Liquid Chromatography Explained: How LC Separates Peptides and How an HPLC System Works

Liquid chromatography is the separation technique behind almost every purity figure printed on a peptide certificate. A liquid carries the sample through a packed column, the components of the sample travel at different speeds, and a detector records each one as it emerges. To judge a purity result rather than simply trust it, you need to know which kind of liquid chromatography produced it.

This guide defines liquid chromatography, compares its six main modes, explains why peptides are usually run in one of them, and walks through the modules of a high-performance liquid chromatography (HPLC) system and the checks that link it to a certificate.

What liquid chromatography is

Liquid chromatography (LC) is a family of separation techniques in which a liquid, the mobile phase, flows through a stationary phase, usually fine particles packed into a column. IUPAC’s definition of chromatography is deliberately general: the components to be separated are distributed between two phases, one stationary and one moving in a definite direction.

The separation itself comes from differential retention. A molecule that interacts strongly with the stationary phase leaves the column later; one that interacts weakly is swept through faster. The time from sample introduction to the top of a component’s peak is its retention time. Pushing components out with the mobile phase is elution; changing the mobile-phase composition during the run is gradient elution.

The detector’s record of this is a chromatogram: signal against time, one peak per separated component. Reading that trace is covered in our guide to reading an HPLC chromatogram; this page is about how the separation is produced.

From a chalk column to HPLC

The method began with plant pigments. In a 1906 paper in the Berichte der Deutschen Botanischen Gesellschaft, the botanist Mikhail Tswett described packing a fine dry powder such as precipitated calcium carbonate into a glass tube, washing a leaf-pigment extract through it, and watching the pigments resolve into coloured bands. He called the banded column a chromatogram; an English translation appeared in 1967.

The theory that made chromatography quantitative came from A. J. P. Martin and R. L. M. Synge. Their 1941 Biochemical Journal paper described a chromatogram employing two liquid phases, the basis of partition chromatography; they shared the 1952 Nobel Prize in Chemistry “for their invention of partition chromatography”. In 1956, van Deemter, Zuiderweg and Klinkenberg analysed why real peaks spread out, attributing non-ideal band broadening to longitudinal diffusion and resistance to mass transfer. Our HPLC vs UPLC comparison picks up how particle size follows from that theory.

Other principles followed: Porath and Flodin’s gel filtration in 1959, the ancestor of size-exclusion chromatography, and Cuatrecasas, Wilchek and Anfinsen’s affinity chromatography in 1968. In 1967 Horváth, Preiss and Lipsky reported “fast liquid chromatography”, separating nucleotides on pellicular ion exchangers, an early step toward the instrumented technique now called HPLC.

Canada has a genuine place in the peptide chapter of this history. In 1986, Robert Hodges’s group at the University of Alberta in Edmonton, part of the Medical Research Council of Canada Group in Protein Structure and Function, measured retention coefficients for individual amino acid residues in model synthetic peptides, so that reversed-phase retention could be predicted from sequence. Hodges and Colin Mant later continued it at the University of Colorado. For more Canadian peptide science, see Canada’s century of peptide discovery.

The main modes of liquid chromatography

Every LC mode uses the same plumbing; what changes is the chemistry of the two phases, and so which property drives the separation.

Reversed-phase and normal-phase

The names are defined by polarity. In IUPAC’s terms, normal-phase chromatography is an elution procedure in which the stationary phase is more polar than the mobile phase, and reversed-phase chromatography is the opposite: the mobile phase is significantly more polar than the stationary phase, for example silica carrying chemically bonded alkyl chains. IUPAC also calls “reverse phase” “an incorrect expression to be avoided”.

HILIC

Andrew Alpert introduced hydrophilic-interaction chromatography in 1990 as a variant of normal-phase chromatography. A hydrophilic column eluted with a mostly organic mobile phase retains more hydrophilic solutes more strongly, so elution order is typically the reverse of reversed-phase, and its selectivity complements other modes; Alpert applied it to peptides, including phosphorylated peptides.

Ion-exchange, size-exclusion and affinity

Ion-exchange separates by charge, useful for impurities that differ from the target by a charged group but hardly in hydrophobicity. Size-exclusion separates by size, and the Hodges group’s methods review describes using it to assess a peptide’s oligomeric state. Affinity chromatography is a purification tool far more often than a purity test. For identity, LC is coupled to a mass spectrometer, a combination covered in LC-MS explained.

Why peptides are usually run in reversed-phase

Reversed-phase HPLC is the workhorse of peptide analysis because it can resolve sequences that differ by a single amino acid, and because its retention behaviour is well understood: it can even be predicted from sequence, as the Alberta work showed. A typical run ramps from a largely aqueous mobile phase toward an organic solvent such as acetonitrile, and peptides come off in order of increasing hydrophobicity.

The acid in the mobile phase matters as much as the column. Trifluoroacetic acid is the usual anionic ion-pairing reagent: it pairs with a peptide’s positively charged groups and sharpens peaks. A 2005 Hodges-group study compared TFA with more hydrophobic perfluorinated acids on ten-residue peptides carrying +1, +3 or +5 net charge: separation improved as the reagent became more hydrophobic and, to a lesser degree, more concentrated, and more highly charged peptides responded more.

