What Is HPLC and How Does It Measure Peptide Purity?

Every purity figure in the Wednesday COA library comes from the same instrument: a high-performance liquid chromatograph. HPLC is the workhorse of peptide analysis, used both to purify crude synthetic material and to measure how clean the finished product is. It is also the method whose output is most often shown on a certificate and least often explained. This note describes what the technique does, how a reversed-phase separation of a peptide actually works, how a chromatogram is turned into a purity number, and where the method’s limits lie. It is written for the researcher who wants to read a certificate with understanding rather than operate the instrument, and it pairs with our note on peptide purity.

Chromatography in one paragraph

All chromatography rests on the same idea. A mixture is carried by a moving fluid, the mobile phase, past a stationary material that the components interact with to different degrees. Components that interact strongly are held back; those that interact weakly move ahead. Given enough distance, the mixture spreads into separate bands. The Russian botanist Mikhail Tswett described the principle in the first years of the twentieth century, using a column of powdered chalk to separate plant pigments into colored bands, which gave the method its name.¹ Every later variant, including HPLC, changes the phases and the engineering but not the logic.

What makes it “high performance”

Early liquid chromatography used gravity to pull solvent through a wide column packed with coarse particles. Separations took hours and resolution was poor. HPLC replaced the gravity feed with a high-pressure pump and the coarse packing with very small, uniform particles, typically 1.7 to 5 micrometers, packed into a narrow steel column. Smaller particles give the sample many more opportunities to interact with the stationary phase per unit length, sharpening the bands dramatically, but they also resist flow, which is why pressures of several hundred bar are needed.^2,3

A modern system has five parts. The pump delivers mobile phase at a precise flow rate, usually around 1 mL per minute for an analytical column, and can blend two solvents in a programmed ratio. The autosampler injects a measured volume of sample, often 5 to 20 microliters. The column, held in a thermostatted oven, performs the separation. The detector records what emerges, and the data system converts the detector signal into a chromatogram and integrates it.³

Reversed-phase separation of peptides

The term reversed-phase is historical. Classical chromatography used a polar stationary phase and a nonpolar mobile phase; reversed-phase inverts this, with a nonpolar stationary phase and a polar (aqueous) mobile phase. For peptides this is by far the dominant mode.^4,5

The stationary phase is silica whose surface has been chemically coated with hydrocarbon chains, most often 18 carbons long (C18) for peptides, sometimes C8 or C4 for larger, more hydrophobic molecules. When a peptide in aqueous solution passes over this surface, its hydrophobic side chains (leucine, phenylalanine, tryptophan and others) partition onto the hydrocarbon layer. The more hydrophobic the peptide, the more strongly it adsorbs and the longer it is retained. Hydrophilic peptides, or fragments rich in charged residues, adsorb weakly and elute early.⁴

To release the adsorbed peptides in order, the mobile phase is made progressively less polar by increasing the proportion of an organic solvent, almost always acetonitrile, over the course of the run. This is a gradient. A typical analytical gradient for a peptide might rise from 5 percent to 65 percent acetonitrile over 30 minutes. As the organic content climbs, each peptide reaches a point at which the mobile phase competes effectively for it and it desorbs, moving down the column as a tight band. Because desorption is sharp, gradients produce narrow peaks even for mixtures spanning a wide range of hydrophobicity.^4,5

One more component is essential. Peptides carry charges, and charged groups interact badly with residual silanol groups on the silica surface, causing broad, tailing peaks. Adding 0.1 percent trifluoroacetic acid to both mobile-phase solvents fixes this. TFA keeps the pH near 2 so that carboxyl groups are neutral, and the trifluoroacetate anion pairs with positively charged amines on the peptide, masking their charge and making the peptide behave as a more uniform hydrophobic entity. The result is sharp, symmetrical peaks.⁴ It is also why peptides purified this way are supplied as TFA salts, a point covered in our synthesis note.

A chromatogram is a race in which every molecule runs the same course; the peptide’s hydrophobicity decides when it crosses the line.

