What HPLC purity actually measures
A purity percentage is not what most people assume. What the number includes, what it quietly excludes, and how to read the chromatogram.
High-performance liquid chromatography (HPLC) is the standard method for assessing peptide purity, and the percentage it produces is the headline figure on almost every certificate. It is a genuinely useful number, and it is also routinely misunderstood.
How the measurement works
The sample is dissolved and pushed under high pressure through a column packed with a stationary material. Different molecules travel through at different speeds depending on how strongly they interact with that packing. By the time everything emerges at the far end, the components have separated out in time.
A detector at the outlet — typically measuring ultraviolet absorbance — records what comes past and when. The result is a chromatogram: a baseline with peaks rising from it, each peak a component of the sample.
Where the percentage comes from
The area under each peak is calculated. The main peak’s area is expressed as a percentage of the total area of all peaks. That is the purity figure.
Read that definition again, because the important part is what it implies: the figure is relative. It tells you what proportion of the detected material is your target compound. It does not tell you what proportion of the vial’s contents is your target compound.
What the number does not include
Anything the detector cannot see does not appear in the calculation. A UV detector set for the peptide bond region responds poorly or not at all to several things that may be present — residual water, salts and counterions among them.
This is why a vial can legitimately be described as highly pure by HPLC while a meaningful fraction of its actual mass is water and acetate. Both statements are true; they are answering different questions. The water content and counterion lines on the certificate are what fill that gap, which is why they are worth reading.
Reading the chromatogram itself
The trace tells you more than the headline number. Look for a main peak that is tall, narrow and symmetrical, returning cleanly to baseline on both sides. Watch for shoulders — a bulge on one side of the main peak — which suggest a closely related impurity that has not fully separated. A scatter of small peaks elsewhere indicates synthesis by-products.
Two batches can carry the same purity figure and look completely different on the trace. The one with a single clean peak and the one with a main peak plus a forest of small ones are not equivalent materials.
Why we publish the trace
Because the number alone is easy to present favourably and hard to argue with. The chromatogram is the evidence behind it, and anyone competent to use the material is competent to look at it.