What do the axes on an HPLC chromatogram mean?

The horizontal axis is retention time in minutes: how long a compound stayed on the column before eluting. The vertical axis is detector signal, usually UV absorbance in milli-absorbance units at 214 nm, the wavelength the peptide bond absorbs. Each resolved peak is produced as a compound elutes, and its area reflects the UV response, which depends on both the amount and how strongly that compound absorbs.

How is purity calculated from a chromatogram?

The software integrates the area under every peak, then divides the main peak's area by the total integrated area. A result of 99% means the target peak accounts for 99% of the signal. It is a ratio of integrated UV areas, so it tells you the target's share of the total peak area, not how many milligrams are in the vial. Because different compounds absorb UV differently, it is also not exactly a mass fraction.

What is a shoulder on a peak?

A shoulder is a partly resolved feature on the side of the main peak, not separated all the way down to the baseline. It often indicates an impurity chemically very close to the target, such as a chain missing one amino acid or one with a single altered residue, because structurally similar molecules elute at similar times. A shoulder can also be a method or overload artefact, so its shape alone does not confirm what it is.

What is co-elution and why does it matter?

Co-elution is when two different compounds leave the column at the same time and merge into a single apparent peak. On the trace it looks like one clean peak, and a purity calculation counts it as one, so an impurity can hide inside the main peak. It is one reason a purity percentage alone can overstate chromatographic purity, and why an orthogonal method such as mass spectrometry or a second gradient adds real information.

Where do the impurities in a peptide come from?

Most are created during synthesis. A failed coupling leaves a chain missing a residue (a deletion sequence); an early stop leaves a truncated chain; the chemistry of activating each residue can flip one to its mirror-image form (epimerization). Others form later: methionine or tryptophan can oxidize, and asparagine or glutamine can deamidate. Some families have a typical position on the trace, but position alone rarely identifies them, and several have no fixed location.

Why can mass spectrometry miss a D-isomer impurity?

A D-isomer, formed when one residue flips to its mirror-image configuration during synthesis, has exactly the same atoms and the same mass as the correct peptide. Mass spectrometry identifies compounds by weight, so it sees no difference. Only the retention-time separation on HPLC can reveal it, and only if the method resolves the two. This is why HPLC and mass spec are complementary rather than redundant.

What causes oxidation and deamidation peaks?

Oxidation adds oxygen to sensitive residues such as methionine, tryptophan or cysteine, increasing the mass by about 16 Da and usually making the molecule more polar so it elutes earlier. Deamidation converts asparagine or glutamine to an acidic form, adding about 1 Da and producing a shoulder very close to the main peak. Both can also increase during improper storage, not only during synthesis.

Why do two labs report different purity for the same peptide?

Because purity is method-dependent. A shallower gradient or a longer column can resolve peaks that a faster method merges, which can lower the reported purity by splitting a co-eluting pair. The detection wavelength changes what is seen, and the analyst's integration and baseline choices move the number. A serious certificate names the method so the figure can be read in context.

What is the void or injection peak at the start of the run?

It is the signal from material that does not interact with the column and washes straight through at the beginning, along with any injection artefact. It is normally excluded from the purity calculation because it is not a retained, resolved compound. A certificate that discloses this exclusion is being transparent about how the number was built.

Is a single sharp peak enough to trust a peptide?

It is good evidence of chromatographic purity, but it is narrow. A clean HPLC peak does not by itself confirm the peptide's identity (that needs mass or sequence data), does not tell you the quantity in milligrams, and says nothing about microbial or endotoxin contamination, which HPLC does not measure. It is one necessary piece of a fuller picture.

What wavelength are peptide chromatograms usually run at?

Most peptide purity work uses UV detection at 214 nm, because the peptide bond absorbs strongly there, so nearly every peptide-related species is visible. Some methods add 280 nm, which mainly detects aromatic residues such as tryptophan and tyrosine. Running both can help characterise which impurities carry aromatic groups.

Why does a longer peptide tend to have more impurities?

Every amino acid added is another coupling step, and every coupling step is another chance for a deletion, a truncation or an epimer to form. A longer, more complex sequence therefore accumulates more opportunities for impurities, which is one reason some peptides are genuinely harder to synthesize to a high purity at scale than others.

How does the EMA threshold for co-eluting peaks work?

The EMA's synthetic-peptide guideline applies the European Pharmacopoeia thresholds: impurities reported from 0.1%, identified from 0.5%, and it states that where co-eluting impurities appear as one peak, a qualification threshold of 1.0% applies unless otherwise justified. It is a rule for approved medicines, not research chemicals, but it captures how analysts read a peak they cannot fully resolve: one observed peak may contain more than one unresolved impurity, so the 1.0% qualification threshold applies to the combined peak unless justified.

Can I see the actual chromatograms behind your purity numbers?

