PepGenexShop our research products

Peptide content vs purity

Peptide content and chromatographic purity are two different fractions of the same sample: chromatographic purity is the share of detected peak area that belongs to the main peptide, while peptide content is the share of the total sample mass that is peptide. In the mass balance calculation McCarthy and colleagues describe, HPLC impurities enter as a percentage of total detected area, whereas counter ions and other impurities enter on a mass-per-mass basis.[1]

What does each figure divide by?

An HPLC area percentage divides the main peak by the sum of the peaks the detector records. A content figure divides the peptide by everything that was weighed, including counter ions, water, residual solvents and inorganic residue.[1]

In the worked example McCarthy and colleagues give, HPLC impurities summing to 0.92% of total detected area and acetic acid at 5.58% by mass combine to 0.93 mg of peptide free base per mg of material on an anhydrous basis. Water contents of 1.5% and 2% then lower the value to 0.92 and 0.91 mg/mg for two laboratories.[1]

How far apart can the two figures be?

Wang and colleagues reported synthetic peptides with purities greater than 99% by reversed-phase liquid chromatography whose peptide content, measured by amino acid analysis with isotope dilution mass spectrometry, ranged from 62.07% to 88.18%.[2]

In a candidate angiotensin II reference material, Melanson and colleagues counted the trifluoroacetate counter-ion as an impurity amounting to nearly 25% by mass, and the assigned value was 691 ± 9 mg/g.[3]

For HPLC-purified batches of the peptide lecirelin, Sázelová and colleagues found purity of 96.4% to 99.9% by two electrophoretic methods. Across the lecirelin batches they analyzed, counter-ions accounted for up to about 11% of the mass as acetic acid, 9% as trifluoromethanesulfonic acid and 3% as trifluoroacetic acid.[4]

Why do counterions not lower a chromatographic purity figure?

Counterions are measured by separate techniques and enter a mass balance on a mass-per-mass basis, apart from the chromatographic area percentage. Sikora and colleagues describe ion chromatography as the first-choice technique for counter-ion determination and note that simple ions that do not absorb ultraviolet light are detected indirectly in capillary electrophoresis.[5],[1]

Erckes and colleagues exchanged trifluoroacetate for chloride in four synthetic peptides and observed no effect on peptide purity at any hydrochloric acid concentration tested, although the counterion changed the salt's composition from an expected 25% trifluoroacetate to an expected 10% chloride by mass.[6]

Which components does each figure count as impurities?

D'Hondt and colleagues include unwanted counter ions such as trifluoroacetate, from the synthesis itself or from later purification, among the peptide-related impurities that may remain in a final peptide product.[7]

A content method can count related peptides as if they were the main peptide. Stoppacher and colleagues estimated 10.4 mg/g of structurally related impurities in an angiotensin I material and calculated that ignoring them would introduce a 1% error into a concentration determined by amino acid analysis.[8]

Melanson and colleagues state that amino acid analysis and quantitative NMR are both susceptible to interference from related peptide impurities, which is why their strategy corrects each result for those impurities before combining them.[3]

How are the two figures combined in a reference standard?

McCarthy and colleagues describe a two-step value assignment: a mass balance assigns a quantitative value to the bulk peptide, and the bulk material is then used as the standard in an HPLC assay that assigns the peptide mass content of each lyophilized vial.[1]

The same report checks the purity of the vialed material to confirm that no new impurities formed during vialing, and states that more than two laboratories are required to assign content.[1]

ICH Q6B states that absolute purity is difficult to establish and that results depend on the method, so a single number is interpreted together with the procedure that produced it.[9]

Limitations

Neither a content figure nor a chromatographic purity figure establishes what the peptide is; ICH Q6B lists identity as its own specification item.[9]

The numbers on this page come from the specific materials and methods in the cited studies, and one of them is a theoretical example; they are not typical values for other peptides.[1],[2],[4],[6]

This page describes what the cited documents say. It is not a statement that any PepGenex material has been manufactured, tested or released under any of them.

Compound profiles

Research peptide profiles in this library. What this page describes applies to peptide lots in general; none of these profiles reports a result of it for any lot.

References

  1. McCarthy D, Han Y, Carrick K, et al. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharm Res. 2023;40(6):1317-1328. PMID 36949371 · DOI 10.1007/s11095-023-03493-1 · PMC10338602
  2. Wang X, Qin W, Qian X, Zhang Y. Accurate quantification of synthetic peptides by amino acid-stable isotope dilution mass spectrometry [article in Chinese]. Se Pu. 2012;30(3):239-244. PMID 22715687 · DOI 10.3724/sp.j.1123.2012.01035
  3. Melanson JE, Thibeault MP, Stocks BB, et al. Purity assignment for peptide certified reference materials by combining qNMR and LC-MS/MS amino acid analysis results: application to angiotensin II. Anal Bioanal Chem. 2018;410(26):6719-6731. PMID 30143839 · DOI 10.1007/s00216-018-1272-7
  4. Sázelová P, Kašička V, Solínová V, Koval D. Determination of purity degree and counter-ion content in lecirelin by capillary zone electrophoresis and capillary isotachophoresis. J Chromatogr B Analyt Technol Biomed Life Sci. 2006;841(1-2):145-151. PMID 16687256 · DOI 10.1016/j.jchromb.2006.04.006
  5. Sikora K, Jaśkiewicz M, Neubauer D, Migoń D, Kamysz W. The Role of Counter-Ions in Peptides-An Overview. Pharmaceuticals (Basel). 2020;13(12):442. PMID 33287352 · DOI 10.3390/ph13120442 · PMC7761850
  6. Erckes V, Streuli A, Chamera Rendueles L, Krämer SD, Steuer C. Towards a Consensus for the Analysis and Exchange of TFA as a Counterion in Synthetic Peptides and Its Influence on Membrane Permeation. Pharmaceuticals (Basel). 2025;18(8):1163. PMID 40872554 · DOI 10.3390/ph18081163 · PMC12389442
  7. D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. PMID 25044089 · DOI 10.1016/j.jpba.2014.06.012
  8. Stoppacher N, Josephs RD, Daireaux A, et al. Impurity identification and determination for the peptide hormone angiotensin I by liquid chromatography-high-resolution tandem mass spectrometry and the metrological impact on value assignments by amino acid analysis. Anal Bioanal Chem. 2013;405(25):8039-8051. PMID 23708692 · DOI 10.1007/s00216-013-6953-7
  9. ICH Q6B Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. U.S. FDA guidance for industry, August 1999. FDA docket FDA-1998-D-0003