Peptide Purity Grades Explained: Crude, Desalted and HPLC-Purified

Published September 2026 · Patriot Labs Research Team · ~18 min read

The short answer

A peptide purity grade describes how much of the peptide in a sample is the intended target sequence, measured by HPLC as the area of the target peak divided by the total area of all peaks. Grades run from crude, which is unpurified and often below 70%, through desalted, to purified specifications quoted as greater than 95% or greater than 98%.

The word that does the quiet work in that sentence is peptide. A purity grade is a statement about the peptide fraction of the sample only: of all the peptide-like material present, what share is the molecule you asked for. It says nothing about how much of the powder in the vial is peptide at all, because water and counterions — which can be a substantial part of the mass — are simply not counted. That second question is answered by a different number, net peptide content, and confusing the two is the single most common mistake made when reading a peptide specification.

The rest of this guide takes the topic in order: what the HPLC purity number is and how it is defined, the ladder of grades from crude to high-purity and what purification step separates each rung, why purity and net peptide content answer different questions, what the impurities behind the missing percent actually are, the analytical methods that generate the figures, and the real limits of a single percentage. Throughout, the theme is the same one that runs through COA testing and how to buy research peptides: a number is only as useful as the method and the documentation behind it.

What a purity grade actually measures

A peptide purity grade is a single figure from analytical high-performance liquid chromatography: the area of the target peptide’s peak as a percentage of the combined area of every peak detected in the run. In the words of one manufacturer’s quality-control guide, “purity is typically assessed by UV detection at 210–220 nm, where the area of the main peak in relation to the total area of all peaks reflects the peptide purity.”^[1]

Three details in that definition matter. The first is the detection wavelength. Peptides are separated on a reversed-phase column and detected by their absorbance of ultraviolet light, and the measurement is usually made near 210 to 220 nm because that is where the amide (peptide) bond of the backbone absorbs.^[1][2] A wavelength that low responds to the peptide backbone itself rather than to a particular side chain, so it detects fragments and by-products that a longer wavelength might miss. Detection at 214 nm is a common industry choice for exactly this reason.^[2]

The second detail is that the figure is an area-normalised percentage — every peak in the chromatogram is added up, and the target’s share of that total is the purity. This makes the number relative: it describes the composition of what the detector saw, not an absolute mass. Anything that does not absorb ultraviolet light at the chosen wavelength, and anything that never comes off the column, is invisible to the calculation. In a regulated setting the related figure called assay is instead measured by comparison against a reference standard, and results are expressed on an anhydrous, counter-ion-free basis so that water and salt do not distort them.^[3]

The third detail is that purity is a measure of how many things are present and in what proportion, not what they are. HPLC separates a mixture into peaks and measures their relative size; it does not, on its own, identify any of them. That is why a purity result is always paired with an identity method — mass spectrometry — that confirms the main peak really is the intended peptide and not a similar-sized impurity sitting where the target should be.^[2][4] Purity and identity are two questions, and one number cannot answer both.

The purity ladder: crude, desalted and purified

The named grades on a catalogue — crude, desalted, and the purified specifications above 90% — correspond to how much chromatographic purification the material received after synthesis. Each rung of the ladder is defined by a processing step, not just by a target number.

Crude. Crude peptide is the mixture that comes directly off solid-phase synthesis after the chain is cleaved from the resin and precipitated, with no purification to separate the target from its by-products. Solid-phase peptide synthesis, introduced by R. B. Merrifield in 1963, builds a chain one protected amino acid at a time on a resin bead, and no coupling is perfectly efficient, so the crude product is always a mixture of the full-length target and a family of closely related sequences.^[5] One large custom-synthesis supplier defines crude simply as purity below 70%.^[6] Crude material is the least expensive grade because the most costly step, preparative chromatography, has been skipped.

Desalted. Desalted grade is crude material that has been passed through a short chromatographic step to strip out synthesis salts, cleavage scavengers and other small-molecule residues, and often to exchange the counterion, without resolving the target sequence from the related peptides that accompany it. The key point, and a frequent source of confusion, is that desalting is a clean-up, not a purification by sequence: it removes non-peptide junk and can improve how a sample handles and dissolves, but it leaves the sequence-level purity close to where the crude started. A desalted peptide and a crude peptide of the same sequence can have similar HPLC purity numbers.

