A university lab manager is pricing a custom sequence for a method-development project. The quote comes back with three lines: crude, desalted, and >95%. The crude line is a fraction of the price of the third, and desalted sits in between. It is natural to read the three as points on one scale, like economy, standard and premium. They are not. Two of those words describe what was done to the material. Only the third is a statement about purity, and even that statement is incomplete until a method is attached to it.
Where the mixture comes from
Solid-phase synthesis builds a peptide one residue at a time on a resin. At the end, a strong acid cocktail cuts the chain from the resin and strips the side-chain protecting groups in the same step. What comes out is not one compound. It is a population: the target sequence, plus deletion sequences from couplings that did not go to completion, truncated chains, species that kept a protecting group, oxidized forms, scavenger adducts and residual cleavage acid.
How much of that population is the target depends heavily on sequence length and difficulty. A short, well-behaved sequence can come off the resin mostly as product. A long or aggregation-prone one may come off with the target as a minority component. The common impurity families are described in peptide synthesis impurities.
Three labels, three different kinds of statement
| Label | What was done | What it tells you about purity |
|---|---|---|
| Crude | Cleaved, precipitated and dried; no chromatographic step | Nothing on its own. Two crude peptides can differ enormously in real purity. |
| Desalted | Crude passed through a short column or solid-phase extraction cartridge to remove salts, scavengers and small molecules | Very little. Peptide-related impurities pass through with the target. |
| >95%, >98% | Preparative reverse-phase chromatography, fraction pooling and lyophilization | A purity claim, meaningful only with the analytical method stated |
The key point about desalting is what it can and cannot separate. It works on size and coarse polarity. Small non-peptide species such as scavengers and salts are removed, which makes the solid cleaner to weigh and less likely to interfere with downstream measurements. But a deletion sequence missing one residue looks almost identical to the target by those criteria, so it stays. The area-percent figure after desalting often looks much the same as before.
A crude label, likewise, should not stand alone. If crude material is sold without any measured value, the word only tells you which step was skipped.
Why each extra point of purity costs more
Buyers are sometimes surprised that the jump from >95% to >98% is priced out of proportion to three percentage points. The reason lies in the separation itself.
In preparative chromatography, the impurities hardest to remove are the ones that elute closest to the target: a single-residue deletion, an oxidized form, a diastereomer. Their peaks overlap the edges of the main peak. To push purity higher, the chemist narrows the collection window and discards fractions from the flanks of the peak, which contain product mixed with those close neighbors. Wider windows give more yield and lower purity; narrower windows give less yield and higher purity.
So a higher grade usually means more of the synthesized material went to waste, a shallower gradient, or a second purification on a different separation principle. Instrument time is part of the cost, but the main driver is how much product was sacrificed to reach the number.
A purity number always belongs to a method
>95% means 95% as measured by a specific analytical method: a column, a gradient, a detection wavelength and a run time. The same lot can return different numbers on different methods, and none of them need be wrong. A steep gradient can hide close-eluting species under the main peak; a detection wavelength that suits one sequence can under-count impurities in another. The mechanics are explained in reverse-phase HPLC and peptide purity.
A grade quoted without any method is therefore a claim about a number rather than a report of a measurement. When you compare quotes, compare the methods behind the grades as carefully as the grades themselves.
What none of the three grades covers
All three labels concern chromatographic purity: the share of UV-absorbing, eluting material that is the target. Several properties sit outside that measurement entirely:
- Peptide content by mass. A lyophilized solid also contains counter-ions and water. Material that is 98% pure by HPLC can still be well under 100% peptide by weight. See purity versus net peptide content.
- Stereochemistry. A residue of the wrong configuration may co-elute with the target and has the same mass, so neither a standard HPLC run nor a mass check will flag it.
- Co-elution in general. Anything hidden under the main peak is counted as main peak.
- Non-chromatographic contaminants. Elemental impurities, residual solvents and biological contaminants are separate questions requiring separate tests.
Matching the grade to the work
Higher is not automatically better value. The right question is what the planned experiment is sensitive to.
- Reference comparisons and quantitative analytical work are directly affected by impurities, so a purified grade with a stated method earns its price.
- Method development, scouting runs and uses where the peptide is a reagent rather than the object of study can often proceed on crude or desalted material, provided a measured value accompanies it.
- Any grade should come with documentation that says which of the three categories it belongs to and, for a purified grade, the measured value and the method that produced it.
Catalog research materials are a different situation from custom synthesis quotes, because the purification decision has already been made. At Battle Born, each product is analyzed by independent reverse-phase HPLC and the result is published for that product, so the figure you read is a measurement rather than a grade label. Our guide on comparing research peptide prices explains how to weigh that figure against cost.
Questions
Is desalted peptide purer than crude?
Cleaner, yes, because salts and small-molecule residues are removed. Purer in the chromatographic sense, usually only slightly, because deletion and truncation sequences pass through desalting with the target.
Does >98% mean the vial is 98% peptide by weight?
No. It is a chromatographic area figure. Counter-ions and water add weight without appearing in the purity calculation.
Why do two suppliers quote the same grade at very different prices?
Sequence difficulty, synthesis scale, how much material was discarded to reach the grade, and the rigor of the analytical method all differ. A grade without a method is also not comparable with a grade that has one.
Is crude material ever the right choice?
For scouting, method development or reagent use it can be, as long as a measured purity is supplied and the experiment is not sensitive to peptide-related impurities.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.