HPLC, LC-MS and UV-Vis: How Peptide Purity Is Actually Measured

In plain English

Labs use different tests to check a peptide sample. Each test answers a different question. Purity asks how much of the peptide material is the right chain of amino acids (the building blocks of proteins). Identity asks whether it is the right molecule at all. Quantity asks how much peptide is really there ^[1].

The main purity test is HPLC (high-performance liquid chromatography, a method that pushes the sample through a column so its parts come out at different times). A detector draws a trace with one big peak and some small ones. Purity is the big peak’s share of the total ^[3]. LC-MS adds a mass spectrometer (a machine that weighs molecules) to check that the big peak has the right mass ^[7].

UV-Vis (a light-absorbing test) is quick but does not separate anything. It cannot tell the right peptide from near copies ^[8].

The short version

• Purity, identity and quantity are three separate questions.
• HPLC measures purity; mass spectrometry confirms identity.
• Every test has blind spots, so labs combine methods ^[1].
• A purity figure says nothing about water or salt in the powder.

Purity, identity and quantity are three different questions

Each question needs a different method:

European guidance on synthetic peptides lists these as separate specification tests. It also lists water content, counter-ion content and residual solvents, because a purity figure says nothing about them ^[1]. The difference between purity and quantity is covered in Purity vs Peptide Content.

How does HPLC measure peptide purity?

High-performance liquid chromatography (HPLC) separates the molecules in a sample by how strongly they interact with a packed column. For peptides, the standard approach is reversed-phase HPLC:

• The column is packed with a hydrophobic material, commonly C18-bonded silica ^[5].
• The sample is eluted with a gradient of water and an organic solvent such as acetonitrile, with an acid additive. Trifluoroacetic acid (TFA), acetic acid and formic acid are routinely used at about 0.1% (v/v) ^[4].
• TFA acts as an ion-pairing reagent. It pairs with positively charged groups on the peptide, which sharpens peaks and changes selectivity between closely related peptides ^[5].
• A UV detector records absorbance as each component leaves the column. Peptide bonds absorb strongly in the far UV, so detection is usually set at about 210–220 nm. For example, one pharmacopoeial reference-standard study monitored peptides and impurities at 220 nm ^[3].

The result is a chromatogram: a trace with one large main peak and, usually, several smaller ones. Purity is calculated as the main peak’s area divided by the total area of all integrated peaks.

What HPLC purity doesn’t show

Area-percent purity is a useful number, but it rests on assumptions:

• Co-elution. Two closely related peptides can leave the column at the same time and appear as one peak. European guidance notes that one method may not separate every impurity, and that additional independent methods may then be needed ^[1].
• Equal detector response. Simple area-percent assumes every impurity absorbs UV light exactly like the main peptide. Truncated, oxidised or deamidated forms may not. If the relative response factor is simply assumed to be 1, impurities can be over- or under-estimated ^[6].
• Non-peptide material is invisible. Water, counter-ions such as acetate or TFA, and inorganic residues are not part of the peptide peak area. Pharmacopoeial value assignment keeps HPLC impurities (as % area) separate from water, counter-ions and solvents (as % w/w) ^[3].
• Identity is not confirmed. A clean single peak shows that the sample is homogeneous under those conditions. It doesn’t prove that the peak is the right peptide.

What does LC-MS add?

Liquid chromatography–mass spectrometry (LC-MS) sends the separated components into a mass spectrometer, which measures their mass-to-charge ratio. This adds two things HPLC-UV can’t provide:

1. Identity. The observed molecular mass of the main peak is compared with the theoretical mass calculated from the sequence.
2. Impurity identification. Each minor peak can be assigned a mass, which often reveals what it is: a deletion sequence, an oxidised form, or a peptide still carrying a protecting group ^{[7, 11]}.

LC-MS has blind spots too. Structural isomers and stereoisomers (for example, a peptide containing one D-amino acid instead of the L-form) have the same mass as the target peptide. Characterising them needs specialised chromatographic and MS techniques ^{[7, 10]}. Leucine and isoleucine also have identical masses. Pharmacopoeial laboratories use amino acid analysis to tell them apart in peptides that contain both ^[3]. Mass spectrometry is also not quantitative on its own unless it is specifically calibrated.

What does UV-Vis spectroscopy measure?

UV-Vis spectrophotometry measures how much light a solution absorbs, without separating its components first. It is quick and inexpensive, and it is mainly used to estimate concentration:

• Absorbance at 280 nm can be predicted from the sequence, but only for peptides that contain tryptophan or tyrosine ^[8].
• Absorbance at 205 nm, where the peptide bond absorbs, can be used for peptides without those residues. A published method predicts the 205 nm absorptivity directly from the amino acid sequence ^[8].