Purified grades. To raise the sequence purity, the crude is run through preparative reversed-phase HPLC, where the target is physically separated from the deletion and truncation sequences and collected. The result is quoted against a floor: greater than 90%, greater than 95%, greater than 98%, and higher still for pharmaceutical material. Greater than 95% is the most common specification for laboratory-grade custom peptides; as one manufacturer puts it, that means “95% of the net peptide content…is composed of your target peptide,” with the remaining few percent typically the deletion or addition sequences that are hardest to separate.^[7] Reaching a higher floor generally means a more aggressive or repeated purification, which lowers yield and raises cost, so the grade is a trade-off between purity, recovery and price rather than a free choice.

Two things are worth keeping in mind as the number climbs. First, the percentage is a floor, not a point value: “>95%” means at least 95%, and a lot may test higher. Second, the same target can be offered at several grades from the same synthesis; the grade reflects how far that particular batch was purified, which is why the grade and the lot-specific test result belong together.

Purity vs net peptide content

Purity and net peptide content are two different percentages that are constantly mistaken for one another: purity is the share of the peptide fraction that is the correct sequence, while net peptide content is the share of the whole powder that is peptide at all. A sample can be greater than 98% pure by HPLC and, at the same time, be well under 100% peptide by mass — the two numbers are not in competition, they describe different things.

Net peptide content, sometimes just called peptide content, is defined by one supplier’s guide as “the percentage of peptides relative to non-peptidic material (mostly counterions and moisture).”^[1] The non-peptide fraction is real and often substantial: a freeze-dried peptide carries bound counterions from purification — commonly trifluoroacetate or acetate — and it holds water, especially if the sequence is hydrophilic.^[1][8] Because that water and salt vary with the purification and the freeze-drying, net peptide content can differ from batch to batch even when the HPLC purity does not.^[8]

The cleanest way to hold the two ideas apart is a single relationship, stated plainly in one technical note: “the absolute amount of correct peptide in a sample is the product of the peptide content and the peptide purity.”^[9] Purity tells you what proportion of the peptide is right; content tells you how much of the powder is peptide; multiply them and you have the fraction of the powder that is actually the molecule you wanted. A material at 98% purity and 80% peptide content delivers roughly 78% of its mass as correct peptide — a figure neither number gives on its own.

This is also why regulated specifications are written the way they are. Assay and content for a peptide drug substance are reported on an anhydrous, counter-ion-free basis, precisely so that water and salt do not inflate or deflate the apparent amount of peptide;^[3] and a full specification treats water and counterions as their own measured attributes rather than folding them into purity.^[3] The counterion question — why trifluoroacetate or acetate is present and how it changes the mass — is covered in its own guide, TFA vs Acetate in Peptides.

What the missing percent is made of

The few percent that separate a high-purity peptide from a perfect one are mostly closely related peptides — molecules that are almost the target, produced as unavoidable by-products of synthesis. Understanding what they are explains both why purification stops where it does and why a purity number needs an identity method beside it.

The largest class is deletion and truncation sequences. Because each amino acid coupling in solid-phase synthesis is slightly less than perfect, a small fraction of chains miss a residue (a deletion sequence) or stop growing altogether (a truncation), and these accumulate over a long sequence.^[5][10] Couplings involving bulky or β-branched residues such as isoleucine, valine and threonine are especially prone to this, which is why some sequences are inherently harder to make pure than others.^[7] Deletion and addition sequences are also the impurities most likely to sit very close to the target on the column, which matters for the purity number itself.^[7]

Alongside these are several classes of chemical modification catalogued in reviews of peptide-medicine impurities:^[10][11]

• Oxidation of sulfur-containing residues (methionine to its sulfoxide and sulfone) and of aromatic residues such as tryptophan, tyrosine and histidine.
• Deamidation at asparagine and glutamine, which can also generate pyroglutamate at a chain terminus.
• Aspartimide formation, a well-known side reaction at aspartate residues — particularly aspartate–glycine motifs — during Fmoc synthesis, which can go on to open into several products including the racemised and iso-aspartate forms.^[10]
• Racemisation and epimerisation, producing stereoisomers with the same mass as the target but a different three-dimensional structure.
• Incomplete removal of protecting groups left over from synthesis.

Two features of this list have practical consequences. First, some of these impurities — a racemised residue, an aspartimide that has re-opened to the same formula, a deamidation that barely changes the mass — are nearly identical to the target in size, so they are hard to separate and sometimes hard to distinguish by mass alone. Second, this is why regulators set impurity thresholds by fraction rather than demanding perfection: pharmacopoeial and guidance frameworks for peptide medicines commonly call for individual impurities to be reported above about 0.1%, identified above roughly 0.5%, and qualified above about 1.0%, with tighter limits where an impurity carries an immunogenicity concern.^[3][11] Research-grade material is not held to those regulatory thresholds, which is one more reason its quality has to be read from its own lot-specific testing rather than assumed. How these same impurities can also grow after manufacture, during storage, is the subject of How Peptides Degrade.

