Peptide purity explained: HPLC and mass spectrometry

Peptide purity is the number that every supplier prints in large type, and it is also the number that is most often misunderstood. A value such as 98% comes from one specific method, reversed-phase HPLC, and it answers one specific question. It does not tell you the amount of peptide in the vial, and it does not prove that the main peak is the right molecule. This guide explains how HPLC separates a peptide sample, what the percentage means, which impurities are typical for synthetic peptides, and why mass spectrometry is the second half of the picture.

How reversed-phase HPLC measures peptide purity

HPLC stands for high-performance liquid chromatography. The sample is dissolved and pumped under high pressure through a column packed with tiny silica particles. In reversed-phase HPLC, these particles are coated with hydrophobic carbon chains, usually C18. The peptide molecules stick to this coating. A solvent mixture, the mobile phase, then flows through the column. At the start, the mobile phase is mostly water with a little acid. Over the run, the share of organic solvent, usually acetonitrile, is increased step by step. This is called a gradient.

Each molecule leaves the column at the point in the gradient where the solvent is strong enough to pull it off the hydrophobic coating. Hydrophilic molecules elute early. Hydrophobic molecules elute late. A detector at the end of the column measures UV absorbance, usually at 214 or 220 nm, where the peptide bond absorbs. The result is a chromatogram: a line with peaks, where each peak is a group of molecules that eluted at the same time. Mant and colleagues describe the method in detail in Methods in Molecular Biology (2007, DOI 10.1007/978-1-59745-430-8_1).

The purity value is calculated from the peak areas:

Purity (%) = area of the main peak / sum of all peak areas × 100

What 98% peptide purity actually means

The percentage has a precise but narrow meaning. It says that 98% of the UV-absorbing material that eluted from the column came out in the main peak. Four limits follow from this definition:

• It is relative, not absolute. The number compares peaks with each other. It says nothing about how many milligrams of peptide are in the vial.

• It only counts what absorbs UV. Water, salts and counter-ions such as TFA do not absorb at 214 nm. They are invisible to the detector and do not reduce the purity value.

• It only counts what the column separates. An impurity that has the same retention time as the product hides inside the main peak. A peptide with one oxidised methionine often elutes very close to the parent peptide.

• It depends on the method. A longer gradient, a different column or a different wavelength can change the number by one or two percent for the same sample.

For most laboratory work, a purity above 95% is sufficient. For quantitative assays or anything that needs a known molar concentration, 98% or higher is the usual target. More important than the number is the chromatogram behind it. How to read that document is covered in our article on how to read a peptide certificate of analysis.

Typical impurities in synthetic peptides

Research peptides are made by solid-phase peptide synthesis. The chain is built one amino acid at a time on a resin bead. Each step has a yield of about 99%, and the errors add up over the length of the chain. D'Hondt and colleagues reviewed the impurity classes of peptide products in the Journal of Pharmaceutical and Biomedical Analysis in 2014 (DOI 10.1016/j.jpba.2014.06.012). The most common ones are:

• Deletion sequences: one amino acid is missing because a coupling step failed. The peptide is one residue shorter and usually elutes slightly earlier or later than the full sequence.

• Truncated sequences: the chain stopped growing early. These are much shorter fragments.

• Insertion sequences: one amino acid was added twice.

• Incompletely deprotected peptides: a protecting group from the synthesis is still attached. These are more hydrophobic and elute late.

• Oxidised and deamidated forms: methionine, cysteine and tryptophan oxidise; asparagine and glutamine deamidate. These form during synthesis, cleavage or storage.

• Diastereomers: one amino acid has flipped from the L form to the D form. These are very hard to separate and often hide in the main peak.

A long peptide such as CJC-1295 has more coupling steps and more chances for deletions than a short pentapeptide like Ipamorelin. This is one reason why purity targets differ between products. The sequences and lengths of both are described in our guide on CJC-1295 and Ipamorelin. Storage also matters, because oxidation and deamidation continue in the vial. Our guide on how to store peptides explains how to slow this down.

Mass spectrometry confirms identity, not peptide purity

Mass spectrometry (MS) ionises the molecules in a sample and measures their mass-to-charge ratio. For peptides, electrospray ionisation (ESI) or MALDI is used. If the measured mass matches the mass calculated from the sequence, the identity of the main component is confirmed.

This is a different question from purity. MS tells you that the molecule with the expected mass is present. It does not tell you how much of the sample that molecule makes up, for three reasons:

• Different molecules ionise with different efficiency. A small impurity that ionises well can produce a large peak, and the product can produce a small one.

• A single peptide appears at several masses because it can carry one, two, three or more protons. The spectrum is not a simple count.

• Impurities with the same mass as the product, such as diastereomers, give the same signal and are invisible.

The reverse is also true. HPLC can show a single clean peak for a molecule that is not the intended peptide. A deletion sequence that is one glycine short can look perfect on a chromatogram. Only the mass reveals the missing 57 Da. This is why a complete analysis combines both methods, and why the best certificates come from LC-MS, where the HPLC column feeds directly into the mass spectrometer and every peak gets its own mass.

Why peptide purity is not net peptide content

A vial labelled 10 mg at 98% purity does not contain 9.8 mg of peptide. The purity value refers only to the peptide fraction. The powder in the vial also contains water, counter-ions and sometimes salts, none of which the HPLC detector sees. Net peptide content is the share of the total mass that is peptide. For a typical lyophilised TFA salt it lies between 70 and 85%. The full calculation looks like this:

Actual peptide (mg) = label weight (mg) × net peptide content (%) × purity (%)

Example: 10 mg × 0.80 × 0.98 = 7.84 mg of the correct sequence. If you plan a concentration for an experiment, this is the number to use. The method for the calculation is explained in our guide on how to reconstitute peptides. Suppliers that fill by net peptide weight, as listed in our lab-tested peptides collection, state this on the certificate.

Frequently asked questions about peptide purity

Sources

• Mant CT, Chen Y, Yan Z, Popa TV, Kovacs JM, Mills JB, Tripet BP, Hodges RS. HPLC analysis and purification of peptides. Methods in Molecular Biology. 2007. DOI 10.1007/978-1-59745-430-8_1

• D'Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, De Spiegeleer B. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis. 2014. DOI 10.1016/j.jpba.2014.06.012

This article describes laboratory handling of research compounds. All products on this site are sold for laboratory research use only and are not intended for human or veterinary use.