Testing methods

HPLC vs. LC-MS: What's the Difference?

Published October 2026 · 6 min read

Almost every Certificate of Analysis you'll see references one or both of these methods. They're related, but they answer different questions — and knowing which one is doing the work on a given COA tells you a lot about how much that document actually proves.

HPLC: how much, and how clean

High-performance liquid chromatography separates the components of a sample as they pass through a column, then measures how much of each component is present using a UV detector. For peptides, this is the standard way to report purity — the percentage of the detected signal that corresponds to the main compound peak, versus impurities, degradation products, or leftover synthesis byproducts.

What HPLC is good at: quantifying purity precisely and detecting related impurities (close chemical cousins of the target compound).

What HPLC alone can't do: definitively prove what the main peak actually is. It separates by size/polarity, not by confirming molecular identity — two different compounds can theoretically elute at a similar retention time.

LC-MS: confirming what it actually is

Liquid chromatography–mass spectrometry adds a mass spectrometer after the same kind of separation step. Instead of just measuring how much light a compound absorbs, it measures the actual mass-to-charge ratio of the molecules — which, combined with fragmentation patterns, confirms the molecular identity against a known reference.

This is why identity testing is almost always done via LC-MS (or MS alone) rather than HPLC: it's answering "is this actually the compound it claims to be," not "how much of it is there."

Why both matter on the same COA

A COA that reports only an HPLC purity percentage, with no identity confirmation method named anywhere, has answered "how much" without ever answering "of what." Both questions matter — a 99.8% pure sample of the wrong compound isn't useful to anyone.

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