How the number is generated

A purity grade is one output of a small battery of analytical methods, each answering a different question, and a complete specification uses several of them together because no single technique covers identity, purity, content and residuals at once.

Reversed-phase HPLC produces the purity figure itself, separating the mixture and reporting the target’s share of total peak area at 210 to 220 nm.^[1][2] Mass spectrometry confirms identity by measuring the molecular weight of the main component, establishing that the dominant peak is the intended peptide and not an impurity of similar retention.^[1][2][4] The two are increasingly run together as liquid chromatography–mass spectrometry, so that every peak in the purity trace can, in principle, be assigned a mass; combining ultraviolet detection with mass detection is now standard practice for profiling the impurities behind the purity number.^[4][11]

Amino acid analysis answers the content question. The peptide is hydrolysed with a strong acid into its constituent amino acids, which are then separated and quantified; comparing the amount of amino acid recovered against the sample weight gives the net peptide content that HPLC purity cannot.^[1][9] Karl Fischer titration measures residual water, and ion chromatography measures the bound counterion — the two largest parts of the non-peptide mass.^[1]

For material intended as a drug substance, standards bodies go further and ask for identity to be established by several orthogonal techniques rather than one. The United States Pharmacopeia’s general chapter on synthetic peptide drug substances calls for identification by multiple orthogonal methods — among them HPLC co-elution with a reference, mass spectrometry, amino acid analysis and, for longer sequences, peptide mapping — so that identity does not rest on a single measurement.^[3][12] Research-grade testing is lighter than this, but the logic is the same: purity, identity and content are separate properties, and a trustworthy picture needs a method for each.

What a purity percentage does not tell you

A purity percentage is genuinely useful, but it is a narrower measurement than it looks, and several of its blind spots are worth naming because they are the ones that mislead buyers most often.

It depends on the method. The purity number is not a fixed property of the material; it is a property of the material as seen by a particular HPLC method. The column, the gradient, the run time and the detection wavelength all change how completely impurities separate from the target and how much each one absorbs, so the same peptide can return slightly different area percentages on two different systems.^[2][13] Recent method work in the peptide-science literature underlines how sensitive purity and separation are to these choices and how carefully a method has to be transferred between instruments to stay comparable.^[13] A purity figure quoted without its method is missing half the information.

Co-elution can hide impurities. If an impurity comes off the column at the same time as the target, its area is counted as part of the main peak, and the reported purity is too high. The impurities most prone to this are precisely the deletion and addition sequences that most resemble the target.^[7] This is the central reason HPLC purity is paired with mass spectrometry, and why peak-purity checks and orthogonal methods exist: a single clean-looking peak is not proof of a single compound.^[4][12]

It is blind to anything it cannot see. Area-normalised purity counts only what absorbs ultraviolet light at the detection wavelength and actually elutes. A non-absorbing contaminant, or one that stays on the column, does not appear in the percentage at all — another reason water, salts and content are measured by separate methods rather than inferred from purity.^[1][3]

It is not identity, content or safety. Purity does not confirm that the main peak is the right molecule (that is mass spectrometry), does not state how much of the powder is peptide (that is net peptide content), and does not by itself speak to whether an impurity matters — regulators assess some peptide impurities specifically for immunogenicity risk rather than treating all impurities as equal.^[11][14] A high number is reassuring, but only in combination with the identity result, the content figure and the documentation that ties them to a specific lot.

Matching grade to the work

The right purity grade depends on what the material is for, not on chasing the highest number for its own sake — though when in doubt, a higher grade carries less risk that a related impurity confounds a result. As a general principle, the more sensitive the measurement and the more a stray peptide could distort it, the higher the grade the work warrants.

At the lower end, crude and desalted material is inexpensive and can be adequate for undemanding screening, for method development, or where the target is going to be identified and separated analytically anyway. Higher-purity grades earn their cost where a closely related impurity would be a problem: quantitative and structure-sensitive work, or studies where an impurity’s own behaviour could be read as the target’s. One supplier’s guidance captures the upper end of that logic by recommending its highest grade, greater than 98%, for its most demanding research applications.^[6] The practical rule is to treat purity as one input to a fitness-for-purpose decision alongside identity and content, rather than as a single quality score.

Whatever grade is chosen, the material remains research material. A research compound that shares its name and sequence with an approved medicine is not that medicine, has not been made or tested to the same standard, and a purity grade — however high — does not change that. The distinction between a research-grade and a pharmaceutical-grade peptide is drawn out in How Peptides Are Made, and the regulatory categories in Peptide vs Drug vs Biologic.