Detection

As components leave the column they pass through a flow cell where a detector measures them. For peptides the standard detector is ultraviolet absorbance. The peptide bond absorbs strongly between 205 and 220 nm, so detection at 214 or 220 nm sees every peptide in the sample in proportion to roughly the number of amide bonds it contains. Aromatic side chains absorb at 254 and 280 nm, so those wavelengths see only peptides containing tryptophan, tyrosine or phenylalanine. A purity measurement intended to capture all sequence-related impurities should be made at the low wavelength, and a certificate should state which was used.^4,5

Increasingly, the HPLC outlet is connected directly to a mass spectrometer (LC-MS). The mass spectrometer measures the molecular weight of whatever elutes at each moment, so each peak in the UV trace can be assigned a mass. This turns a chromatogram from a set of anonymous peaks into a list of identified species: the main peak at the expected mass, a small peak 16 daltons heavier that is an oxidation product, a peak one residue lighter that is a deletion sequence.^6,7 Our planned note on mass spectrometry covers this.

Reading a chromatogram

The output is a plot of detector signal against time. Its features have names.³

Feature	Meaning	What to look for on a COA
Retention time	Time from injection to peak maximum; characteristic of a compound under fixed conditions	Main peak retention time consistent between lots run on the same method
Peak area	Integrated signal under a peak; proportional to amount for compounds with similar absorbance	Basis of the purity calculation
Peak width and symmetry	Narrow, symmetrical peaks indicate good column performance and adequate ion pairing	Broad or tailing main peak suggests method problems that may hide impurities
Resolution	Degree of separation between adjacent peaks; a value above about 1.5 means baseline separation	Impurity peaks clearly separated from the main peak rather than as shoulders
Baseline	Signal in the absence of analyte; should be flat and low	Drifting or noisy baseline complicates integration of small peaks
Solvent front / injection peak	Unretained material eluting at the column dead time	Usually excluded from the purity calculation; check whether it was

Purity is computed as the area of the main peak divided by the sum of all integrated peak areas, multiplied by 100. Every choice in that computation matters: where the baseline is drawn, how small a peak must be to count, and whether the solvent front is included. Pharmaceutical laboratories fix these choices in a written method; research-grade certificates should at minimum state the wavelength, column and gradient so a reader can judge how discriminating the analysis was.^5,8

Analytical versus preparative HPLC

The same technique serves two purposes in peptide production. Preparative HPLC uses wide columns, high flow rates and large sample loads to separate grams of crude peptide into fractions, which are then pooled to reach a purity target and lyophilized. Analytical HPLC uses narrow columns and microgram sample loads to measure the purity of the result. A certificate of analysis reports the analytical run. Because preparative fractions are pooled according to how the operator chooses to cut the main peak, the purity of the final product reflects a decision as well as a chemistry: cutting narrowly gives higher purity and lower yield, cutting broadly the reverse.^4,9

Making the measurement trustworthy

An HPLC purity number is only as good as the method behind it, and two documents define what a trustworthy method looks like. The United States Pharmacopeia’s general chapter <621> sets out how chromatographic systems are described and how their fitness is confirmed before each run, through system suitability tests that check resolution, peak symmetry (tailing factor), and the reproducibility of replicate injections.⁸ The International Council for Harmonisation’s Q2(R2) guideline, finalized in 2023 and adopted by FDA in 2024, describes how an analytical procedure is validated: its specificity (it measures what it claims to and separates it from likely impurities), linearity and range, accuracy, precision, and detection and quantitation limits.¹⁰

Research-grade material is not required to meet these standards, but they define the questions a careful reader can ask of any certificate. Was the method shown to separate the target from its expected impurities? Was it run at a wavelength where those impurities are visible? Is the reported figure the result of one injection or several? Is the laboratory independent of the manufacturer? Wednesday’s planned note on third-party testing addresses the last question directly.

How HPLC and mass spectrometry work together on a certificate

A complete research-grade certificate pairs the chromatogram with a mass spectrum, and the two are read together. The chromatogram establishes that the sample is dominated by one species and shows how much of the UV-absorbing material is something else. The mass spectrum establishes that the dominant species has the molecular weight calculated from the intended sequence. Neither result alone is sufficient: a single clean peak of the wrong mass is a pure sample of the wrong compound, and a correct mass in a sample with many peaks is the right compound in poor condition. The Wednesday COA library presents both for each lot, with the method conditions listed, so that the purity figure can be read against the chromatogram that produced it rather than taken on its own.

