When a supplier prints "99.2% purity" on a certificate of analysis, the buyer is expected to trust that number. But the number is not the evidence. The evidence is the chromatogram, the squiggly trace that the HPLC instrument produces during the actual run. A research buyer who can read a chromatogram, even at a basic level, is much harder to fool than one who only reads the summary figure.
This guide walks through what a real HPLC purity report looks like, what the visual features mean, and how to spot the documented patterns of fabricated reports that have shown up in the unregulated peptide market. The goal is not to turn you into an analytical chemist. It is to give you enough literacy to ask the right questions and catch obvious problems.
Why this matters: the COA is the supplier's primary verification document. Learning to read the chromatogram inside it is the difference between trusting a number and verifying a number.
The audience is the research buyer who has received a COA for a compound like BPC-157, GHK-Cu, or Retatrutide and wants to actually evaluate it rather than file it away unread. None of this article addresses how the compounds are used. It is entirely about reading the document.
Anatomy of an HPLC chromatogram
A chromatogram is just a two-dimensional plot. Once you know what each axis means, the rest follows.
The two axes
- Horizontal axis (X): retention time, measured in minutes, from sample injection to detection
- Vertical axis (Y): detector response, usually in milli-absorbance units, at a specific UV wavelength (typically 214 nm or 220 nm for peptides)
What a peak is
Each compound in the sample shows up as a peak, roughly bell-shaped, rising from the baseline at its characteristic retention time and falling back down. The chromatography software measures the area under each peak. The purity of the main compound is reported as the area of the dominant peak divided by the total area of all peaks, expressed as a percentage.
What a credible chromatogram includes
- A flat or very gently sloping baseline
- Sharp, symmetric peaks
- Clear separation between adjacent peaks
- Axis labels with units
- A header identifying: the sample, batch number, column used, mobile phase, flow rate, injection volume, detection wavelength, and date of the run
- A software signature or report identifier
A chromatogram missing any of these standard fields is incomplete. It might still be real, but it is harder to verify.
How to read the purity number and the conventions behind it
Purity by HPLC is calculated as the area of the principal peak divided by the total area of all peaks above the integration threshold, times one hundred.
The integration threshold matters
The integration threshold is a software setting that decides how small a peak must be before it is ignored. Different labs use different thresholds, usually 0.05% to 0.1% of the principal peak height. A threshold set too high will hide legitimate impurity peaks and inflate the apparent purity number.
Wavelength matters too
A purity figure should always state the wavelength at which it was measured. Peaks absorb light differently at different wavelengths, so purity at 214 nm and purity at 280 nm for the same sample can be different numbers. A bare "99% purity" with no wavelength is incomplete documentation.
What is a credible purity figure?
- 99% at 214 nm for a well-synthesised research peptide is a credible result
- Above 99.9% should prompt scrutiny, because that is at the upper limit of what well-tuned HPLC systems routinely deliver, and it is sometimes the product of integration parameters that excluded real impurity peaks
- Reported at multiple wavelengths or by multiple methods is the strongest signal, because consistency across orthogonal methods beats any single high number
Rule of thumb: a credible supplier reports purity with its wavelength and methodology. A supplier reporting only a bare percentage figure is showing you less than they should.
Visual features that distinguish a credible chromatogram
A few visual features separate a credible chromatogram from a poor or fabricated one. None require analytical training to spot.
The baseline
The baseline should be flat, or have a gentle drift if the run uses gradient elution (where the mobile phase composition changes during the run). What is not normal:
- Spikes or oscillations (suggest detector noise)
- Step changes (suggest instrument malfunction)
- Wild swings (undermine the integration entirely)
Peak shape
The principal peak should be sharp and symmetric. Width at half-height should be small relative to the retention time. The technical term for asymmetry is tailing, reported as a tailing factor.
- Tailing factor near 1: symmetric, good
- Tailing factor above 2: column degradation or sample overload, suspect result
Other peaks should exist
This one surprises people. A credible chromatogram shows small impurity peaks near the main peak. Real synthesis produces real impurities. They should be visible above the baseline at low levels.
Red flag: a chromatogram that shows only one tall peak with absolutely nothing else visible is suspicious. It implies either an integration threshold so high that real impurities are hidden, or a chromatogram that has been edited. Honest synthesis leaves an honest trace.
How to detect fabricated or edited reports
Fabricated HPLC reports show up in the unregulated peptide market and have several recognisable patterns. The buyer does not need analytical training to catch most of them, just attention and a willingness to cross-reference.
Six fabrication patterns
- Template chromatograms reused across products. Compare COAs for different compounds from the same supplier. If the chromatograms look identical with only the header text changed, all of them are untrustworthy.
- Missing axis labels, units, or instrument metadata. Real chromatography software always outputs these. Their absence means the image was edited or generated outside genuine software.
- Pixel-level inconsistencies in the image. Look for visible compression artifacts around peak edges, mismatched font rendering between the chromatogram and surrounding text, or resolution that varies across the image.
- Mathematical mismatch between the reported purity and visible peak areas. A buyer can eyeball the peak areas and check whether the reported percentage is plausible. If the math does not work, the report is fabricated.
- Missing batch numbers, dates, or analyst identification. Real QC documentation always carries this provenance.
