Research & Educational Content Only — Not Medical Advice. All compounds referenced are for in-vitro and in-vivo laboratory research use only. Not intended for human or unsupervised veterinary application.
What the HPLC Trace Actually Tells You
An HPLC purity report is only useful if you know how to read it. Most don't. Here's what the peaks, percentages, and retention times are actually measuring.
High-performance liquid chromatography separates compounds by how strongly they interact with a stationary phase as a mobile phase carries them through a column. In reverse-phase HPLC — the most common method for peptide analysis — the stationary phase is nonpolar and the mobile phase is an aqueous-organic gradient. More hydrophobic molecules stick to the column longer. They elute later. The result is a timeline of what left the column and when, measured by UV absorbance at a fixed wavelength, usually 220 nm for peptides.
Retention time is the x-axis: how long after injection a compound exits the column. It's reproducible under identical conditions and serves as a partial identity marker. If a compound elutes at a meaningfully different time than the reference, something is wrong — wrong compound, wrong column condition, or degradation that changed the molecule's polarity. Retention time alone isn't identity confirmation, but an unexpected shift is a red flag worth investigating before any other analysis.
Purity percentage is calculated from peak areas, not peak heights. The software integrates the area under each detected peak and reports what fraction of total area belongs to the main compound. A 98.5% purity reading means the target compound accounts for 98.5% of the total UV signal. What the remaining 1.5% is — synthetic byproducts, degradation fragments, residual solvents — is not specified by purity alone. That's why a full trace matters more than a single number.
Impurity peaks appear as smaller peaks that are clearly resolved from the main peak, or occasionally as shoulders on the main peak that suggest co-eluting species. Noise is different: it's the baseline variation that occurs even when nothing is eluting. A legitimate impurity peak rises meaningfully above the baseline and has a defined shape. Anything below a signal-to-noise ratio of roughly 3:1 is not reliably quantifiable and shouldn't be reported as a discreet peak. If you're looking at a COA and the reported impurity peaks seem suspiciously absent, it's worth asking whether they were detected but excluded or genuinely not present.
If a COA arrives without the actual trace — just a purity number — ask for it. A reputable lab will provide the chromatogram. The trace shows you what the software saw before it calculated the percentage: the raw separation, the baseline, the shape and resolution of each peak. Purity numbers can be selectively reported or misrepresented; the trace is harder to fake and considerably more informative. For any serious research application, the raw data should be available, and if a supplier declines to provide it, that is itself data.
For Research & Educational Purposes Only — Not Medical Advice
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