Purity Does Not Equal Identity
You can be extremely certain about how much of something you have, and completely wrong about what it is.
Key takeaways
- Chromatography separates. Mass spectrometry identifies. They answer different questions and neither substitutes for the other.
- A 99% purity result on a misidentified compound is a precise measurement of the wrong thing.
- Deletion sequences — a peptide missing one residue — are chemically similar enough to be difficult to separate and are a known failure mode of synthesis.
- If a certificate reports purity but no expected-versus-observed mass, identity was never established.
Imagine a sealed box. One instrument tells you the box contains a single kind of object, evenly and consistently, with almost nothing else mixed in. A second instrument tells you what that object is.
Purity testing is the first instrument. Identity testing is the second. In this industry the first result is quoted constantly and the second is often not run at all — which means the confident number people rely on is answering a question nobody asked.
Two different questions
| Purity (HPLC) | Identity (MS) | |
|---|---|---|
| Question | How much of the detected sample is one component? | What is that component? |
| Physical basis | Separation by interaction with a column | Mass-to-charge ratio of ionised molecules |
| Typical output | A percentage and a chromatogram | Expected mass vs observed mass |
| Fails silently when | Impurities co-elute or do not absorb | Not run at all |
HPLC sorts a mixture by physical behaviour. Two compounds that behave similarly on the column come off at similar times. The detector records a peak; it does not know what made it. Chromatography is a sorting process being read as an identification, and it was never designed to be one.
Mass spectrometry ionises the molecule and measures its mass-to-charge ratio. Because the theoretical mass of a peptide can be calculated directly from its sequence, an observed mass that matches the expected mass is meaningful evidence about what the molecule is. That is the check that closes the gap.
How a pure sample ends up being the wrong molecule
This is not a hypothetical failure. There are ordinary, well-documented routes to it.
Deletion sequences
Peptides are commonly built one amino acid at a time. If a single coupling step is incomplete, some fraction of the chains continue on missing that residue. The result is a peptide that is nearly the target — same sequence, one residue short.
Nearly-identical molecules behave nearly identically on a column. A deletion sequence can elute close enough to the main peak to sit underneath it, contributing to the area of the peak it is meant to be distinguished from. Mass spectrometry catches it immediately, because the missing residue changes the mass by a specific, calculable amount.
Substitution and mislabelling
A vial can be filled with a different compound entirely — through a labelling error upstream, a substitution somewhere in the supply chain, or deliberate misrepresentation. If that compound happens to be clean, the purity test returns an excellent number. It is a completely accurate measurement. It is also irrelevant.
Modified variants
Oxidation, truncation, and incomplete deprotection all produce species related to the target. Some are separable and show up as their own peaks. Others are not, and do not.
What this changes about how you read a certificate
Once you separate the two questions, a certificate becomes much easier to evaluate. You are no longer looking for a number to be high. You are checking whether both questions were asked.
- Find the identity section. If there is no expected mass and no observed mass, identity was not established — regardless of how good the purity figure is.
- Check that the two masses agree, and that the document states the expected value rather than only the measurement.
- Then read the purity result, with its method, as a statement about homogeneity.
- Confirm both belong to the lot you actually have.
This is the entire discipline in four steps, and it is why "99% pure" as a standalone claim should read to you as an incomplete sentence. Pure what?
Sources
We link to primary sources — PubMed, ClinicalTrials.gov, FDA, NIH, peer-reviewed journals, and laboratory documentation. Entries marked needs research are placeholders: the claim in the text is not yet backed by a verified citation and should be read as unconfirmed until it is.
Solid-phase peptide synthesis: incomplete coupling and deletion sequence formation
PubMed
Needs research
Cite a primary or review source on deletion sequences in SPPS. Do not cite from memory.
Mass spectrometry for peptide identity confirmation — methodology review
Peer-reviewed analytical chemistry literature
Needs research
Co-elution and resolution limits in reversed-phase HPLC of closely related peptides
PubMed
Needs research
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