The measurement behind the number

HPLC turns a question about a vial into a picture: a trace with one tall peak and, ideally, very little else. The purity percentage is a summary of that picture, and like all summaries it is more useful when the original is available. Understanding how the separation works, why the wavelength matters, and what the method cannot detect is what makes a certificate legible. The remaining notes in the Lab & Quality series build on this one to cover identity by mass spectrometry, net peptide content and endotoxin testing, the other measurements that complete the description of what is in the vial.

Frequently asked questions

What is HPLC in simple terms?

High-performance liquid chromatography is a technique that separates the components of a mixture by pumping them under pressure through a column packed with fine particles. Different components stick to the packing to different degrees and emerge at different times, producing a series of peaks. The size of each peak indicates how much of that component was present.

How does HPLC measure peptide purity?

The peptide sample is separated on a reversed-phase column, and each component is detected by its ultraviolet absorbance as it elutes. The area under the main peptide peak is divided by the total area of all peaks to give a purity percentage. This compares the target peptide with its sequence-related impurities, such as deletion or oxidized variants.

What is reversed-phase HPLC?

Reversed-phase HPLC uses a nonpolar stationary phase, usually silica coated with C18 hydrocarbon chains, and a polar aqueous mobile phase. Hydrophobic molecules are retained longer than hydrophilic ones. A gradient of increasing acetonitrile releases them in order of hydrophobicity. It is the standard mode for peptide analysis and purification.

What does the wavelength on an HPLC report mean?

It is the ultraviolet wavelength at which the detector measured absorbance. At 214 or 220 nm the peptide backbone itself absorbs, so all peptide impurities are visible. At 254 or 280 nm only aromatic side chains absorb, so impurities lacking those residues may not appear and purity can read artificially high.

Can HPLC tell you what a peptide is?

Not on its own. HPLC shows how many components are present and in what proportion, but a peak’s position does not identify it. Identity is confirmed by mass spectrometry, which measures the molecular weight of the main peak and compares it with the value calculated from the intended sequence. A good certificate of analysis reports both.

References & further reading

1. Ettre LS, Sakodynskii KI. M. S. Tswett and the discovery of chromatography I: early work (1899–1903). Chromatographia. 1993;35(3-4):223-231. doi:10.1007/BF02269707
2. Aguilar MI. HPLC of peptides and proteins: basic theory and methodology. Methods Mol Biol. 2004;251:3-8. doi:10.1385/1-59259-742-4:3 / PMID 14704434
3. Fekete S, Veuthey JL, Guillarme D. New trends in reversed-phase liquid chromatographic separations of therapeutic peptides and proteins: theory and applications. J Pharm Biomed Anal. 2012;69:9-27. doi:10.1016/j.jpba.2012.03.024 / PMID 22475515
4. Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods Mol Biol. 2007;386:3-55. doi:10.1007/978-1-59745-430-8_1 / PMID 18604941
5. Vergote V, Burvenich C, Van de Wiele C, De Spiegeleer B. Quality specifications for peptide drugs: a regulatory-pharmaceutical approach. J Pept Sci. 2009;15(11):697-710. doi:10.1002/psc.1167
6. 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
7. United States Pharmacopeia. General Chapter <1503> Quality Attributes of Synthetic Peptide Drug Substances. USP-NF. doi.usp.org
8. United States Pharmacopeia. General Chapter <621> Chromatography. USP-NF. Harmonized text official 1 December 2022. usp.org
9. Isidro-Llobet A, Kenworthy MN, Mukherjee S, et al. Sustainability challenges in peptide synthesis and purification: from R&D to production. J Org Chem. 2019;84(8):4615-4628. doi:10.1021/acs.joc.8b03001
10. International Council for Harmonisation. Validation of Analytical Procedures Q2(R2). Step 4, November 2023; adopted by FDA as guidance, March 2024. fda.gov / Federal Register