Net peptide content versus purity

What an area percentage measures

For HPLC purity expressed by area, the main peak area is compared with areas detected and integrated by that method. The result describes the distribution of chromatographic signal, rather than weighing every solid component. McCarthy and colleagues explicitly separate impurities determined as a percentage of total area from contributions measured as mass per mass, such as counterions and other residues. These percentages therefore have different denominators (McCarthy et al., 2023; PMID: 36949371).

A high area result can coexist with a lower peptide content by mass. There is no contradiction: the denominators differ. Moreover, “HPLC” does not necessarily mean area normalization; a quantitative assay using a standard with assigned content can determine quantity. When reading the certificate, we recommend distinguishing “area purity” from “assay content”, even when both use chromatography and appear beside similar percentage values (McCarthy et al., 2023; PMID: 36949371; Li et al., 2019; PMID: 30640042).

Water, counterions and other contributions

Material mass can include residual water, counterions, residual solvents, peptide-related impurities and inorganic residues. Value assignment to standards considers these contributions through separate determinations. McCarthy’s work describes Karl Fischer water measurements, acetic acid and TFA measurements using specific methods, residual solvents and residue on ignition to complete material assessment. The total amount on a balance thus cannot be assumed to represent peptide alone (McCarthy et al., 2023; PMID: 36949371).

The salt form contributes mass, and its composition should not be assumed constant across preparations. Erckes and colleagues measured differences in counterion content across sequences and following salt exchange. For that reason, we recommend avoiding a fixed conversion factor between TFA, acetate and peptide mass for arbitrary samples. The relevant information is the analytical result for the studied preparation and its calculation basis, including what was actually quantified (Erckes et al., 2025; PMID: 40872554).

What net content means

In this article, net target-peptide content means peptide mass, excluding water and counterions, per mass of material as received. A certificate can express that result as a mass fraction, percentage or quantity per container. McCarthy distinguishes assigned bulk-material content from final content per vial. We recommend requesting an explicit definition when “peptide content” does not clarify whether it quantifies the target or a total peptide fraction (McCarthy et al., 2023; PMID: 36949371).

An “as-is” basis includes moisture in the analysed sample; an anhydrous basis applies a water correction. In the standards study, moisture is determined when preparing analyses because it can vary with handling conditions. Comparing percentages without checking these bases mixes different results. Nor should a content value already expressed for material as received be corrected for water again, since that would repeat a correction (McCarthy et al., 2023; PMID: 36949371).

How it is determined

Published strategies include mass balance, HPLC with an external standard, quantitative nuclear magnetic resonance and amino acid analysis. Li and colleagues compared these quantification techniques in an interlaboratory study using a model peptide. Results and their uncertainties depend on the method and standard used; the study does not establish one technique as a universal solution for every sequence or matrix, or eliminate the need for suitable calibration (Li et al., 2019; PMID: 30640042).

Amino acid analysis quantifies residues after peptide hydrolysis, with attention to recovery and composition. It does not necessarily distinguish the target from every peptide impurity sharing those residues. Mass balance, in turn, requires measuring contributions not attributable to the target: subtracting only water and counterion does not itself demonstrate that everything remaining is the desired sequence. The scope of the measurement must remain explicit (Hoofnagle et al., 2016; PMID: 26719571; McCarthy et al., 2023; PMID: 36949371).

Laboratory solutions and reading the COA

If f is the measured target-peptide mass fraction in material as received, m the weighed mass, M its molar mass excluding counterions and V the final volume, the relationship is c = m × f / (M × V), using consistent units. This is a balance relationship for analytical solutions, rather than product data. It requires an appropriate f; automatically replacing it with area purity is unjustified (McCarthy et al., 2023; PMID: 36949371).

That relationship assumes adequate dissolution and recovery; adsorption losses or undissolved material need additional assessment. When reviewing the COA, we recommend identifying purity and content methods and units, salt form, water, anhydrous or as-is basis, and quantification standard. If content is missing, record the uncertainty and request its determination rather than deriving a net percentage from the chromatogram or assuming an undocumented correction factor (Hoofnagle et al., 2016; PMID: 26719571; McCarthy et al., 2023; PMID: 36949371).

Verified references

• McCarthy D et al. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharmaceutical Research, 2023. PMID: 36949371. DOI: 10.1007/s11095-023-03493-1.
• Li C et al. Survey of peptide quantification methods and comparison of their reproducibility: A case study using oxytocin. Journal of Pharmaceutical and Biomedical Analysis, 2019. PMID: 30640042. DOI: 10.1016/j.jpba.2018.12.028.
• Hoofnagle AN et al. Recommendations for the Generation, Quantification, Storage, and Handling of Peptides Used for Mass Spectrometry-Based Assays. Clinical Chemistry, 2016. PMID: 26719571. DOI: 10.1373/clinchem.2015.250563.
• Erckes V et al. 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. PMID: 40872554. DOI: 10.3390/ph18081163.