Peptide catalogues sort products into tiers: crude, desalted, 90%, 95%, 98%, 99%. The numbers look like a simple ladder of quality, but they describe something narrower than many buyers assume. This article explains what peptide purity grades measure, what makes up the remaining few percent, why that remainder can matter more than its size suggests, and how to match a grade to the experiment in hand, using the synthetic pentapeptide ipamorelin as an example.
What a purity percentage measures
A purity grade is almost always an HPLC area percentage. The sample is separated on a reversed-phase column, the eluate is monitored by UV absorbance around 214-220 nm, and purity is the target peak's area divided by the total integrated area. Three consequences follow:
- It is relative. Species that do not absorb at the detection wavelength, such as inorganic salts, water and most counterions, are not counted.
- It is method-dependent. Changing the column, acid modifier or gradient can change which impurities resolve from the main peak.
- It says nothing about identity. A pure peak of the wrong sequence still scores well. Identity requires mass spectrometry.
A separate figure, net peptide content, answers a different question: what fraction of the powder's weight is peptide at all. It is determined by amino acid analysis or elemental nitrogen and commonly falls between 60 and 90% for lyophilized salts. A lot can therefore be 99% pure and 75% peptide by weight. When a stock concentration needs to be accurate, content matters as much as purity, and the two should never be used interchangeably in calculations.
Our walkthrough of HPLC purity testing covers the chromatography in detail.
Common peptide purity grades and typical uses
The tiers below reflect widespread convention among peptide chemists and core facilities rather than a formal standard:
| Grade | Typical composition | Commonly used for |
|---|---|---|
| Crude | Often 50-70% target, many synthesis by-products | Initial screening, sequence feasibility |
| Desalted | Crude with small molecules removed | Non-quantitative screening |
| 75-85% | Main species dominant, related peptides present | Antibody generation, semi-quantitative screens |
| 90-95% | Low levels of closely related peptides | Enzyme substrate work, qualitative in-vitro assays |
| 95-98% | Minor related impurities | Quantitative in-vitro assays, receptor binding, cell signalling |
| 98% and above | Trace impurities only | Reference standards, structural studies (NMR, crystallography), sensitive quantitative work |
Pharmacopoeial monographs for approved peptide drugs go further, setting limits on individual named impurities as well as total purity, along with specifications for water, counterion content and assay [3]. Research grades rarely reach that level of detail, which is why the method and chromatogram behind the number matter.
What the remainder contains
The impurities in a synthetic peptide are not random. A review of peptide-related impurities sorts them into predictable families [2]:
- Deletion sequences, missing one residue because a coupling or deprotection step was incomplete.
- Insertion sequences, carrying an extra residue from excess reagent.
- Diastereomers, from racemization at a chiral centre during activation or deprotection.
- Protection adducts, where a side-chain protecting group survived final cleavage.
- Degradation products, such as oxidized methionine, deamidated asparagine, pyroglutamate or diketopiperazine.
Many of these are one residue or one stereocentre away from the target. They often share its charge, similar hydrophobicity, and sometimes a similar ability to bind the same receptor. Where they come from is explained in our solid-phase peptide synthesis overview.
Why a few percent can matter
Consider a stock prepared at 1 mM from a 95% pure peptide. Up to 50 uM of that solution is something else. If one related impurity is inactive, the only effect is a small concentration error. If it is a partial agonist, an antagonist or a more potent analogue, it can shift a concentration-response curve or produce activity that the target peptide does not have.
This is not only theoretical. When one group re-tested research peptides ordered at 95% purity or better, only 44% met the specification, most impurities were deletion sequences, and one sample's main component had the wrong structure [1]. The authors noted that related impurities can mask or distort experimental findings.
Counterions add a separate layer. Peptides purified with TFA carry trifluoroacetate, which does not appear in the purity figure. In fetal rat osteoblast cultures, trifluoroacetate at 10^-8 to 10^-7 M reduced cell numbers, and TFA salts of several peptides gave less proliferation than hydrochloride salts [4]. A 99% pure TFA salt and a 99% pure acetate salt of the same peptide are not interchangeable in every cell assay.
