Mass spectrometry identity: what a one-dalton match proves

ESI and MALDI in plain terms, why a peptide shows up at several m/z values at once, monoisotopic against average mass, and a worked example on a stocked fifteen-residue peptide from its PubChem entry.

By Touchstone PeptidesSeptember 22, 20266 minute read

A mass spectrometry line on a certificate answers one question: does the molecule in the vial weigh what the named sequence should weigh. A match within about a dalton confirms that the elemental composition is consistent with the sequence on the label, which rules out most of the ways a supplier could send the wrong compound. It does not confirm the order of the residues, and it cannot see differences that do not change the mass. Purity and identity are separate assays, for the reasons set out in purity is not potency, and a certificate needs both.

How the molecule gets into the instrument

A mass spectrometer measures mass-to-charge ratio, m/z, so the molecule must first be given a charge and moved into the gas phase without being destroyed. Two soft ionisation methods do this for peptides.

In electrospray ionisation the sample solution is pushed through a capillary held at a high electric potential, a few kilovolts, which draws the liquid into a cone and then into charged droplets. As the droplets travel toward the inlet the solvent evaporates and they shrink until ions are released. The characteristic feature is that a peptide picks up more than one proton, so one compound appears at several m/z values at once, as [M+2H]²⁺, [M+3H]³⁺ and so on.

In MALDI the sample is co-crystallised with a light-absorbing matrix and hit with a laser pulse. It is the standard approach for peptides and proteins, and in contrast to electrospray it typically produces singly charged ions: as Cramer and colleagues put it in Angewandte Chemie, MALDI typically generates singly charged peptide ions whereas nanoelectrospray easily provides multiply charged ions, even for peptides as low as 1000 Da in mass.

Neither method is better in the abstract. A certificate should say which one produced the number it reports, because what you expect to see on the trace depends on it.

Monoisotopic and average mass are two different numbers

Carbon is mostly ¹²C with about one per cent ¹³C, and nitrogen, oxygen, sulfur and hydrogen have their own minor isotopes. A peptide therefore has no single mass.

The monoisotopic mass is calculated from the exact masses of the most abundant isotope of each element: one specific molecular species. The average mass is the isotope-abundance-weighted mean across all of them. For small molecules the two are close. For a large one they diverge visibly: in the tutorial literature, ubiquitin's monoisotopic mass is 8559.6167 Da against an average mass of 8564.7568 Da.

Which one a certificate should quote depends on the instrument. At high resolving power the individual isotope peaks are resolved and the monoisotopic peak is a physically well-defined species, so the observed value is compared with a calculated monoisotopic mass. At low resolution the isotope structure collapses into a single envelope and the average mass is the meaningful comparison. A report that pairs a high-resolution observed value with an average calculated value, or the reverse, is comparing two different quantities.

Resolution itself is defined as R = (m/z)/(Δm/z), with Δm/z the peak width at a defined fraction of its height, typically half maximum. A quadrupole instrument gives mass accuracy of the order of 0.1 to 0.5 Da; time-of-flight and Orbitrap instruments routinely reach the low parts-per-million range, typically 3 to 5 ppm or better.

A worked example: a fifteen-residue peptide

BPC-157 is a convenient case: short enough to reason about, with a public record. PubChem's entry, CID 9941957, gives the molecular formula C62H98N16O22, an average molecular weight of 1419.5 and a monoisotopic mass of 1418.70416 Da, verified against PubChem on September 21, 2026.

From those two figures, the arithmetic a certificate is implicitly doing:

Quantity	Value
Molecular formula	C62H98N16O22
Average mass	1419.5 Da
Monoisotopic mass	1418.70416 Da
Calculated [M+H]⁺, monoisotopic	1419.711
Calculated [M+2H]²⁺, monoisotopic	710.359
Calculated [M+3H]³⁺, monoisotopic	473.909

A proton adds about 1.00728 to the mass and one to the charge, which is why the doubly charged ion sits near half the singly charged value rather than exactly at half. The isotope peaks give the charge away without any other information: neighbouring isotope peaks are separated by about 1 Da divided by the charge, so 0.5 for a doubly charged ion and 0.33 for a triply charged one.

At this mass a one-dalton tolerance is loose. One dalton at m/z 1419.7 is about 700 ppm, which is a hundred times looser than a high-resolution instrument's accuracy of a few ppm. "Within 1 Da" is a sensible acceptance criterion on a bench instrument and a very generous one on a modern high-resolution machine, and the certificate should make clear which kind of instrument produced it.

What a mass match cannot see

Mass is a property of composition, so anything that leaves composition unchanged is invisible to it.

Sequence order. The same residues in a different order have the same formula and the same mass. An intact-mass match is consistent with the sequence; it does not read it.
Leucine and isoleucine. Both are C6H13NO2 with a monoisotopic mass of 131.09463. No mass measurement at any resolution can separate them.
D- and L-amino acids. Enantiomeric and diastereomeric substitutions do not change the formula at all.
Isomers generally. The tutorial literature is explicit that isotopomers with identical elemental and isotopic composition are not distinguishable by MS alone, and that constitutional isomers often fragment to product ions at the same m/z values, with only the relative intensities differing.

Some near-misses are also small enough to be missed by a loose tolerance. Asparagine and aspartic acid differ by about 0.98 Da, and lysine and glutamine differ by about 0.036 Da: the first vanishes inside a 1 Da window, and the second needs high resolution to see at all.

When a composition match is not enough

Three assays close the gaps, in ascending order of effort.

Tandem mass spectrometry. MS/MS selects one precursor ion, breaks it apart and scans the fragments. Cleavage of the amide bonds yields b- and y-ions, the series used to read sequence, and this is what turns a composition match into sequence evidence. On a certificate it appears as an LC-MS/MS or intact-plus-fragment identity line, and it is priced as an upgrade rather than as the default.

Amino acid analysis. The chain is hydrolysed and the liberated residues are quantified, giving both composition and an absolute quantity of peptide. It is the classical basis for net peptide content.

Chiral or orthogonal methods. Where the D or L question matters, it takes a method chosen for that question. No amount of mass accuracy substitutes.

Reading the identity panel

The method, named: ESI, MALDI, LC-MS, LC-MS/MS.
The calculated mass and whether it is monoisotopic or average.
The observed value, as m/z, with the charge state stated.
The acceptance criterion, in Da or ppm, with the instrument type behind it.
The same lot number as the purity page, because two assays on different lots describe two different materials.

Reading a chromatogram covers the purity half of the certificate, and how to read a certificate of analysis covers the page as a whole. Terms used above are defined in the peptide paperwork glossary.

Sources

For laboratory research use only. Not a drug, not a supplement, and nothing here is a claim about what any of this material does in a person or an animal.

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