The purity figure for each batch we sell comes from its lab report, and those reports are published on our lab reports page with the testing lab named and a verification link where the lab provides one. The purity overview page lists the figures side by side. Where a report includes the full trace, it is in the linked document.

Does a good chromatogram mean the vial is safe to use?

No. Purity is a materials-quality measurement of a research chemical, not a safety assessment. HPLC does not test sterility, endotoxin or biological safety, and none of these products are approved for human use. A chromatogram tells you about the composition of a sample, nothing more.

How to Read an HPLC Chromatogram: Peaks, Shoulders, and Where Impurities Come From

Q: Does TFA show up as a peak on the chromatogram?

Not usually. Trifluoroacetic acid is the acid modifier in the mobile phase and the counterion paired with the peptide in the dried salt. It does not typically appear as a meaningful impurity peak in the 214 nm trace. Its relevance is to net peptide content, the share of the vial's weight that is actually peptide rather than counterion and water, which is a separate measurement from HPLC purity.

A certificate reduces a whole chromatogram to a single number: "purity 99.2%". That number is an area calculation over a trace with a shape, and the shape is where the information is. A shoulder on the main peak, a cluster of small peaks late in the run, two things hiding under one apex: each has a cause worth understanding. Most trace back to how the peptide was made, some to how it was stored or handled, and some to the method itself.

This article is about reading the trace, not comparing techniques. We already cover HPLC versus mass spectrometry, grades, and counterions separately. Here the question is narrower and more practical: when you look at the plot behind the number, what are you actually looking at?

TL;DR: what the trace is telling you

The x-axis is time, the y-axis is absorbance. A peptide chromatogram plots detector signal (usually UV at 214 nm) against retention time in minutes. Each peak is detector signal from material eluting at a given moment. Purity is one peak's area as a share of all the area. "99.2%" means the main peak is 99.2% of the integrated signal. It is a ratio, not a mass. A shoulder often marks a near-identical impurity. Deletion sequences, deamidation and oxidation products elute close to the target because they are chemically close to it. Co-elution is the trap. Two compounds under one peak read as pure. This is exactly why the EMA applies its qualification threshold to the whole co-eluting peak, and why a second method matters. The one impurity mass spec cannot flag: a mirror-image isomer. Same molecular formula and mass, different stereochemistry. Only HPLC retention time separates it, and only if the method resolves the two.

The two axes, and the main peak

A reversed-phase HPLC run pushes the sample through a column with a gradient: the solvent starts mostly water and becomes progressively more organic. Compounds that barely interact with the column leave early; more hydrophobic ones are held back and leave later. A detector at the end measures how much UV light the eluting stream absorbs, typically at 214 nm, the wavelength the peptide bond itself absorbs. The plot is that absorbance against time.

The tallest, largest peak is normally the target peptide. Purity by HPLC is an area calculation: the software integrates the area under every peak, and the main peak's area divided by the total integrated area is the purity percentage. Two consequences follow immediately, and both matter when you read a real certificate:

It is a proportion, not a quantity. A 99% pure peptide can still be underfilled. Purity says nothing about how many milligrams are in the vial. That is a separate measurement, and we walk through the difference in reading an actual certificate field by field.
What gets excluded changes the number. The injection or void peak at the very start, where unretained material washes through, is normally left out of the calculation. So are peaks the analyst attributes to the solvent or the blank. Change what is integrated and the percentage moves, which is one reason the same sample can produce different purity figures in different labs.

Shoulders, tailing, and co-elution

Once you can see the main peak, the impurities are the other analyte peaks, after you set aside the void, injection, solvent and blank signals a lab documents. Their position is a clue to what they probably are, not proof.

A shoulder is a partly resolved peak sitting on the flank of the main one, often close enough that the two are not separated back to baseline. Shoulders commonly come from impurities that are chemically almost the target: a chain missing one amino acid, or one where a single residue has been chemically altered. Such species are close in structure, so they are close in retention time. A shoulder can also be a method or overload artefact, so its shape alone does not identify what it is.

Tailing is when a peak drags out on its trailing side instead of coming down symmetrically. It is often a column or method artefact rather than a separate compound, but a heavy tail can also hide a small impurity eluting just after the main peak.

Co-elution is the one to respect. Two different compounds can leave the column at the same time and merge into a single apparent peak. On the trace they look like one clean peak, and the purity number counts them as one. This is the failure mode a purity percentage cannot warn you about by itself, and it is precisely why regulators treat co-eluting peaks more cautiously.

