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Mass spectrometry answers identity: it weighs the molecule and compares that against the mass calculated for the compound on the label. Purity cannot answer that.
Mass spectrometry ionises a sample and measures the mass-to-charge ratio of what results. Because a peptide's mass follows directly from its amino-acid sequence, the measured mass can be compared against the mass calculated for the compound the label claims. A match is strong evidence of identity; a mismatch means the material is something else, however pure that something else may be.
A chromatography trace shows that one component dominates. It does not name that component. A sample can be 99% one substance and be 99% of the wrong substance, and nothing in the purity figure would reveal it. Identity testing closes that gap, which is why a report carrying both is a complete result and a report carrying one is half of one.
A usable identity result states two numbers: the mass expected for the named compound and the mass observed. The comparison is the finding. A report that gives an observed value with nothing to compare it against has published a measurement rather than a conclusion, and the reader is left to look up the expected value themselves.
Mass alone cannot distinguish molecules that share a formula but differ in arrangement, and it says nothing about how much of the material is present. It is an identity test, not a quantity test and not a purity test. Each of the three answers a different question, and a seller publishing only one has left the other two open.
The informative combination is an identity confirmation plus a purity figure from the same sample on the same date. Together they say: this is the compound claimed, and this proportion of what was detected was it. In the published set behind this site, 181 reports carry a purity percentage and the remainder are identity or content tests reporting a result without one — which is normal, and not a gap in the record.
Measured masses are rarely exactly the calculated figure. Instrument calibration, isotope distribution and the charge state all shift the observed value slightly, so results are reported within a tolerance rather than to the decimal. A result a fraction of a unit from the expected mass is an ordinary confirmation; a result several units away is a different molecule.
Fragmenting the molecule and measuring the pieces can confirm the order of the amino acids rather than only the total mass. This matters because two different sequences built from the same amino acids weigh the same, so total mass alone cannot tell them apart. Sequence confirmation is more work and appears less often on commercial certificates, which is worth knowing when a report says only that the mass matched.
Identity testing is the cheapest insurance against the worst outcome, which is not an impure product but the wrong product. If a seller publishes only purity figures, the question worth asking is not about the percentage but about whether identity was confirmed at all, and on which batches.
Most commercial certificates report a single observed mass against an expected one and stop there. That is a legitimate identity confirmation and it is also the cheapest version of one. What is typically absent is the spectrum itself, any statement of the tolerance applied, and whether fragmentation was used to confirm the sequence rather than only the total mass. None of those omissions makes the result wrong; together they mean the certificate is reporting a conclusion rather than showing the evidence for it, which is worth noticing when the same seller publishes full chromatograms for purity.
The claims on this page about why identity is measured by mass, and what a mass result settles can be checked against sources outside this site: