised.

Do peptides need to be refrigerated?

A sealed, dry vial is far more forgiving than most buyers expect, and cold and dark is still the right default. The chemistry behind that is simple. The reactions that degrade peptides — chain cleavage, deamidation of asparagine and glutamine, oxidation — mostly need water and are faster when warm. Freeze-drying removes the water; cold storage slows what is left.

Manning and colleagues reviewed those degradation routes across peptide and protein products and describe temperature and residual moisture as the two dominant variables (PMID 20143256). Lawson and colleagues later modelled the deamidation step in detail (PMID 36549007). The practical reading for a sealed vial, such as the GHK-Cu lyophilised vial: keep it dry, keep it dark, keep it cold, and keep it sealed until it is used.

How long do peptides last before they go bad?

A sealed, lyophilised vial kept frozen is a multi-year item; the same vial kept warm and damp is not. Shelf life is set by storage conditions rather than by the calendar, which is why a single number quoted with no conditions attached tells you very little.

Two things drive the timeline. Moisture is the first. Water is a reactant in the main degradation routes. A vial that has been opened, or one whose seal has failed, is on a much shorter clock. Temperature is the second, and its effect compounds over months rather than days. The Manning review sets out the underlying chemistry for peptide and protein products generally (PMID 20143256). Where a manufacturer states a retest date, that date is the figure to work from, not a rule of thumb from a forum.

My peptides arrived warm with no ice — are they ruined?

Almost certainly not, and here is the reasoning rather than a reassurance. A sealed lyophilised powder is not a cold-chain item in the way a solution is. It has already had its water removed, which is what makes short periods at room temperature a much smaller event than the same exposure would be for a liquid.

Three things are worth checking on arrival:

  • The vial is sealed and the stopper is seated.
  • The cake inside looks like a dry solid, not a syrup or a puddle.
  • The lot code matches the Certificate of Analysis.

A vial that arrives visibly melted, wet, or unsealed is a different situation from one that simply arrived warm. Transit conditions and what happens if a vial arrives damaged are covered under shipping temperature and vial condition on arrival.

Why is there so little powder in my vial?

Because ten milligrams of freeze-dried peptide is genuinely a very small amount of solid — often a thin film or a light cake rather than a visible heap. Freeze-drying leaves an airy, low-density structure, so the volume you see has almost no relationship to the mass on the label.

The figure printed on the vial is the mass of peptide sealed inside it. It is not a volume, and it is not a scoop size. A vial that looks nearly empty is the normal case at the smaller fill weights. A vial that looks generously full is more often a sign of added bulking material than of a better deal.

This page is one section of the full Artemis Labs research-buyer FAQ.

References

  1. Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. “Stability of protein pharmaceuticals: an update.” Pharm Res. 2010;27(4):544–575. PMID 20143256 · doi:10.1007/s11095-009-0045-6
  2. Lawson KE, et al. “Computing the Differences between Asn-X and Gln-X Deamidation and Their Impact on Pharmaceutical and Physiological Proteins.” J Phys Chem A. 2023;127(1):57–70. PMID 36549007 · doi:10.1021/acs.jpca.2c06511
  3. Yoshida K, et al. “Impurity profiling of synthetic cyclic peptides based on orthogonality between hydrophilic-interaction and reversed-phase liquid chromatography.” J Chromatogr A. 2025;1745:465748. PMID 39922152 · doi:10.1016/j.chroma.2025.465748