How it works
“Reversed-phase” describes the polarity. The stationary phase — typically silica particles bonded with C18 alkyl chains — is non-polar, while the mobile phase is a polar water/organic mixture, usually water and acetonitrile with a small amount of an acidic modifier such as trifluoroacetic acid. This is the reverse of the historical “normal-phase” arrangement, which is where the name comes from.
Separation is driven by a solvent gradient. The run begins mostly aqueous and the proportion of organic solvent rises over time; more hydrophobic molecules stay bound to the C18 phase until the mobile phase becomes organic enough to release them, so each component elutes at a characteristic retention time. Peptides that differ by even a single residue or a small modification often resolve into separate peaks.
The detector is usually a UV absorbance detector set near 214 nm, the wavelength at which the peptide backbone’s amide bonds absorb. Its signal is plotted against time as the chromatogram, and the area under each peak is proportional to how much of that component passed the detector.
Purity is computed as area percent: the target peak’s area divided by the summed area of all integrated peaks, expressed as a percentage. It is an arithmetic ratio of what the method resolved — not an absolute count of every possible impurity, since anything that co-elutes with the target or does not absorb at the detection wavelength is not counted.
Why it matters
Purity by RP-HPLC is the single number most often used to summarise a peptide preparation, because it condenses “how much of this is the thing I asked for” into one comparable figure. It underpins release decisions, batch-to-batch comparison, and stability monitoring over time.
Because the figure is method-dependent, a purity value is only meaningful alongside the method that produced it — the column chemistry, the gradient, and the detection wavelength. Two laboratories reporting “98%” on different methods are not necessarily reporting the same thing, which is why a certificate states the method, not just the result.
Reading the chromatogram itself, rather than only the headline number, is what distinguishes a clean preparation from one whose single impurity sits just under the reporting threshold. The shape, symmetry, and baseline separation of the main peak carry information the percentage alone does not.
A purity value is only meaningful alongside the method that produced it.
At Lineará
RP-HPLC purity is the headline analytical result on every Lineará certificate of analysis: each lot reports a target-peak area percent alongside the method used to measure it.
The compound pages describe identity and purity as measured rather than asserted — RP-HPLC is the purity half of that pair, with ESI-MS confirming identity.