Worked example: ipamorelin
Ipamorelin is the pentapeptide Aib-His-D-2-Nal-D-Phe-Lys-NH2, developed as a growth hormone secretagogue receptor agonist. In primary rat pituitary cells it released growth hormone with an EC50 of about 1.3 nmol/L, similar to GHRP-6, and in swine it showed selectivity for growth hormone release over ACTH and cortisol [5].
Its structure illustrates why purity grade and method matter together:
- Two D-amino acids. D-2-naphthylalanine and D-phenylalanine are essential to the design. Racemization during synthesis could produce L-epimers with an identical mass. Only chromatography can separate them, so a mass-only identity check cannot rule them out.
- A non-proteinogenic residue. Aib (alpha-aminoisobutyric acid) is sterically hindered and couples slowly, raising the risk of a des-Aib deletion impurity.
- Nanomolar potency. With activity in the low nanomolar range, even a small fraction of an active impurity could contribute measurably in receptor or cell-based assays.
For quantitative work on a peptide like this, a grade of 98% or higher, a chromatogram that resolves closely eluting species, and a mass spectrum confirming 711.9 Da are a sensible minimum.
Choosing a grade for an experiment
A few questions help:
- Is the readout quantitative? EC50, Kd or kinetic constants call for 95% or higher.
- Is the target sensitive to close analogues? Receptor pharmacology usually is; bulk immunization usually is not.
- Is the peptide a reference or standard? Use the highest available grade and record its content.
- Is the assay cell-based? Check the salt form as well as purity.
- Will data be compared across lots? Keep the grade and method constant, and log each lot. Our guide to reading a certificate of analysis shows where these details appear, and per-lot chromatograms are available on our lab reports page.
Key takeaways
- Peptide purity grades are HPLC area percentages; they exclude water, salts and counterions and do not confirm identity.
- The remaining percent is mostly deletion sequences, diastereomers, protection adducts and degradation products closely related to the target.
- Related impurities can be biologically active, so a few percent can shift quantitative in-vitro data.
- 95% or higher is the usual floor for quantitative assays; 98% or higher suits reference work and potent peptides such as ipamorelin.
- Always read the grade alongside the method, the mass data and the salt form.
This article summarizes published research for informational purposes. All Ascent Sciences products are for laboratory research use only and are not for human or animal consumption.
References
- Verbeke F, Wynendaele E, Braet S, et al. Quality evaluation of synthetic quorum sensing peptides used in R&D. J Pharm Anal. 2015;5(3):169-181. PubMed
- D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. PubMed
- Vergote V, Burvenich C, Van de Wiele C, et al. Quality specifications for peptide drugs: a regulatory-pharmaceutical approach. J Pept Sci. 2009;15(11):697-710. PubMed
- Cornish J, Callon KE, Lin CQ, et al. Trifluoroacetate, a contaminant in purified proteins, inhibits proliferation of osteoblasts and chondrocytes. Am J Physiol. 1999;277(5):E779-E783. PubMed
- Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PubMed
Frequently asked questions
Does 98% purity mean 98% of the vial's weight is peptide?
No. The purity figure is the share of UV-absorbing HPLC peak area belonging to the target peptide. Counterions, water and salts are outside that measurement, so net peptide content by weight is usually lower.
What is in the other 2% of a 98% pure peptide?
Mostly peptide-related impurities from synthesis and storage: deletion sequences missing a residue, truncated chains, diastereomers, incompletely deprotected species and oxidation or deamidation products.
Which purity grade should be used for cell-based assays?
A common convention is 95% or higher for quantitative in-vitro work such as receptor binding or cell signalling, with 98% or higher preferred where small impurities could interfere or where the peptide serves as a reference standard.
Are two 98% purity figures from different labs directly comparable?
Only if the methods are similar. Column, mobile phase modifier, gradient and wavelength all affect the value, so a purity grade should always be read alongside the method that produced it.
All Ascent Sciences products are for laboratory research use only and are not for human or animal consumption. This article summarizes published research and is not medical advice. See our Research Use Agreement.