HPLC purity and mass spectrometry: what each one shows

What HPLC measures

High-performance liquid chromatography pushes a dissolved sample through a column packed with material that separates compounds by how strongly each one interacts with it. Different compounds take different amounts of time to pass through, called the retention time, so the target peptide, its degradation products, and any leftover synthesis byproducts come out as separate peaks on a chart. A detector measures how much material is in each peak, and "purity" is usually reported as the target peak's share of the total peak area, often written as percent by area. A 99% HPLC purity result means 99% of everything the detector saw was the target peak. It doesn't mean the peptide is 99% identical to a reference standard in some other sense.

A clean-looking HPLC result can still miss a real problem: the method separates compounds by how they behave on that particular column, not by mass, so two chemically similar but different molecules can co-elute, leaving at the same time and getting counted as a single peak. That gap is what mass spectrometry closes.

What mass spectrometry confirms

Mass spectrometry measures the mass-to-charge ratio of ionized molecules in the sample. Every peptide sequence has a calculable expected mass, and MS checks whether the dominant signal in the sample actually matches that number. This is an identity check, not a quantity check. MS can tell you that you probably have the right molecule; on its own, it doesn't say how much of it you have, or how pure it is, the way HPLC does.

Why a certificate should show both

HPLC without MS can report a clean purity number for the wrong molecule. MS without HPLC can confirm the right molecule is present without saying whether it's sitting in 20% impurities. Together, the two cover quantity and identity. A method line reading "HPLC-MS" or "HPLC-UV-MS" states that both were run as part of the same analysis.

What endotoxin and identity lines mean, if they appear

On certificates for injectable-intent products, you'll sometimes see a separate endotoxin result, checking for bacterial contamination, alongside or instead of a repeated identity line. Neither is interchangeable with purity or mass identity; each is testing something else again. If one vendor's certificates show an endotoxin result on some products and an identity result on others, the testing isn't standardized product to product. That's worth noting as a gap rather than treating it as an automatic failure.

What we do with this

Our "method stated" column tracks specifically whether a vendor names HPLC, mass spectrometry, or both, for each product it sells, not just somewhere on its website in general. See the homepage's method section for the full scoring rules, or how to read a certificate of analysis for the rest of the checklist.