How the EMA treats co-eluting peaks

The EMA's guideline on synthetic peptides (EMA/CHMP/CVMP/QWP/367182/2025, in force since 1 June 2026) works from the European Pharmacopoeia thresholds: impurities are reported from 0.1%, identified from 0.5%, and it states that "when co-eluting impurities are observed as one peak the qualification threshold of 1.0% applies unless otherwise justified". In plain terms: because one observed peak can contain more than one unresolved impurity, the guideline applies the 1.0% qualification threshold to the combined co-eluting peak unless there is justification to do otherwise. That guideline governs approved medicines, not research chemicals, and none of the products discussed here fall under it. It is simply a published example of how the analytical community treats a peak it cannot fully resolve. We go through the whole document in our guide to the EMA guideline.

Where the impurities actually come from

The small peaks on a peptide chromatogram are not random noise. Each family has a chemical origin, and many are created during synthesis, one coupling step at a time, though oxidation and deamidation can also grow later during storage. Understanding the origin is what turns "there are some little peaks" into "that is a deletion sequence, and here is why it is there".

Impurity family	Chemical origin	Where it shows on the trace	Can mass spec tell it apart?
Deletion sequence	A coupling step failed, so the chain is missing one residue	Close to the main peak, often a shoulder; direction depends on the missing residue	Yes, it is lighter by that residue's mass
Truncated sequence	The chain was capped or stopped early	Varies with the residues lost	Yes, clearly lighter
Deamidation	Asparagine converts to aspartate or isoaspartate, glutamine to glutamate or iso-glutamate, adding about 1 Da	A shoulder very close to the main peak	Only just: a 1 Da shift is easy to miss
Oxidation	Methionine, tryptophan or cysteine picks up oxygen, adding about 16 Da	Usually earlier, because the product is more polar	Yes, plus 16 Da
Epimerization (D-isomer)	A single residue flips to its mirror-image form during synthesis	A separate peak, sometimes well resolved, sometimes a shoulder	No: identical mass, only retention time separates them
Incomplete deprotection	A protecting group was not fully removed	Often later (protecting groups are usually hydrophobic), but it depends	Yes, heavier by the group's mass
Aggregates and adducts	The peptide sticks to itself or to a scavenger molecule	Variable, often broad or late	Sometimes, depending on the species

Deletion sequence

Chemical origin: A coupling step failed, so the chain is missing one residue
Where it shows on the trace: Close to the main peak, often a shoulder; direction depends on the missing residue
Can mass spec tell it apart?: Yes, it is lighter by that residue's mass

Truncated sequence

Chemical origin: The chain was capped or stopped early
Where it shows on the trace: Varies with the residues lost
Can mass spec tell it apart?: Yes, clearly lighter

Deamidation

Chemical origin: Asparagine converts to aspartate or isoaspartate, glutamine to glutamate or iso-glutamate, adding about 1 Da
Where it shows on the trace: A shoulder very close to the main peak
Can mass spec tell it apart?: Only just: a 1 Da shift is easy to miss

Oxidation

Chemical origin: Methionine, tryptophan or cysteine picks up oxygen, adding about 16 Da
Where it shows on the trace: Usually earlier, because the product is more polar
Can mass spec tell it apart?: Yes, plus 16 Da

Epimerization (D-isomer)

Chemical origin: A single residue flips to its mirror-image form during synthesis
Where it shows on the trace: A separate peak, sometimes well resolved, sometimes a shoulder
Can mass spec tell it apart?: No: identical mass, only retention time separates them

Incomplete deprotection

Chemical origin: A protecting group was not fully removed
Where it shows on the trace: Often later (protecting groups are usually hydrophobic), but it depends
Can mass spec tell it apart?: Yes, heavier by the group's mass

Aggregates and adducts

Chemical origin: The peptide sticks to itself or to a scavenger molecule
Where it shows on the trace: Variable, often broad or late
Can mass spec tell it apart?: Sometimes, depending on the species

The epimerization row is the one worth pausing on. A D-isomer has exactly the same molecular formula and exactly the same mass as the correct peptide. A mass spectrometer, which identifies compounds by weight, sees nothing wrong. Only the retention-time separation on HPLC catches it, and only if the method resolves it. This is the concrete reason "we ran mass spec, it is the right peptide" and "the HPLC is 99% pure" are answering different questions, and why certificates that carry both cover more than either alone.

One thing that is not a chromatogram peak, despite being everywhere in peptide chemistry: TFA (trifluoroacetic acid). It is the acid added to the mobile phase and the counterion that pairs with the peptide's charges in the dried salt. It does not usually appear as a meaningful impurity peak in the 214 nm trace. Its relevance is to net peptide content, how much of the vial's weight is actually peptide versus counterion and water, which is a different measurement from purity entirely.

Why the same peptide gives two different purity numbers

If a purity figure were an absolute property of the material, every lab would report the same number. They do not, and the reasons are all in the method:

The column and the gradient. A shallower gradient or a longer column can resolve peaks that a faster method merges. Better resolution can lower a reported purity, because it splits a co-eluting pair the faster method counted as one clean peak.
The detection wavelength. 214 nm sees the peptide backbone; 280 nm mainly sees aromatic residues. The same sample looks different at each.
The integration choices. What the analyst includes or excludes, and where they draw the baseline under a tailing peak, moves the percentage.