- Supplier cannot provide raw data files on request. Raw chromatography data files are produced by the instrument software and contain the underlying numerical traces. A supplier with genuine reports can produce them. A supplier with fabricated reports cannot.
The raw data test is the strongest single check. Asking for the underlying data file is a polite, professional request that separates legitimate suppliers from fraudsters. Take the absence of an answer as your answer.
What HPLC alone does not tell the buyer
HPLC purity is a powerful technique but it is not the whole picture. Understanding what it does not cover keeps you from over-trusting a single number.
The co-elution problem
HPLC separates compounds by how they behave on the column, which depends on molecular size, hydrophobicity, and interactions with the stationary phase. Two different compounds with similar behaviour will come off the column at the same time. They will appear as a single peak. The reported purity number will look great. The actual identity of the compound is unverified.
Why mass spectrometry is the partner method
Mass spectrometry measures the molecular weight of the eluting compound. If the dominant peak has the expected molecular weight of the target peptide, the identity is confirmed. If it has a different molecular weight, the buyer has discovered a near-isobaric impurity or a substituted compound.
What this means for COA evaluation
The combination of HPLC and mass spectrometry is the analytical standard for peptide identity and purity confirmation. Reputable suppliers issue COAs that include both. A COA that reports only HPLC purity without mass spectrometry data is incomplete and should be supplemented by independent testing before any high-value procurement decision.
Method parameters that distinguish credible chromatography
Beyond the chromatogram image, the method parameters in the report header give the buyer additional verification leverage. A complete header lets a second analyst reproduce the run. An incomplete header reduces the document to an assertion.
What the column section should include
- Manufacturer
- Dimensions (length and internal diameter)
- Particle size
- Stationary phase chemistry
For peptide work, a typical column is a C18 reversed-phase, 150-250 mm long, 2.1-4.6 mm internal diameter, with 1.7-5 micrometre particles.
What the mobile phase section should include
- Composition (usually water and acetonitrile)
- Ion-pairing agent (typically trifluoroacetic acid or formic acid)
- Gradient profile (percent organic at start, percent at end, run duration)
- Flow rate (typically 0.2-1.0 mL/min for analytical columns)
What the sample section should include
- Injection volume (typically 5-25 microlitres)
- Sample concentration matched to the detector's linear range
Why this matters for buyer evaluation
Reports from contract analytical laboratories with regulated-industry experience uniformly include all of this information. Reports from less mature operations frequently omit it. Absence is not proof of fabrication, but it is a marker that the supplier's documentation infrastructure is below the market norm for peptide QC.
What to do with this knowledge: when comparing suppliers, set complete method documentation as a baseline. Suppliers below that bar are easy to deprioritise.
References
- [1] Snyder LR, Kirkland JJ, Dolan JW (2010). Introduction to Modern Liquid Chromatography, 3rd Edition. Wiley. PMID n/a
- [2] Mant CT, Hodges RS (2002). Analysis of peptides by high-performance liquid chromatography. Methods in Enzymology. PMID 12073334
- [3] United States Pharmacopeia (2022). General Chapter 621: Chromatography. USP-NF. PMID n/a
- [4] Dolan JW (2003). Peak tailing and resolution. LC GC North America. PMID n/a
- [5] Aguilar MI (2004). HPLC of peptides and proteins: methods and protocols. Methods in Molecular Biology. PMID 14970507
Frequently asked questions
What does a credible HPLC chromatogram look like?
A flat baseline, sharp and symmetric peaks, clear separation between adjacent peaks, axis labels with units, and a header identifying the sample, batch, column, mobile phase, flow rate, detection wavelength, and run date. Software signatures or report identifiers should be present.
What wavelength is typically used for peptide HPLC?
214 nanometres or 220 nanometres for backbone amide detection. 280 nanometres is also used when aromatic residues are present. Purity figures should always be reported with the wavelength at which they were measured because absorbance is wavelength-dependent.
Is a purity figure above 99.9 percent always credible?
It warrants scrutiny. Such figures are at the upper limit of what well-tuned HPLC systems routinely deliver and are sometimes produced by integration thresholds set high enough to exclude legitimate impurity peaks. Consistency across orthogonal methods is a stronger indicator than any single high number.
Should a chromatogram ever show no impurity peaks at all?
No. Real synthesis produces real impurities, and a credible chromatogram shows them honestly above the integration threshold. A trace that displays only one peak with no visible smaller peaks is suspicious and suggests editing or a misconfigured integration threshold.
How can a buyer detect a fabricated chromatogram?
Look for generic templates reused across products, missing axis labels or instrument metadata, pixel-level inconsistencies in the image, mathematical mismatch between the reported purity and visible peak areas, missing batch or date information, and inability of the supplier to provide raw data files on request.
Why is HPLC alone insufficient to confirm compound identity?
HPLC separates by retention behaviour, and two different compounds with similar retention will co-elute as a single peak, inflating apparent purity. Mass spectrometry combined with HPLC confirms that the dominant peak matches the expected molecular weight of the target compound.
What is the tailing factor and why does it matter?
The tailing factor is a numerical descriptor of peak asymmetry. Values near one indicate symmetric peaks. Values substantially above two indicate column degradation or sample overload and suggest the chromatographic result may not be reliable for accurate integration.
