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HPLC purity explained: what "99% pure" does and does not mean

By the PeptideLabCheck team · Last reviewed 14 November 2026 · 10 sources

How does HPLC separate a sample?

HPLC stands for high performance liquid chromatography. It separates a mixture into its parts. A pump pushes a liquid, called the mobile phase, through a packed tube called the column ¹. The sample is injected into that flow.

Each compound in the sample interacts with the material inside the column, called the stationary phase. The stronger the attraction, the slower the compound moves ¹. So different compounds leave the column at different times. A detector placed after the column records them as they come out ¹.

A well-documented mode for peptides is reversed-phase HPLC ^{2, 3}. The column packing is silica with carbon chains attached, labelled C8 or C18 ². Peptides leave the column in order of increasing hydrophobicity ². Hydrophobicity is how strongly a molecule avoids water. The mobile phase contains a rising share of a solvent called acetonitrile ². Trifluoroacetic acid (TFA) is widely added to the mobile phase to improve the separation ^{2, 8}.

What does a chromatogram show?

A chromatogram is the graph the instrument draws. The horizontal axis is time. The vertical axis is the strength of the detector signal ¹.

When nothing is leaving the column, the line is flat. This is the baseline ¹. When a compound leaves the column, the signal rises and falls. That shape is a peak ¹. The retention time is the time from injection to the top of a peak ¹. The peak area is the area enclosed by the peak and the baseline ¹.

For peptides, the detector usually measures how much UV light the sample absorbs. Peptide bonds are the links between amino acids. They absorb strongly in the far UV, so peptides are generally detected at 210–220 nm ². In one example, United States Pharmacopeia (USP) scientists monitored a peptide and its impurities at 220 nm ³. A few amino acids also absorb at 250–290 nm, but not every peptide contains them ².

How is the purity percentage calculated?

The usual method is called area percent, or area normalisation. The software measures the area of every reported peak. It divides the area of one peak by the sum of all the peak areas and multiplies by 100 ⁴. Impurities are reported as a percent of the total detected area ³. Area percent is commonly used to describe purity ⁵.

So "99% pure by HPLC" has a narrow meaning. Under that method, at that wavelength, 99% of the total peak area belonged to the main peak. It is a ratio of signals. It is not a weight.

What does "99% pure" not tell you?

Endotoxins are substances called lipopolysaccharides. They are found in the outer membrane of gram-negative bacteria ⁹. Sterility is freedom from the presence of viable (living) microorganisms ¹⁰. These are two separate questions. An FDA technical guide notes that sterilizing filters do not remove endotoxins ⁹.

Why can a 99% pure peptide be less than 99% peptide by weight?

Synthetic peptides are commonly obtained as salts. Peptide synthesis and purification rely on TFA, so peptides are obtained as TFA salts ⁸. The TFA acts as a counter-ion. A counter-ion is an ion of opposite charge that pairs with the charged groups on the peptide ⁸. Freeze-drying, also called lyophilization, leaves the peptide and its counter-ion together as a solid salt ⁸. TFA can be exchanged for another counter-ion, such as chloride or acetate ⁸.

Counter-ions add weight. In a 2025 study, TFA made up as much as 35% of the total weight of the peptide salts tested ⁸. Water adds weight too, and is measured separately ³.

USP scientists describe a "mass balance" approach for peptide material. Peptide-related impurities, counter-ion, water and residual solvents are each measured and subtracted from 100% ³. The share of the weight that is peptide is often called net peptide content. HPLC purity covers only the first of those subtractions. So a powder can show 99% HPLC purity while a large share of its weight is counter-ion and water.

Can the HPLC purity number itself mislead?

Yes, in three ways.

• Unequal detector response. Area percent treats every peak as if the detector responded to each compound equally. In practice, response often differs between compounds ⁵. European Pharmacopoeia guidance controls an impurity against its own reference standard when its response factor falls outside the range 0.8 to 1.2 ⁴.
• Hidden peaks. Two compounds can leave the column together and look like one peak. This is called co-elution ⁶. Software checks for it cannot give a definitive answer, especially when the impurity is at a low level or absorbs UV light much like the main compound ⁶.
• Method dependence. How well peptides separate depends on method settings such as the solvent gradient ². A purity figure belongs to the method that produced it. That is why a report should name the method.

