Key Takeaways
- •Coupling efficiency compounds. At 99% per step, crude purity falls from about 98% on a tripeptide to about 65% on a 44-mer, purely from chain length.
- •The dominant impurity is a deletion sequence: the target peptide missing one residue. It is nearly identical to the target, which is exactly why it is hard to separate.
- •Purification is subtraction, and it is imperfect. A high final purity on a long chain means more was removed, not that less went wrong.
- •Peptides purified by reversed-phase HPLC come out as trifluoroacetate salts. TFA is not an incidental trace, it is the counterion paired to every basic site in the molecule.
- •TFA has measurable biological activity of its own. Published work found trifluoroacetate at 10⁻⁸ to 10⁻⁷ M reduced cell numbers and thymidine incorporation in osteoblast and chondrocyte cultures.
- •Purity, water and counterion are three separate numbers, and only the first is usually published.
KPV is three amino acids long. Tesamorelin is forty-four. Both can be sold with a certificate reading ">99% by HPLC", and the two documents describe manufacturing achievements that are not remotely comparable.
At a per-coupling efficiency of 99%, which is a good number, a tripeptide comes off the resin around 98% correct before any purification at all. A 44-residue chain built at the same efficiency comes off at roughly 65%. Everything between that figure and the number on the certificate had to be removed by purification, and the material removed is chemically almost identical to the material kept.
That is the part of a peptide's story that no certificate shows: not what survived, but how much had to be taken away, and what the removal could not catch. This article covers where synthetic peptide impurities come from, why chain length changes the meaning of a purity claim, and the counterion that sits in the vial and appears on almost no retail certificate.
Volta's catalogue runs from KPV at three residues to tesamorelin at forty-four, at a single stated purity standard across the range, so the argument below applies to our own documentation as much as anyone's. All material is supplied for laboratory research use only.
How the chain is actually built
Solid-phase peptide synthesis anchors the first residue to an insoluble resin bead and adds the rest one at a time, working from the C-terminus back. Each cycle is the same three moves: remove the protecting group from the growing chain's free end, couple the next protected residue, wash away the excess.
The reason it works is that the product stays bolted to the bead, so every reagent and by-product can be rinsed off between steps. The reason it is imperfect is that each coupling is a chemical reaction with a yield below 100%, and the chains that fail a step do not leave. They stay on the bead and carry on to the next cycle.
Two things are done about that. Difficult sequences get help: pseudoproline dipeptides, alternative side-chain protection, additives, reduced resin loading on long chains. And after each coupling, unreacted chains are frequently capped, usually with acetic anhydride, which permanently blocks them so they stop growing. Capping does not raise the yield. It converts a chain that would have become a deletion sequence, missing one internal residue, into a short truncated fragment that is much easier to separate later. It is a trade of a hard purification problem for an easy one.
Why length changes what a purity number means
Crude purity before any purification is roughly the per-coupling efficiency raised to the power of the number of couplings. One residue means no couplings; n residues means n − 1 of them.
| Compound | Residues | Couplings | Crude ceiling at 99%/step | At 98%/step |
|---|---|---|---|---|
| KPV | 3 | 2 | 98.0% | 96.0% |
| GHK-Cu | 3 | 2 | 98.0% | 96.0% |
| Ipamorelin | 5 | 4 | 96.1% | 92.2% |
| TB-500 | 7 | 6 | 94.1% | 88.6% |
| Melanotan II | 7 | 6 | 94.1% | 88.6% |
| BPC-157 | 15 | 14 | 86.9% | 75.4% |
| MOTS-c | 16 | 15 | 86.0% | 73.9% |
| Sermorelin | 29 | 28 | 75.5% | 56.8% |
| Semaglutide | 31 | 30 | 74.0% | 54.5% |
| Tirzepatide | 39 | 38 | 68.3% | 46.4% |
| Retatrutide | 39 | 38 | 68.3% | 46.4% |
| Tesamorelin | 44 | 43 | 64.9% | 41.9% |
These are ceilings, not predictions. Real syntheses vary with sequence, and a difficult stretch, an aggregating region or a sterically awkward residue can drop a single step well below the nominal figure. Modifications add further steps: semaglutide, tirzepatide and retatrutide all carry fatty-acid chains attached through linkers, and retatrutide and tirzepatide both include non-standard residues.
The reading that matters: the same certificate number on a 3-mer and a 44-mer represents very different amounts of purification. A ">99%" tripeptide barely needed any. A ">99%" 44-mer required removing roughly a third of the crude material, selectively, from a mixture whose components differ from the target by as little as one residue.
Which is why an identical purity figure across an entire catalogue, at identical pricing tiers, is a question worth asking rather than a reassurance. It is achievable. It is just much more expensive at one end of the catalogue than the other.
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What the other few percent actually is
Not dust, and not a foreign compound. Almost all of it is closely related to the target, which is the entire difficulty.
| Impurity | Origin | Why it is hard to remove |
|---|---|---|
| Deletion sequence | A coupling failed; the chain continued without that residue | Differs from the target by one residue out of many. On a long peptide the two can co-elute |
| Truncated sequence | A chain was capped and stopped early | Much shorter, so usually well separated. This is what capping buys you |
| Insertion sequence | A residue coupled twice | Same problem as a deletion, in the other direction |
| Racemised residue | A stereocentre inverted during activation | Identical mass, near-identical retention. Effectively invisible to routine analysis |
| Incomplete deprotection | A side-chain protecting group survived cleavage | Mass differs by a known amount, so it is detectable when looked for |
| Oxidation | Methionine, cysteine or tryptophan oxidised in air | Mass shift of +16 per oxygen. Detectable, and often a storage problem rather than a synthesis one |
Notice which of these a mass spectrometer catches. Deletions, truncations, insertions, retained protecting groups and oxidation all shift the mass, so an MS trace can see them. Racemisation does not shift the mass at all, and a deletion on a long chain may sit under the main peak in the chromatogram. The impurity that a purity figure is least able to describe is the one closest to the target.