Temperature is the other lever. Testing peptides between 5 and 80 °C, Chen, Mant and Hodges found that amphipathic helical peptides responded more strongly than random-coil peptides, attributed to helices unfolding during the run, and that temperature altered selectivity between peptides differing by one amino acid.

How reversed-phase purity is calculated and reported is covered in our HPLC peptide testing guide, and what a purity percentage does and does not mean is explained in peptide purity explained.

How an HPLC system works

An HPLC system, often called a liquid chromatography machine, is a chain of modules, each controlling one variable. Walking through the HPLC instrumentation module by module shows where a purity result can go wrong.

Solvent reservoirs and degassing

Bottles hold the mobile-phase components, typically acidified water and an acidified organic solvent for peptide work. Dissolved gas is removed first, because bubbles disturb flow and detection.

Pump and gradient

The pump delivers a steady flow and blends the solvents into the programmed gradient. If the organic fraction arrives late or unevenly, retention times shift and close peaks can merge.

Autosampler and sample introduction

The autosampler draws a measured volume of sample into a loop, and a switching valve places that loop in the flowing mobile phase so the sample is swept onto the column. Variation in the volume loaded shows up as variation in peak areas, and carry-over from the previous sample is checked with blank runs.

Column and column oven

The stationary phase follows from the mode, particle size sets peak sharpness, and the oven holds a temperature that can change selectivity.

Detector

For peptides the detector is usually a UV absorbance detector set at a low wavelength. Kuipers and Gruppen measured the molar extinction coefficient of the peptide bond at 214 nm as 923 M⁻¹ cm⁻¹. Some residues absorb far more strongly: tryptophan roughly 30 times more than a peptide bond, and phenylalanine, tyrosine and histidine roughly six times more. So UV area percentage is a practical measure of purity, not an exact molar ratio.

Data system

The software draws the chromatogram and integrates each peak. Purity by area normalisation is the main peak’s area divided by the total integrated area, so where the baseline is drawn and which small peaks are counted are part of the method.

How does HPLC work, in one run

1. The column is equilibrated in the starting mobile phase.
2. The autosampler loads the sample and the gradient starts.
3. Components leave the column in order of retention and are recorded as peaks.
4. The data system integrates the peaks; the column is washed for the next sample.

System suitability and method validation

Because every module is a variable, regulated laboratories run a system suitability test before samples to confirm the complete system meets defined criteria that day. Typical checks are resolution between critical peaks, peak symmetry and the repeatability of replicate runs.

In the United States Pharmacopeia these requirements sit in General Chapter <621> Chromatography. A harmonized version, coordinated with the European Pharmacopoeia, became official on 1 December 2022. That revision added a definition of system sensitivity, a general peak-symmetry requirement of 0.8 to 1.8, and changed how signal-to-noise is measured.

System suitability checks the instrument on the day. Method validation checks that the procedure itself measures what it claims to. The ICH Q2(R2) guideline, in effect in the European Union since 14 June 2024, sets out how procedures for assay, purity and impurities, and identity are validated. Our guide to analytical method validation explains what those characteristics mean for a purity number.

From instrument to certificate

A certificate of analysis compresses a whole run into a few lines. A useful HPLC section should answer most of these questions:

• Which mode and column? Reversed-phase is expected for peptides.
• Which mobile phase and gradient? The acid used and the gradient range affect which impurities are resolved.
• Which wavelength? Near 214 nm detects the peptide backbone.
• Is the chromatogram shown? A trace shows whether small peaks were integrated.
• Is identity confirmed separately? Retention time alone does not prove identity; mass spectrometry does that job, as described in reading a peptide mass spectrum.
• Does the batch number match? A report is evidence only for the batch it names; see lot numbers and batch traceability.

Our peptide COA guide covers the rest of a certificate, and for multi-component products, how peptide blends are tested explains why a single purity figure can mislead.

What’s established and what isn’t

Sourcing peptides for chromatography work in Canada

If you are buying research peptides in Canada, the instrument knowledge above becomes a few practical questions.

• Ask how purity was measured. Mode, column, gradient and wavelength should be stated or available. Our HPLC testing guide explains what a purity figure covers.
• Check identity separately. HPLC counts components; mass spectrometry confirms the main one is the right molecule.
• Account for counter-ions. Lyophilized peptides can carry counter-ions and water, so weighed mass is not necessarily peptide mass; see net peptide content, TFA and acetate.
• Store material so the result stays true. A clean chromatogram describes the powder when it was tested; the peptide storage guide covers keeping it that way.

GHRP-2A hexapeptide containing tryptophan and D-phenylalanine, residues that absorb strongly in low UVView product GHRP-6Differs from GHRP-2 at its first two residues, a close pair for comparing retentionView product SNAP-8Eight residues and no aromatic side chains, so its UV signal comes mainly from peptide bondsView product LL-37A 37-residue cationic, amphipathic peptide, the type whose retention responds to ion-pairingView product

Frequently asked questions