Quick Answer
Peptide purity is the percentage of the target peptide relative to all detectable compounds in the sample, typically measured by reverse-phase High-Performance Liquid Chromatography (RP-HPLC). A purity of 99% means that 99% of the UV-absorbing material detected is the target peptide, with the remaining 1% consisting of impurities such as truncated sequences, deletion sequences, or side products.
Purity vs. Content: A Critical Distinction
One of the most commonly confused concepts in peptide research is the difference between purity and content:
- Purity: The percentage of the target peptide among all detected substances in a chromatographic analysis. Measured by HPLC.
- Peptide content (net content): The actual mass of the peptide as a proportion of the total powder weight. This accounts for water, salts (e.g., TFA counter-ions), and other non-peptide components. Measured by amino acid analysis, elemental analysis, or UV spectrophotometry.
For example, a peptide with 99% HPLC purity might have a net peptide content of only 80%, meaning that 20% of the powder's weight is water, TFA salts, or other non-peptide material. When calculating concentrations for an experiment, researchers must account for net peptide content, not just purity.
How Purity Is Measured
Reverse-Phase HPLC
Reverse-phase HPLC (RP-HPLC) is the standard analytical method for determining peptide purity. The peptide sample is injected into a chromatographic column where compounds are separated based on their hydrophobicity. As different compounds elute from the column at different times, they are detected by UV absorbance (typically at 214 nm, which detects peptide bonds). The resulting chromatogram shows a series of peaks, and the area of the main peak (target peptide) is compared to the total area of all peaks to calculate purity.
Mass Spectrometry Confirmation
While HPLC measures purity, mass spectrometry (MS) confirms identity. MS determines the molecular weight of each peak in the chromatogram, allowing researchers to identify whether impurities are truncated sequences (missing one or more amino acids), deletion sequences, or side products from the synthesis.
Common Impurities in Synthetic Peptides
Impurity TypeCauseDetectionTruncated sequencesIncomplete coupling during SPPSHPLC + MSDeletion sequencesMissing one or more amino acidsHPLC + MSDeamidation productsAsn/Gln side-chain modificationHPLC + MSOxidation productsMet/Cys/Trp oxidationHPLC + MSTFA saltsCounter-ions from HPLC purificationElemental analysisResidual waterMoisture absorbed during handlingKarl Fischer titration
Purity Grades
Research peptides are commonly available in several purity grades:
- Crude (Desalted): ~50–70% purity; minimal purification after synthesis
- 85% purity: Suitable for screening assays where some impurity tolerance exists
- 95% purity: The most common research grade; suitable for most in vitro studies
- 98–99% purity: High-purity grade for sensitive assays, structural studies, and quantitative work
How Purity Affects Research
The required purity level depends on the experimental application:
- In vitro binding assays: 90–95% is typically sufficient
- Cell-based assays: ≥95% recommended to minimize confounding effects from impurities
- Structural studies (NMR, crystallography): ≥98% recommended
- Mass spectrometry standards: ≥98–99% for accurate calibration
Conclusion
Peptide purity, measured by RP-HPLC, represents the proportion of the target peptide among all detected compounds. It is distinct from peptide content, which accounts for non-peptide material in the powder. Researchers must consider both purity and net peptide content when preparing solutions and designing experiments, and should select the appropriate purity grade for their specific application.
Sources & References
- Snyder LR, Kirkland JJ, Dolan JW. Introduction to Modern Liquid Chromatography, 3rd ed. Wiley; 2010.
- Bhatt NP, Patel K, Borchardt RT. Chemical pathways of peptide degradation. I. Deamidation of adrenocorticotropic hormone. Pharm Res. 1990;7(9):593-599.
- Sigma-Aldrich. Peptide Purity and Analysis Technical Bulletin. https://www.sigmaaldrich.com/technical-documents/articles/peptide-analysis.html