Key facts

• Peptide bonds absorb light strongly in the far UV, and peptides are generally detected at 210–220 nm ².
• In reversed-phase HPLC, peptides leave the column in order of increasing overall hydrophobicity ².
• HPLC impurities are measured as a percent of the total detected peak area ³.
• ICH Q6A states that identification solely by a single chromatographic retention time is not regarded as specific ⁷.
• Mass spectrometry is used to determine the mass of a peptide, and MS/MS fragmentation is used to confirm its amino acid sequence ³.
• In one peer-reviewed study, TFA counter-ion reached up to 35% of the total weight of the peptide salts examined ⁸.
• Bacterial endotoxins are lipopolysaccharides found in the outer membrane of gram-negative bacteria, and they are not removed by sterilizing filters ⁹.
• Sterility can be defined as freedom from the presence of viable microorganisms ¹⁰.

Common questions

Sources

1. "What is HPLC (High Performance Liquid Chromatography)?", Shimadzu Corporation. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/basic/what_is_hplc.html — Used for how HPLC works (pump, mobile phase, column, stationary phase, detector) and for the parts of a chromatogram (axes, baseline, peak, retention time, peak area).
2. Fields GB, Mant CT, Chen Y, et al. "HPLC Analysis and Purification of Peptides." Methods in Molecular Biology 386, 3–55 (2007); full text hosted by PubMed Central, U.S. National Library of Medicine. https://pmc.ncbi.nlm.nih.gov/articles/PMC7119934/ — Used for peptide UV detection at 210–220 nm, C8 and C18 reversed-phase columns, elution by hydrophobicity, the acetonitrile gradient, and TFA as an ion-pairing additive.
3. McCarthy D, Han Y, Carrick K, et al. "Reference Standards to Support Quality of Synthetic Peptide Therapeutics." Pharmaceutical Research 40, 1317–1328 (2023), Springer. https://pmc.ncbi.nlm.nih.gov/articles/PMC10338602/ — Used for impurities reported as a percent of total detected area, the mass balance approach (peptide impurities, counter-ion, water, residual solvents), mass spectrometry for identity, monitoring at 220 nm, and content per vial measured against a standard.
4. "How should the test requirements be applied in related substances tests?", EDQM FAQs, European Directorate for the Quality of Medicines & HealthCare (Council of Europe). https://faq.edqm.eu/x/_AGL — Used for the normalisation (area percent) calculation and the 0.8 to 1.2 response factor range, with impurity reference standards outside it.
5. Borman P, Lane S, Carmody C, et al. "Single Calibrant Quantification with HPLC — Fact or Fantasy?", LCGC International, 1 March 2006. https://chromatographyonline.com/view/single-calibrant-quantification-hplc-fact-or-fantasy — Used for the statements that area percent is commonly used to characterize purity and that response factors differ between compounds.
6. Stoll DR, Rutan SC, Venkatramani CJ. "Peak Purity in Liquid Chromatography, Part I: Basic Concepts, Commercial Software, and Limitations", LCGC International, 1 February 2018. https://www.chromatographyonline.com/view/peak-purity-liquid-chromatography-part-i-basic-concepts-commercial-software-and-limitations — Used for co-elution and the limits of software peak-purity checks.
7. ICH Topic Q6A, Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances (CPMP/ICH/367/96), European Medicines Agency, May 2000. https://www.ema.europa.eu/en/documents/scientific-guideline/ich-q-6-test-procedures-and-acceptance-criteria-new-drug-substances-and-new-drug-products-chemical-substances-step-5_en.pdf — Used for identification not being specific by a single retention time, and for assay, water content (Karl Fischer), sterility and endotoxin (LAL) being separate tests.
8. 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 18, 1163 (2025), MDPI. https://doi.org/10.3390/ph18081163 — Used for why synthetic peptides are obtained as TFA salts, what the counter-ion pairs with, exchange to chloride or acetate, and the measured TFA content of up to 35% of total weight.
9. "Bacterial Endotoxins/Pyrogens", Inspection Technical Guide No. 40, U.S. Food and Drug Administration, 20 March 1985. https://www.fda.gov/ICECI/Inspections/InspectionGuides/InspectionTechnicalGuides/ucm072918.htm — Used for the definition of endotoxins, the LAL test name, and the note that sterilizing filters do not remove endotoxins.
10. "Sterility testing", World Health Organization. https://www.who.int/teams/health-product-policy-and-standards/standards-and-specifications/norms-and-standards/sterility-testing — Used for the definition of sterility and for how a sterility test detects microbes through growth.