A purity percentage on a Certificate of Analysis looks simple — "99.4% purity" reads as a single, clean fact. But that number is the output of a specific analytical process, and understanding how it's generated says a lot about what it actually tells you, and just as importantly, what it doesn't.
Why Testing Requires Two Different Questions
Analytical chemistry treats "what is this?" and "how much of it is pure?" as two separate questions, answered by two different techniques. A sample can be extremely pure and still be the wrong compound entirely — high purity says nothing about identity on its own. This is why a rigorous testing process uses both High-Performance Liquid Chromatography (HPLC) for purity and mass spectrometry (MS) for identity, rather than treating either as sufficient by itself.
How HPLC Actually Works
HPLC separates a sample into its individual chemical components based on how they interact with two different materials: a "stationary phase" (a solid material packed into a column) and a "mobile phase" (a liquid solvent pushed through that column under pressure).
Different molecules in the sample travel through the column at different speeds, depending on how strongly they interact with the stationary phase versus how readily they move with the solvent. Each component exits the column — and reaches a detector — at a different point in time, called its retention time. The detector records this as a series of peaks on a graph called a chromatogram, where each peak represents one distinct component of the sample.
The height and area of each peak correspond to how much of that component is present. Purity percentage is calculated by comparing the area of the peak matching the target compound against the total area of all peaks in the sample — in other words, what fraction of everything detected is the compound you're looking for, versus leftover synthesis byproducts, degradation products, or other impurities.
How Mass Spectrometry Confirms Identity
Mass spectrometry answers a different question: not "how much," but "what is this, specifically." A mass spectrometer ionizes molecules in the sample (gives them an electric charge) and then measures their mass-to-charge ratio as they pass through a detector. Because every molecule has a specific, calculable molecular weight, the result can be compared directly against the expected weight for the target compound.
This is what actually confirms identity. HPLC alone can show a single, extremely clean peak — but without mass spec confirming that the peak corresponds to the correct molecular weight, a "highly pure" result could just as easily describe a pure sample of the wrong substance.
What the Purity Percentage Does — and Doesn't — Mean
A purity result is a measurement of chemical composition: what proportion of the detected material is the intended compound. It is not a measure of:
- Biological activity or potency. Purity describes what's in the vial chemically — it says nothing about how a compound behaves in any biological system.
- Sterility. A sample can be chemically pure and still carry microbial contamination; that's a separate test entirely (see endotoxin testing below).
- Safety for any particular use. Purity testing evaluates the sample itself, not any downstream application of it.
It's a precise, useful number — but it answers a narrower question than it's sometimes assumed to.
Endotoxin Testing: A Different Method Entirely
Where HPLC and mass spec examine the chemical sample itself, endotoxin testing looks for a specific class of contaminant: byproducts of bacterial cell walls that can persist depending on manufacturing conditions. The standard method is a Limulus Amebocyte Lysate (LAL) assay, which reacts specifically to these bacterial byproducts and produces a measurable result independent of the compound's own purity. A sample can pass purity and identity testing cleanly while still requiring endotoxin testing to rule out this separate category of contamination — which is why thorough documentation lists it as its own distinct result, not folded into a general "purity" figure.
Reading Results With This in Mind
A complete, trustworthy analysis includes identity confirmation (mass spec), a purity percentage (HPLC), and ideally mass/endotoxin data where relevant — each answering a distinct question, not duplicating the others. A COA showing only a purity number, with no identity confirmation named, has left the more fundamental question unanswered.
Our own Lab Results page lists exactly which of these tests were performed for each product, and is explicit where any weren't — rather than presenting a single purity figure as though it covers everything.
This article is provided for general research and educational purposes. All products referenced are sold strictly for laboratory research use.