Reading purity on a certificate of analysis

On a certificate of analysis, a purity figure is only as meaningful as the context printed next to it: the method, the lot it applies to, and the laboratory that ran it. A bare “98%” with none of that attached is a marketing claim; a purity result tied to a lot number, a stated HPLC method and an accompanying mass-spectrometry identity is a checkable fact.

A useful COA therefore shows more than one number. It reports purity by HPLC and identity by mass spectrometry, because the first counts peaks and the second names them.^[2][4] It states the lot, so the document can be matched to the physical material. Ideally it names the testing laboratory and whether that laboratory is independent of the seller. And where net peptide content, water or counterion are reported, it becomes possible to reason about how much of the powder is actually peptide rather than salt and moisture.^[1][9] What each of these entries proves, and how to spot a COA that proves less than it appears to, is the subject of Understanding Peptide COA Testing.

For reference on what independent documentation looks like in practice: Patriot Labs is a first-responder-owned, US-based company, and every batch it sells is tested by an independent US laboratory by HPLC and mass spectrometry, with the certificate of analysis published on the product page — the same pairing of purity and identity this section recommends looking for from any vendor.

What this guide does not establish

This guide explains what purity grades mean in general; it does not evaluate any specific product, and several of its statements carry limits worth making explicit.

• Grade names are conventions, not a single standard. “Crude,” “desalted” and the purified floors are widely used, but there is no universal regulatory definition of “research grade,” and exact cut-offs and terminology vary between suppliers.
• Typical numbers are illustrative. Figures such as crude being below 70% or greater than 95% being the common laboratory specification are representative industry statements, not fixed rules; a given batch or supplier may differ.
• Regulatory thresholds describe medicines, not research material. The reporting, identification and qualification thresholds cited here come from frameworks for peptide drug substances and are given for context; research-grade material is not held to them.
• Nothing here describes any use in a person or animal. Purity, content and impurities are discussed as analytical properties of the material only.
• Nothing here describes Patriot Labs’ supply chain beyond the testing facts stated above.

Frequently asked questions

What do peptide purity grades mean?

A peptide purity grade describes how much of the peptide in a sample is the intended target sequence. It is measured by analytical HPLC as the area of the target peak divided by the total area of all peaks detected, usually with ultraviolet detection near 210 to 220 nm. Grades run from crude, which is unpurified and often below 70%, through desalted, to purified specifications quoted as greater than 95% or greater than 98%.

What does >95% purity mean for a peptide?

Greater than 95% purity means that in the analytical HPLC run, the target peptide’s peak accounts for more than 95% of the combined area of all detected peaks, with the remaining few percent made up of closely related peptide impurities such as deletion and truncation sequences. It is a statement about the peptide fraction only. It does not describe how much of the total powder is peptide, because water and counterions are not counted in that percentage.

What is the difference between peptide purity and net peptide content?

Purity is the percentage of the peptide fraction that is the correct sequence, measured by HPLC peak area. Net peptide content is the percentage of the whole powder that is actually peptide, measured by amino acid analysis, with the rest being water and counterions such as trifluoroacetate or acetate. A sample can be greater than 98% pure by HPLC and still be well under 100% peptide by mass. The absolute amount of correct peptide in a sample is the product of the peptide content and the purity.

What is crude peptide purity?

Crude peptide is material taken straight from synthesis and cleavage with no chromatographic purification. Its purity is whatever the synthesis produced, commonly below 70%, and it contains the full mixture of deletion sequences, truncations and other by-products alongside the target. Crude grade is the least expensive option and is typically used for undemanding screening work where the target is later identified analytically.

What does desalted peptide grade mean?

Desalted grade means the crude peptide has been passed through a short chromatographic step that removes synthesis salts, scavengers and residual small molecules, and may exchange the counterion, but does not separate the target sequence from closely related peptide impurities. Its sequence purity is therefore similar to crude material; desalting cleans up the sample without meaningfully raising the HPLC purity number.

Is higher peptide purity always better?

Higher purity reduces the chance that a related impurity affects a result, and demanding applications generally call for the highest available grade. But the right grade depends on the work: a purity number alone says nothing about identity, mass or how much peptide is in the powder, so a very high percentage on its own is not a complete measure of quality. Purity should be read alongside the identity result, the net peptide content and the testing documentation.

How is peptide purity measured?

