High performance liquid chromatography is where a purity percentage comes from. A pump pushes the sample through a packed column, the components travel at different speeds, and a detector records them as they leave. The number on a certificate is a measurement of that recording, which means it is only as meaningful as the conditions that produced it.

How the separation happens

The column is packed with a stationary phase. A solvent, the mobile phase, is driven through it under high pressure, carrying the sample along. Each component interacts with the stationary phase to a different degree, so each spends a different amount of time held back. That time is the retention time, and it is what separates one compound from another. (Reference: Dong, 2006)

For peptides it is almost always reversed phase

The word reversed is historical. The stationary phase is non-polar, usually silica with C18 alkyl chains bonded to it, and the mobile phase is polar. Peptides stick to the non-polar surface and are then washed off by gradually increasing the proportion of organic solvent, typically acetonitrile. More hydrophobic sequences hold on longer, so they come off later. (Reference: Snyder et al., 2009)

Why there is acid in the mobile phase

Peptide methods usually include a small amount of trifluoroacetic acid. It has two jobs: it keeps the pH low so that ionisable groups stay in one consistent state, and it acts as an ion-pairing agent, associating with protonated basic sites on the peptide. The result is sharper peaks and better separation of closely related sequences. The effect depends on concentration and on which acid is used, and it has been studied systematically. (Reference: Shibue et al., 2005)

This connects to something that appears later on the certificate. That same trifluoroacetate can remain bound to the material after purification, which is why a peptide is typically isolated as a salt and why part of the powder you weigh is counterion rather than peptide.

What the detector sees

Most peptide work uses ultraviolet detection, and the wavelength is not arbitrary.

  • Around 214 nm the amide bond itself absorbs. Every peptide has amide bonds, so this wavelength detects essentially any sequence.
  • At 280 nm the absorbance comes mainly from aromatic side chains, tryptophan and tyrosine. A peptide with no aromatic residues is close to invisible at this wavelength.

That difference has a consequence worth knowing. Two runs of the same sample at different wavelengths can produce different purity figures, because they are not detecting the same things with the same sensitivity.

Gradient and isocratic runs

An isocratic method holds the mobile phase composition constant for the whole run. A gradient method changes it, usually raising the organic proportion steadily over time. Peptides are almost always run as gradients, because a single fixed composition that releases one sequence promptly will hold another on the column far too long, or wash both off together with no separation at all.

The shape of that gradient is a real variable. A shallow gradient spreads peaks out and resolves species that a steep one merges into a single peak. Two laboratories can analyse the same batch, both report in good faith, and arrive at different purity figures because one gave the separation more room than the other. It is another reason the conditions belong next to the number.

What the trace shows

If a certificate includes the trace, look at it. One tall, symmetrical peak on a flat baseline is the picture you want. Small peaks close to the main one are common in synthesis and usually correspond to closely related species, such as truncated or modified sequences. A shoulder on the main peak means the method resolved it with little margin, which is exactly why the method conditions belong on the document. (Reference: Kazakevich & LoBrutto, 2006)

Area percent is not weight percent

This is the most common misreading of an HPLC result. Purity is normally reported as the area of the target peak divided by the total area of all detected peaks. It answers a question about the detected material: of everything this method saw, what share was the target?

It does not answer what share of the powder is peptide. Counterion, absorbed water and anything without a chromophore at the chosen wavelength are not in that calculation at all. A batch can be 99 percent pure by area and still be well below 99 percent peptide by mass, with no contradiction between the two figures. They measure different things, which is why a thorough certificate of analysis reports both.

The conditions are part of the result

A purity figure with no method behind it is a statement, not a measurement. The column, the gradient, the flow rate, the detection wavelength and the run time all change what the number comes out as. Reported together, they let someone else repeat the analysis. Reported alone, the percentage asks to be taken on trust. Guidance on validating analytical procedures exists precisely because a result is inseparable from the procedure that produced it. (Reference: Swartz & Krull, 2012)

What HPLC cannot do

  • It separates, it does not identify. A detector reports a peak at a retention time. It has no opinion about what the molecule is, which is why purity is paired with mass spectrometry.
  • Things can hide under the peak. An impurity that elutes at the same time as the target is counted as target. Deletion sequences, which differ from the intended peptide by a single residue, are the usual candidates.
  • Response is not uniform. Two compounds at the same concentration do not necessarily give peaks of the same area.

Used for what it is good at, it remains the standard way to establish how much of a batch is the thing it claims to be, and the number it produces is one of the two results that should carry a batch number on any vial you can check.

Check this against a real batch

Every compound in the catalog lists its formula, molecular weight and CAS number, and every batch we ship has a certificate you can look up by number.

Frequently asked questions

What is HPLC used for in peptide work?

It separates the components of a sample and quantifies them, which is how a purity percentage is produced. It is the standard method for establishing how much of a batch is the target compound.

What is retention time?

How long a compound takes to travel through the column and reach the detector. Components that interact more strongly with the stationary phase are held back longer, and that difference is what separates them.

Why is reversed-phase HPLC used for peptides?

The stationary phase is non-polar and the mobile phase is polar, so peptides bind to the column and are released as the proportion of organic solvent rises. More hydrophobic sequences elute later, which separates closely related peptides well.

Why is trifluoroacetic acid added to the mobile phase?

It holds the pH low so ionisable groups stay in a consistent state, and it acts as an ion-pairing agent that sharpens peaks. The same trifluoroacetate can remain with the material afterwards as a counterion.

Does 214 nm or 280 nm matter for a purity result?

Yes. At around 214 nm the amide bond absorbs, so essentially any peptide is detected. At 280 nm the signal comes mainly from tryptophan and tyrosine, so a peptide without aromatic residues is barely visible. The same sample can give different purity figures at different wavelengths.

Is 99 percent by HPLC the same as 99 percent peptide?

No. HPLC purity is the target peak area as a share of total detected peak area. Net peptide content is the share of the weighed powder that is peptide rather than counterion and water. A batch can be 99 percent pure by area and well below that by mass.

References

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