Compound Quality

A Purity Percentage Is Peak Area On A Chromatogram. Here Is What That Number Can And Cannot See.

99% is a measurement, not an adjective. It comes from one specific instrument, run one specific way. Understand what the machine is looking at and you understand exactly where the number stops being useful.

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Almost every listing in this category prints a purity figure. Very few explain where it comes from, which means very few readers can tell a rigorous number from a decorative one.

So here is the actual procedure, in plain language.

A sample of the compound is dissolved and pushed through a chromatography column. Different molecules travel through at different speeds, so they come out separated. A detector on the far end watches what emerges, usually reading ultraviolet absorbance in the 210 to 220 nanometre range, which is where these bonds absorb.

The output is a chart of peaks. The intended molecule makes one peak. Everything else makes its own.

Purity is the area of the target peak divided by the total area of all the peaks. That is the whole calculation. A result of 99% means the target peak accounts for 99% of the detected area.

1The percentage is a share of what the detector saw

Read that definition again, because the important word is "detected".

The number is a ratio of areas on a chromatogram. It is a share of the things that made a peak. Anything that does not absorb in that wavelength range, or does not come off the column at all, contributes nothing to the denominator.

It is not a share of the powder in the vial. It is a share of the signal.

Purity describes the active fraction. It says nothing about anything outside it.

2Three things a purity number cannot tell you

This is the useful part. A high purity result is silent on all three of the following.

Method note. HPLC purity is integrated from peak area, typically at 210 to 220nm. Identity is confirmed by mass spectrometry. Net content is determined separately. Sources: Bachem analytical quality control guidance; PMC10338602, reference standards for synthetic therapeutics. Accessed 12 September 2026.

Identity. The chromatogram sees a peak. It does not know what the label says. A vial of the wrong compound, cleanly made, returns a high purity figure for the wrong molecule. Only mass spectrometry connects the peak to a name.

Quantity. Purity is a ratio and ratios do not count things. A container can be 99% pure and hold less material than the label claims, because counterions and absorbed moisture take up weight without lowering a purity figure.

Everything else in the container. Residual solvents, salts and moisture are largely invisible to the method that produces the purity number.

One instrument, one question. A supplier reporting only purity has answered a third of what matters.

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3Why 99% and 98% are further apart than they look

One percentage point sounds like rounding. On a chromatogram it is a doubling.

A 99% result means 1% of the detected area is something other than the target. A 98% result means 2%. The impurity fraction is twice the size.

And those impurities are not random dust. In synthetic production they are usually closely related sequences: a chain missing one amino acid, a chain with one extra, a partially modified variant. Structurally similar, which is exactly why they are the hardest ones to separate out.

Compare the impurity fraction, not the purity fraction. 1% and 2% is a doubling, and the language hides it.

4What a complete document actually contains

A certificate worth reading covers identity, purity and content as three separate lines, each with its own method named, and it belongs to a specific batch.

Structure shown is the standard format for a certificate of this type. The real documents are published per batch on the catalog.

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5The five second version

If you only do one thing before ordering, do this.

• Count the lines on the certificate. One line is a claim. Three or more is a document.
• Check each line names a method. HPLC, mass spectrometry. A result without a method is a sentence, not a measurement.
• Check the batch reference. It should match what is shipping to you.
• Check who signed it. A third party laboratory, not the seller.
• Convert the purity to an impurity figure. 99% means 1%. That is the number you are actually comparing.