Peptide purity is measured by reversed-phase high-performance liquid chromatography. The sample is separated on a column, the components are detected by ultraviolet absorbance near 210 to 220 nm where the peptide bond absorbs, and the purity is reported as the area of the target peak as a percentage of the total peak area. Identity is confirmed separately by mass spectrometry, because HPLC alone shows how many components are present, not what they are.

Why do two labs report different purity for the same peptide?

Because the purity number depends on the method. The column, the gradient, the detection wavelength and the run length all change how well impurities separate from the target and how strongly each one absorbs, so the same material can give slightly different area percentages on two systems. An impurity that co-elutes with the target on one method may resolve into a separate peak on another. This is why a purity figure is most meaningful when the method and the testing laboratory are stated alongside it.

References

1. Bachem. Quality Control of Amino Acids & Peptides: A Guide (purity by UV at 210–220 nm; net peptide content; counterion by ion chromatography; water by Karl Fischer; amino acid analysis; identity by mass spectrometry). https://www.bachem.com/knowledge-center/quality-control-of-amino-acids-peptides-a-guide/
2. Amino Foundry. Peptide Purity Verification: HPLC and Mass Spectrometry Analytical Methodology (RP-HPLC purity at 214 nm; identity by mass spectrometry). https://aminofoundry.com/blog/peptide-purity
3. Na, D. H., et al. (2026). Regulatory and analytical considerations for the quality assessment of peptide drugs. Journal of Pharmaceutical Investigation. doi:10.1007/s40005-026-00817-2. https://link.springer.com/article/10.1007/s40005-026-00817-2
4. BioPharm International. The Benefits of Combining UHPLC-UV and MS for Peptide Impurity Profiling. https://www.biopharminternational.com/view/benefits-combining-uhplc-uv-and-ms-peptide-impurity-profiling
5. Merrifield, R. B. (1963). Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. Journal of the American Chemical Society, 85(14), 2149–2154. doi:10.1021/ja00897a025
6. GenScript. Custom Peptide Synthesis (crude defined as purity below 70%; RP-HPLC; >98% for demanding applications). https://www.genscript.com/peptide.html
7. AmbioPharm. Typical Purity Percentage of Custom Peptides (>95% by RP-HPLC of net peptide content; deletion/addition sequences co-eluting with the target). https://www.ambiopharm.com/faq/typical-purity-percentage-of-custom-peptides/
8. LifeTein. What Is Net Peptide Content? (net peptide content vs purity; counterions and moisture; batch-to-batch variation). https://www.lifetein.com/What-is-net-peptide-content.html
9. Merck / Sigma-Aldrich. Peptide Sample Amount Determination (peptide purity at 214 nm vs peptide content by amino acid analysis; absolute amount = content × purity). https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/protein-biology/protein-quantitation/explanation-of-peptide
10. Behrendt, R., White, P., & Offer, J. (2016). Advances in Fmoc solid-phase peptide synthesis. Journal of Peptide Science, 22. doi:10.1002/psc.2836
11. D’Hondt, M., Bracke, N., Taevernier, L., Gevaert, B., Verbeke, F., Wynendaele, E., & De Spiegeleer, B. (2014). Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis, 101, 2–30. doi:10.1016/j.jpba.2014.06.012
12. United States Pharmacopeia. General Chapter ⟨1503⟩ Quality Attributes of Synthetic Peptide Drug Substances (identification by multiple orthogonal methods: HPLC co-elution, mass spectrometry, amino acid analysis, peptide mapping). https://doi.usp.org/USPNF/USPNF_M12935_02_01.html
13. Streuli, A., Erckes, V., Nardone, B., Bedard, V., Beland, F., & Steuer, C. (2026). Improvement of Analysis and Transferability in Peptide Purification: From HPLC to FPLC and Back Again. Journal of Peptide Science. doi:10.1002/psc.70090. https://onlinelibrary.wiley.com/doi/full/10.1002/psc.70090
14. U.S. Food and Drug Administration (May 2021). ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin: Guidance for Industry. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/andas-certain-highly-purified-synthetic-peptide-drug-products-refer-listed-drugs-rdna-origin

For related reading, see Understanding Peptide COA Testing for what a certificate of analysis proves, How Peptides Are Made for the synthesis and purification chemistry behind the grades, TFA vs Acetate in Peptides for the counterion that sits in the non-peptide fraction, How Peptides Degrade for how impurities can grow after manufacture, and What Are Peptides? for the basics.

All Patriot Labs products are sold strictly for in-vitro research and laboratory use only. Not for human or veterinary consumption. This guide is educational and describes how peptide purity is defined and measured in general terms; it is not medical advice, does not describe how to use any product, and makes no claim about any specific product.