None of this makes purity meaningless. It makes it method-dependent, which is why a serious certificate names the method, or at least the procedure, so the number can be read in context. A bare percentage with no method behind it is a number you cannot fully interpret, a point we make when we go through what each field on a real certificate proves.

What a clean chromatogram does not prove

A single sharp peak at 99% is good analytical news, and it is also narrow news. It does not confirm the peptide's identity on its own (that needs mass or sequence confirmation), it says nothing about how many milligrams are in the vial, and it does not touch microbial or endotoxin contamination, which HPLC never measures. A purity trace is one instrument's view of one sample. It is necessary, not sufficient.

From the trace back to the synthesis

Read enough chromatograms and a pattern appears: the impurities you see are a fingerprint of how the molecule was built. Deletion and truncation peaks come from imperfect couplings. Epimers come from the chemistry of activating each residue. The longer and more complex the sequence, the more coupling steps, and the more chances for each of these to appear. That is the direct link between the trace and the synthesis route, and it is why the same peptide can be genuinely harder to make cleanly at scale. We follow that thread in our article on how a peptide is actually made, from solid-phase to liquid-phase synthesis.

Products and Categories Referenced

Every batch we sell has a lab report behind the number on the lab reports page, and where a report includes the full trace, it is in the linked document. You can see the purity figures side by side on our purity overview.

Metabolic Researchmetabolic

GIP/GLP-1/Glucagon agonists and metabolic pathways

Retatrutidemetabolic

First-ever triple-action weight management peptide targeting three receptors at once: GLP-1, GIP, and glucagon. Shown exceptional results in Phase 2 trials - up to 24% weight reduction. The most advanced metabolic peptide available.

GLOWregeneration

3-in-1 skin peptide blend: GHK-Cu 50mg + BPC-157 10mg + TB-500 10mg. Targets collagen synthesis, tissue regeneration, and skin repair for comprehensive dermatological research.

BPC-157regeneration

Gastric pentadecapeptide (15 amino acids) known for exceptional tissue repair properties. Promotes wound healing, angiogenesis, and cytoprotection across tendons, muscles, gut, and nerves. Over 30 years of preclinical research.

TB-500regeneration

Full-length 43-amino-acid Thymosin Beta-4, a naturally occurring repair protein, independently confirmed by a third-party CoA from Janoshik. Promotes cell migration and new blood vessel formation for systemic tissue healing. Especially researched for muscle, tendon, and cardiac repair.

SS-31longevity

Mitochondria-targeted tetrapeptide (Elamipretide) that stabilizes cardiolipin and prevents ROS formation at the source.

Longer, harder-to-synthesize sequences

Retatrutidemetabolic

Tesamorelingrowth

Modified GHRH analog for lipodystrophy and metabolic liver research

Multi-peptide blends we stock

GLOWregeneration

3-in-1 skin peptide blend: GHK-Cu 50mg + BPC-157 10mg + TB-500 10mg. Targets collagen synthesis, tissue regeneration, and skin repair for comprehensive dermatological research.

KLOWregeneration

4-in-1 anti-aging peptide blend: GHK-Cu 50mg + BPC-157 10mg + TB-500 10mg + KPV 10mg. Targets collagen synthesis, tissue regeneration, skin repair, and anti-inflammatory pathways.

Frequently Asked Questions

FOR RESEARCH USE ONLY. Not for human consumption. Nothing in this article is medical advice or a therapeutic claim. Chromatographic purity describes the composition of a research chemical and is not a safety assessment for any use in humans.

The two axes, and the main peak

Shoulders, tailing, and co-elution

Where the impurities actually come from

Why the same peptide gives two different purity numbers

From the trace back to the synthesis

Products and Categories Referenced

Frequently Asked Questions

What do the axes on an HPLC chromatogram mean?

How is purity calculated from a chromatogram?

What is a shoulder on a peak?

What is co-elution and why does it matter?

Where do the impurities in a peptide come from?

Why can mass spectrometry miss a D-isomer impurity?

What causes oxidation and deamidation peaks?

Does TFA show up as a peak on the chromatogram?

Why do two labs report different purity for the same peptide?

What is the void or injection peak at the start of the run?

Is a single sharp peak enough to trust a peptide?

What wavelength are peptide chromatograms usually run at?

Why does a longer peptide tend to have more impurities?

How does the EMA threshold for co-eluting peaks work?

Can I see the actual chromatograms behind your purity numbers?

Does a good chromatogram mean the vial is safe to use?

Research context for English-speaking buyers