Purity
The relative result reported for detected components under the stated purity analysis.
Laboratory analysis guide
HPLC Purity vs Compound Identity
Reported HPLC purity and compound identity describe different analytical questions. This guide explains how to distinguish them and how to read each result within the limits of a peptide laboratory report.
What HPLC purity measures
High-performance liquid chromatography separates detected components under the conditions described by the laboratory. A reported HPLC purity percentage generally describes the relative chromatographic response assigned to the reported component within that analysis.
The value is tied to the stated sample, method and detection conditions. It should be read from the original report together with any chromatogram, qualifiers or limitations the laboratory provides.
What the percentage does not establish
A purity percentage does not by itself establish that the detected component is the compound named on the label. It also does not independently report the sample’s net content, endotoxin result or sterility result.
Those are separate analytical questions. No conclusion about them should be inferred from an HPLC purity figure alone.
Identity requires its own evidence
Identity testing is intended to assess whether an observed property or set of properties is consistent with the named compound. A laboratory may use a separate analytical method because identity and chromatographic purity answer different questions. Read only the methods and conclusions actually recorded in the report.
Keep each test category separate
Identity
Whether the measured characteristics are consistent with the compound named in the report.
Net content
The reported quantity of material in the tested sample.
Endotoxin
A separate test category that should be treated as present only when explicitly reported.
Sterility
A separate test category that should not be inferred from purity or identity data.
Reading a peptide HPLC chromatogram
In reversed-phase HPLC, peptides are commonly separated on a non-polar column such as C18 using an increasingly organic mobile phase, often aqueous acetonitrile with TFA or formic acid. A UV detector commonly measures peptide-bond absorption around 210–220 nm.
Axes and peaks
- x-axis: time (minutes) since sample injection.
- y-axis: detector response, often milli-absorbance units (mAU), at the stated wavelength.
- Main peak: the component assigned as the target by the laboratory; the trace alone cannot establish that assignment. A tall, symmetrical and well-separated peak is easier to interpret.
- Nearby minor peaks: may include deletion sequences, truncated chains, incompletely deprotected material or oxidised/deamidated forms.
- Early peaks: may be the injection front, salts or very polar material; the solvent front is commonly excluded from integration.
- Blank/system peaks: can appear in solvent-only injections; comparison with a blank helps identify them.
Tailing (a long trailing edge) or fronting (a sloping leading edge) may reflect column issues or overloading. A shoulder on the main peak can indicate an unresolved impurity and an overstated purity result.
Retention time
Retention time is the time of a peak's apex. It is useful for batch comparison under a fixed column, mobile phase, gradient, flow rate and temperature. A method change can shift retention time, and different compounds can elute at similar times. Retention time alone is not an identity fingerprint; interpret it with the separate identity evidence described above.
Area-% purity worked example: example data
Example data only, not a Pepy batch result or certificate.
purity (%) = area of main peak ÷ total area of all integrated peaks × 100
The software draws a baseline under each peak and measures its area.
Peak | Retention time (min) | Area | Area % |
|---|---|---|---|
1 | 11.2 | 1,200 | 0.8 |
2 (main) | 12.6 | 148,500 | 98.6 |
3 | 13.1 | 900 | 0.6 |
Total | 150,600 | 100.0 |
For these example data, 148,500 ÷ 150,600 × 100 = approximately 98.6% (rounded to one decimal place).
Area-% assumes comparable UV response per unit mass. Related peptide impurities often absorb similarly, but not identically. Water, salts and counter-ions such as TFA or acetate may not contribute to the UV signal. HPLC area-% is therefore not peptide content by mass; content requires its own measurement, such as amino acid or nitrogen analysis. See laboratory concentration and dilution calculations.
Integration choices matter
The same raw trace can produce different reported values depending on:
- Baseline placement
- Minimum integrated peak size or reporting threshold
- Exclusion of the solvent front and blank peaks
- How overlapping peaks are split
Request the integrated trace and peak table as well as the method. ICH Q2(R2) covers validation principles such as specificity, linearity, accuracy and precision. Values from different methods or integration rules are not directly comparable.
Quick chromatogram checklist
- Is the method stated, including column, gradient and wavelength?
- Is there one dominant peak with its retention time given?
- Are minor peaks visible and included in a peak table?
- Do the peak-table area percentages agree with the front-page purity?
- Is there a shoulder or poorly resolved peak on the main peak?
- Does the baseline look realistic, with some noise?
- Is mass spectrometry evidence also provided for identity?
Use these checks alongside the CoA verification guide and printable checklist and batch reports. See the testing methods glossary for terminology.
Chromatogram sources
- ICH Quality Guidelines, including Q2(R2)
- United States Pharmacopeia General Chapter 621, Chromatography. USP-NF, subscription access via https://www.usp.org/
- Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods in Molecular Biology. 2007;386:3–55. https://doi.org/10.1007/978-1-59745-430-8_1
For laboratory research use only. Not for human or veterinary use.
Reading HPLC information with Pepy batch documentation
First match the compound, strength and batch number shown in the report to the batch documentation being reviewed. Then identify which test categories are explicitly listed and read each result within the scope stated by the laboratory.
A blank or absent category is not a reported result. The document for one identified batch should not be used to draw conclusions about another batch or future material.
Educational laboratory reference only. Conclusions should be limited to the tests, sample and batch information explicitly reported in the original laboratory document.