The counterion nobody mentions
Reversed-phase HPLC, the standard purification for synthetic peptides, runs with trifluoroacetic acid in the mobile phase. TFA is what makes the separation work: it ion-pairs with the peptide's basic sites and sharpens the peaks.
It also comes home with the product. Peptides purified this way are isolated as trifluoroacetate salts, with roughly one TFA molecule paired to each cationic site: every lysine, every arginine, every histidine, plus the free N-terminus where there is one. A lysine-rich sequence therefore carries proportionally more of it. TFA pairing is persistent; removing it takes a dedicated exchange step, typically repeated lyophilisation against hydrochloric acid or an ion-exchange operation, and that step costs money and yield.
Two consequences.
It occupies mass. A vial's contents are peptide plus water plus counterion plus residual salts and solvents. The counterion is part of what the balance weighed. A purity percentage is a ratio within the peptide fraction and says nothing about how large that fraction is, which is the separate question of net peptide content.
It is not biologically inert. This is the part that matters for cell work and is almost never mentioned on a product page. Published work found trifluoroacetate at 10⁻⁸ to 10⁻⁷ M reduced cell numbers and thymidine incorporation in fetal rat osteoblast cultures within 24 hours, with comparable effects in articular chondrocyte cultures and neonatal mouse calvariae. Counterion identity has also been shown to affect the measured activity and cytotoxicity of antimicrobial peptides.
So a sensitive cell-based assay can be reading the counterion rather than the peptide. If a result depends on a modest effect size in culture, the salt form is a variable in the experiment, and "we used the same peptide" is not the same as "we used the same salt form". The solvent compatibility tool and the salt form converter cover the arithmetic of switching between them.
The four numbers that would actually describe a vial
| Number | Answers | Typical availability |
|---|---|---|
| HPLC purity, area percent | How much of the peptide fraction is the target | Almost always published |
| Water content (Karl Fischer) | How much of the mass is water | Rarely published |
| Counterion content | How much of the mass is TFA or acetate | Rarely published |
| Net peptide content | How much target peptide the vial actually holds | Rarely published |
The first without the others is the least informative of the four, and it is the one the market has standardised on. The net peptide content calculator works the arithmetic when the inputs exist, and the COA explainer walks a certificate field by field.
Volta publishes HPLC purity and mass-spectrometry identity per lot, and states a >99% purity standard. We do not currently publish water content or counterion content, which means the net peptide content of our vials is not something a reader can compute from what we give them. That is a real gap, it is shared by essentially every retail supplier in this market, and naming it is more use than a badge.
What to ask a supplier
Four questions, in order of how much they reveal:
- What salt form is this, and was a counterion exchange performed? A supplier who knows the answer is a supplier whose material has a specification.
- What is the water content? Lyophilised peptides are hygroscopic. Water is frequently several percent of the mass.
- What is the net peptide content, or the inputs to compute it? This is the number that makes price per milligram meaningful.
- Was the purity determined on this lot? A method and a date, tied to the lot on the vial.
If a compound is long, add a fifth: what does the impurity profile look like? On a 40-residue chain the answer is genuinely interesting, and a supplier who has the chromatogram is in a different category from one who has a number.
For the documentation side, how to tell if a peptide supplier is legitimate covers traceability, and what the human evidence actually shows covers the separate question of whether a compound is worth studying at all.
Frequently Asked Questions
Why is a longer peptide harder to make pure?
Because errors compound. Every coupling has a yield below 100%, and a chain that fails one step continues through the rest of the synthesis as a slightly wrong molecule. At 99% per coupling, a three-residue peptide is about 98% correct before purification and a 44-residue peptide about 65%. The longer chain needs far more purification to reach the same final number.
What is a deletion sequence and why does it matter?
It is the target peptide missing one internal residue, produced when a coupling fails and the chain carries on. It matters because it is the impurity most similar to the product: on a long peptide it can elute at almost the same time and sit partly under the main peak, so it is both the most likely impurity and the hardest one for a purity measurement to see.
Does TFA in a peptide vial matter?
For cell-based work it can. Peptides purified by reversed-phase HPLC are isolated as trifluoroacetate salts, and trifluoroacetate has documented effects on cultured cells at low concentrations, including reduced proliferation in osteoblast and chondrocyte cultures. For work where a modest effect size in culture is the readout, salt form is an experimental variable rather than a packaging detail.
Can I tell the salt form from the certificate?
Usually not, because most retail certificates do not state it. If it is unstated, the default assumption for a reversed-phase purified peptide is a trifluoroacetate salt, since that is what the standard process produces. Acetate salts exist and are usually the result of a deliberate, separately paid-for exchange step, which a supplier who performed it has every reason to advertise.
Does higher purity mean more peptide in the vial?
No, and this is the most valuable distinction in the article. Purity is a ratio within the peptide fraction. Net peptide content is how large that fraction is once water, counterion and salts are subtracted. A vial can be 99% pure and still contain substantially less peptide than the label states, because purity does not describe the water and the counterion at all.
Is capping a good thing or a sign of a poor synthesis?
A good thing, and standard practice. Capping blocks chains that failed a coupling so they stop growing, converting a hard-to-remove deletion sequence into an easy-to-remove short fragment. It does not improve the yield of correct product; it makes the impurities that do form far easier to separate out.
