HPLC Explained: How Labs Measure Peptide Purity

What does HPLC actually do?

A pump pushes a liquid, the mobile phase, at high pressure through a narrow steel column packed with very fine particles, the stationary phase. A small amount of sample is injected into the flowing liquid. As the sample travels, its different molecules interact with the packing to different degrees, so they spread out into separate bands. A detector at the far end records each band as it passes.

The "high performance" comes from those fine particles and the pressure needed to push liquid through them: smaller particles separate more sharply. The pharmacopeias' chromatography chapter (USP <621> in the US) calls the technique pressurized liquid chromatography, noting that it's commonly called high pressure or high-performance liquid chromatography. The result, plotted against time, is a chromatogram.

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Swipe sideways to see the whole trace.

• 1The main peak, labeled BPC-157. Its share of the included peak area is the purity figure: 99.621% on this certificate.
• 2Its retention time: where the peak sits on the time axis, just after 14 minutes on this run.

Open the full certificate (PDF)

Why do different molecules come out at different times?

Peptides are usually analyzed by reversed-phase HPLC. Its packing carries hydrophobic chains, while the mobile phase is mostly water. Molecules interact with the packing to different degrees and elute in order of increasing overall hydrophobicity. “C18” refers to 18-carbon chains bonded to silica and is a common peptide packing. Mant and colleagues' methods review describes C8 and C18 silica packings as the favored choices for peptides, which come off in order of increasing overall hydrophobicity.

During the run, the lab gradually increases an organic solvent, usually acetonitrile. This gradient releases molecules from the packing in sequence. An acid such as trifluoroacetic acid is added around 0.05% to 0.1% to improve peak shape.

Think of it as a race in which each runner stops at the trackside cafés a different number of times. The comparison breaks down at the finish: there's no winner, and the order depends on the conditions, so a different column or gradient can reshuffle it.

How does the detector see a peptide?

Most peptide methods use ultraviolet detection. Peptide bonds absorb strongly below about 220 nm; 214 nm is widely used. A diode array detector (DAD) records spectra over time, allowing the lab to select a reporting wavelength and assess spectral uniformity.

As each band passes, absorbance rises and falls to form a peak. Its position is the retention time.

How is HPLC purity calculated?

The usual figure is an area percent. Software measures the area under each peak:

Purity (%) = main peak area ÷ total included peak area × 100

A main peak holding 99.6% of the total area gives a purity of 99.6%. The pharmacopeias' harmonized chromatography text describes exactly this normalization, and it leaves out peaks that come from solvents or the mobile phase, injection disturbances and peaks known not to originate from the sample, along with any below a set reporting threshold.

That calculation carries an assumption: that everything counted responds about equally at the chosen wavelength. For close relatives of the main peptide that's usually reasonable, but it isn't exact. Some side chains absorb far more strongly at 214 nm than a peptide bond does (tryptophan about 30 times as much, by Kuipers and Gruppen's measurements), and pharmacopeia methods apply correction factors when a response differs by more than about 20%.

It also means purity is a share of the signal, not a share of the weight. what a purity figure does and doesn't count explains why that distinction matters.

What does the method line on a certificate mean?

The certificate of analysis library includes a certificate for a batch of BPC-157 with the line: “Method: RP-HPLC | Column: C18 | Detection: DAD @ 214 nm.”

This identifies reversed-phase HPLC, a C18 column and diode-array detection reported at 214 nm. The certificate reports 99.621% purity against a specification of >98%, meaning the main peak accounted for 99.621% of the included integrated area under that method. how to read a certificate of analysis explains the rest of the document.

BPC-157 on this siteBPC-157 ReferenceBPC-157 ResearchRegeneration Peptides

What can a method change reveal?

As an illustrative example, the first method may report a very high purity result because a closely related impurity co-elutes with the main peptide peak. Switching to a column with different selectivity or using a thinner gradient can separate that hidden component, reducing the reported main-peak area. A different wavelength may also reveal an impurity that responded weakly at the original setting. In that case, the first method did not detect a new impurity—it failed to resolve or adequately represent one that was already present.

How is the reporting wavelength chosen?

The wavelength should be selected during method development to provide adequate sensitivity for the target peptide and relevant impurities, acceptable baseline behavior and suitable detector response. Around 214 nm is useful because peptide bonds absorb strongly, but area percent is not automatically equal to molar or weight percent. Aromatic residues—especially tryptophan, tyrosine and phenylalanine—can increase absorbance, so an impurity enriched in these residues may appear disproportionately large, while a component with fewer aromatic residues may appear smaller than its actual amount.

How do I check that the main peak is really one compound?

A clean-looking chromatographic peak is a good sign, but it does not prove that only one compound is present. Start by checking the peak shape and, when available, the DAD peak-purity or spectral-uniformity results. Next, use LC-MS to confirm that the peak has the expected mass. If there is still doubt—especially if you see a shoulder, an unusual spectrum or a change after modifying the method—an impurity may be co-eluting with the main peptide. In that case, analyze the sample using a second chromatographic method that separates compounds differently. When the peak shape, DAD assessment, LC-MS result and second method all agree, you can be more confident that the main peak represents a single compound.

What can't HPLC tell you?

It cannot establish identity from retention time alone. A peak at the expected time is consistent with the target peptide, but different molecules can arrive together. Identity therefore needs complementary evidence, usually LC-MS.

HPLC also misses or excludes some material. Water and counter-ions add powder weight but are not represented like resolved peptide impurities; solvent, mobile-phase and sub-threshold peaks may be excluded.

It also cannot separate what the method does not resolve. An impurity that travels with the peptide can hide within the main peak, which is why a second method with different selectivity may reveal what the first missed.