A deletion sequence or an oxidised variant absorbs light in almost the same way as the target peptide. UV-Vis therefore reports total absorbing material, not the share that is the correct sequence. It is not a test of purity in the chromatographic sense.

Other methods used in peptide testing

Why orthogonal methods matter

“Orthogonal” methods measure the same attribute by different physical principles, so one method’s blind spot is covered by another. European guidance recommends at least two orthogonal methods for identity, such as mass, relative retention time, LC-MS, peptide mapping, amino acid analysis or NMR. It also recommends mass spectrometry from early development to reduce the risk of co-eluting impurities going unnoticed ^[1].

The international guideline on analytical validation, ICH Q2(R2), sets out what a method must demonstrate before its results can be relied on. This includes specificity (it measures the intended analyte without interference), range, accuracy and precision ^[2]. A purity result from an unvalidated or undescribed method is weaker evidence than one produced by a validated method that is named on the report.

Method comparison at a glance

What a well-documented purity result includes

• The method named (for example, “RP-HPLC, UV 220 nm”), not just a percentage.
• The chromatogram, with integrated peaks visible.
• Mass spectrometry confirming the main peak’s identity.
• The batch or lot number that matches the material.
• Ideally, separate results for peptide content, water and counter-ion, since purity doesn’t cover them.

For how these appear on a report, see How to Read a Peptide Certificate of Analysis. For what the minor peaks usually are, see Common Impurities in Synthetic Peptides.

What this means in Australia

In Australia, the relevant rules are those for therapeutic goods and for testing laboratories.

• Peptides are therapeutic goods. Peptide products are regulated as therapeutic goods under the Therapeutic Goods Act 1989. Unapproved peptide products are not on the ARTG (the Australian Register of Therapeutic Goods) and have not been assessed by the TGA for safety, quality or effectiveness ^[13].
• Medicines are made under GMP. Manufacturers of therapeutic goods generally need a licence, and good manufacturing practice (GMP) applies. The TGA sums up GMP as: quality cannot be tested into a batch; it must be built in at every stage of manufacture ^[15]. Testing a finished batch does not replace that system.
• Scheduling is compound by compound. Some research peptides are named in the Poisons Standard. For example, BPC-157 and TB-500 are Schedule 4 (prescription-only) and Appendix D, clause 5 (possessing them without authority is illegal) ^[12].
• “Research use only” labels. The TGA has stated that a “research use only” disclaimer does not make supply lawful. It does not change a product’s regulatory status, permit importation, or remove advertising or supply obligations ^[13]. The TGA also counts applying a label to a blank vial as a step in manufacture ^[14].
• Laboratory accreditation. In Australia, NATA accredits testing laboratories to ISO/IEC 17025. The standard covers method validity, staff competence, traceability of measurements, equipment calibration, quality control and reporting of results ^[16]. When reading a purity result, it is worth checking whether the laboratory’s accreditation covers the method reported.

This is general information, not legal advice. Laws change; check the current Poisons Standard and TGA guidance.

Frequently asked questions

How is peptide purity measured?

Usually by reversed-phase HPLC with UV detection at about 210–220 nm. Purity is reported as the main peak’s share of the total peak area, ideally with the chromatogram attached ^{[3, 5]}.

Is HPLC or mass spectrometry better for peptides?

They answer different questions. HPLC separates and measures purity; mass spectrometry confirms identity by mass. Combined as LC-MS, they are complementary, and guidance recommends orthogonal methods ^{[1, 7]}.

Can UV-Vis spectroscopy measure peptide purity?

Not in the chromatographic sense. Without a separation step, UV-Vis measures all absorbing material together, so closely related impurities are counted as if they were the target peptide ^[8].

Does 99% HPLC purity mean the powder is 99% peptide?

No. HPLC purity only compares peptide-related peaks with each other. Water, counter-ions and other non-peptide material are measured separately ^{[1, 3]}.

Why can two labs report different purity for the same batch?

Differences in column, gradient, detection wavelength and integration settings can change how peaks separate and are counted. Response-factor assumptions also affect impurity figures ^[6]. Comparing results is most meaningful when the method is stated in full.

