Chromatography sorts a sample by stickiness. Mass spectrometry weighs it, and the size of any gap usually names what went wrong.
A chromatography run sorts a sample by how sticky each part of it is. That is useful, but it cannot tell you what any of those parts actually are. Two different substances can travel at the same speed. A single clean peak is a clue, not proof.
Mass spectrometry answers the other question. It weighs the molecule.
The sample is given an electric charge, turning each molecule into an ion. What the instrument then reads is the mass-to-charge ratio, written m/z. It is a ratio because a peptide usually picks up more than one charge. The same molecule can appear several times in the raw data, once for each charge it carried.
Software works backwards from those signals to a single number: the mass of the molecule itself. That number is reported in daltons, shortened to Da. One dalton is roughly the mass of a single hydrogen atom, so this is a very fine ruler.
Then comes the comparison that matters. The sequence you ordered has a mass you can calculate exactly. The lab checks the measured mass against that calculated one.
There are two ways to calculate the expected number, and mixing them up wastes a lot of time.
Monoisotopic mass uses the lightest common form of each atom. Average mass blends all the natural forms of each atom by how often they occur. The two differ by a small amount that grows with the size of the peptide.
Identity checks on peptides normally use monoisotopic mass, because the instrument can resolve individual isotopes at this size. A mismatch of a few daltons often turns out to be nothing more than the wrong column of the calculator.
When the measured mass does not match, the size of the gap usually names the cause. These four come up constantly:
| Difference from expected | Most likely cause |
|---|---|
| +16 Da | Oxidation of a methionine or tryptophan |
| +42 Da | Acetylation at the N-terminal end |
| +71 Da | An acrylamide adduct |
| −18 Da | Loss of water, commonly a glutamate closing into pyroglutamate |
One near neighbour is worth knowing. A glutamine at the N-terminal end can close into pyroglutamate as well. But it loses ammonia rather than water, so it shows as −17 Da. Seeing −17 and −18 as the same event is a common slip.
Three problems show up this way.
Identity errors. The mass is simply wrong for the sequence on the label. Nothing else needs discussing until that is resolved.
Incomplete cleavage. Peptides are built with protecting groups on the side chains, which acid is meant to strip off at the end. If any survive, each one adds its own mass, and the peptide arrives heavier than it should be.
Chemical modification. Oxidation and water loss are damage. They tell you something happened to the material after it was made, during handling or storage, rather than during synthesis.
None of these are visible on a purity chart. A modified peptide still absorbs light, still forms a peak, and can still be counted inside the 99%. Weighing it is what separates clean from correct.
This article explains published research and the rules around it. It carries no dose, no protocol, and no claim that any compound treats, cures or prevents a condition in anyone. Where the evidence is thin we say so. See our editorial standards.
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