References

1. European Medicines Agency. Guideline on the development and manufacture of synthetic peptides. EMA/CHMP/CVMP/QWP/367182/2025. Adopted 4 December 2025; effective 1 June 2026. ema.europa.eu. Accessed 11 October 2026.
2. International Council for Harmonisation. ICH Q2(R2): Validation of analytical procedures. 2023. database.ich.org. Accessed 11 October 2026.
3. McCarthy D, Han Y, Carrick K, et al. Reference standards to support quality of synthetic peptide therapeutics. Pharm Res. 2023;40(6):1317–1328. doi:10.1007/s11095-023-03493-1. PMID: 36949371
4. 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. doi:10.3390/ph13120442. PMID: 33287352
5. Åsberg D, Langborg Weinmann A, Leek T, et al. The importance of ion-pairing in peptide purification by reversed-phase liquid chromatography. J Chromatogr A. 2017;1496:80–91. doi:10.1016/j.chroma.2017.03.041. PMID: 28363419
6. Kumar Kuril A. The critical need for implementing RRF in the accurate assessment of impurities in peptide therapeutics. Anal Chem. 2025;97(24):12480–12485. doi:10.1021/acs.analchem.5c02149. PMID: 40499007
7. Lian Z, Wang N, Tian Y, Huang L. Characterization of synthetic peptide therapeutics using liquid chromatography-mass spectrometry: challenges, solutions, pitfalls, and future perspectives. J Am Soc Mass Spectrom. 2021;32(8):1852–1860. doi:10.1021/jasms.0c00479. PMID: 34110145
8. Anthis NJ, Clore GM. Sequence-specific determination of protein and peptide concentrations by absorbance at 205 nm. Protein Sci. 2013;22(6):851–858. doi:10.1002/pro.2253. PMID: 23526461
9. Li C, Bhavaraju S, Thibeault MP, et al. Survey of peptide quantification methods and comparison of their reproducibility: a case study using oxytocin. J Pharm Biomed Anal. 2019;166:105–112. doi:10.1016/j.jpba.2018.12.028. PMID: 30640042
10. Badgujar D, Paritala ST, Matre S, Sharma N. Enantiomeric purity of synthetic therapeutic peptides: a review. Chirality. 2024;36(3):e23652. doi:10.1002/chir.23652. PMID: 38448043
11. D’Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2–30. doi:10.1016/j.jpba.2014.06.012. PMID: 25044089
12. Therapeutic Goods (Poisons Standard—October 2026) Instrument 2026 (F2026L01327). Federal Register of Legislation. legislation.gov.au. Accessed 11 October 2026.
13. Therapeutic Goods Administration. Understanding your responsibilities when importing, compounding and supplying unapproved peptide products. Safety advisory, 13 April 2026. tga.gov.au. Accessed 11 October 2026.
14. Therapeutic Goods Administration. Unapproved peptide product promoters and suppliers are put on notice. Media release, 20 July 2026. tga.gov.au. Accessed 11 October 2026.
15. Therapeutic Goods Administration. Good manufacturing practice (GMP); and Manufacturing. tga.gov.au; tga.gov.au. Accessed 11 October 2026.
16. National Association of Testing Authorities, Australia (NATA). Laboratory accreditation: ISO/IEC 17025. nata.com.au. Accessed 11 October 2026.

Important information and disclaimer

General information only. This article is published for general educational and informational purposes. It summarises published scientific literature and public regulatory information as at the “last reviewed” date shown above. It is not medical, health, pharmaceutical, veterinary or legal advice, and it must not be relied on as a substitute for advice from a qualified professional about your circumstances.

Not for human or animal use. Nothing in this article recommends, endorses or describes the use of any substance in humans or animals. Findings from cell, animal or early human studies cannot be assumed to apply to people. The compounds discussed have not been evaluated by the Therapeutic Goods Administration (TGA) for safety, quality or efficacy unless the article expressly states otherwise. Any research materials offered by Iconic Peptides are supplied solely for in vitro and laboratory research.

No therapeutic claims. Iconic Peptides makes no claim that any product treats, cures, prevents, alleviates or diagnoses any disease, condition or symptom. References to published studies, clinical trials or medicines approved by regulators are factual descriptions of the scientific and regulatory record. They are not representations about any product we supply.

Regulatory status. Many peptides are scheduled substances under the Poisons Standard. Some are prescription-only (Schedule 4), and some may not be possessed without authority (Appendix D). The TGA has stated that a “research use only” label does not make supply lawful, change a product’s regulatory status, or remove advertising or supply obligations. State and territory laws may also apply. You are responsible for knowing and complying with all laws that apply to you. Regulatory information in this article may change after the review date.

Accuracy and third-party sources. We take care to cite and verify every source, but we do not warrant that this article is complete, current or error-free. Citing a study or organisation does not mean that its authors or that organisation endorse Iconic Peptides.

Limitation of liability. To the maximum extent permitted by law, Iconic Peptides, its owners, employees and contributors exclude all liability for any loss, damage, injury or expense, whether direct, indirect or consequential, arising from use of or reliance on this article or any misuse of any product. Nothing in this disclaimer excludes, restricts or modifies any right or remedy under the Australian Consumer Law that cannot lawfully be excluded.

Age restriction. This website is intended for adults aged 18 and over.