verification
Mass Spectrometry and Peptide Identity: What a Mass Match Establishes and What It Does Not
A mass measurement establishes composition, not the order of residues. This is where the reported number comes from, why monoisotopic and average masses produce false mismatches, and how to read an offset.
No — a mass spectrometry result confirms mass, not sequence order. Mass is a function of composition alone and carries no positional term, so every rearrangement of the same residues gives the same number. A match between an observed and a theoretical mass is consistent with the intended peptide and does not demonstrate it. Demonstration requires fragmentation, a separate experiment 1.
What the measurement is
A mass spectrometer converts the analyte into gas-phase ions, separates them by mass-to-charge ratio, and detects them 1. It never measures mass directly. It measures m/z, and a mass follows only once the charge is known.
Peptides are ordinarily ionised by electrospray, acquiring charge by protonation as the sample is sprayed from solution. The consequence is that a peptide does not appear as one peak. It appears as a series of multiply charged ions, each at an m/z of approximately (M + 1.008n)/n for n charges. A peptide of nominal mass 3000 Da therefore gives signals near m/z 3001, 1501 and 1001 at one, two and three charges, the charge count rising with chain length and with the basic residues available to carry it.
That charge-state envelope is solved for a single neutral mass, since adjacent charge states constrain the assignment. This step is deconvolution and it is done in software, so the figure on a certificate is derived rather than observed. A mis-assigned charge state gives a mass wrong by a large multiplicative factor, which is conspicuous. An adduct passes through deconvolution intact and emerges as a small, plausible-looking offset.
Monoisotopic against average mass
Two masses can be calculated from one formula and they are not interchangeable. The monoisotopic mass sums the lightest stable isotope of each element — carbon-12 at exactly 12.000, hydrogen-1, nitrogen-14, oxygen-16, sulfur-32. The average mass sums the standard atomic weights, abundance-weighted over the natural isotope distribution, so carbon enters at about 12.011.
The two diverge with molecular size, driven mainly by the natural abundance of carbon-13: roughly half a dalton to a dalton per kilodalton for ordinary peptide compositions. Below a kilodalton the gap is a fraction of a unit. At several kilodaltons it is several units — larger than the tolerance any competent instrument is held to, and so large enough to look like a failure.
This is the commonest source of an apparent mismatch. A certificate quotes an observed monoisotopic mass, the reader calculates an average mass from the formula, the two differ by a few daltons and the material is treated as suspect. Both figures are correct; they answer different questions. The basis must be stated on both sides and like compared with like.
Which basis applies is a property of the instrument, not a preference. Where the analyser resolves individual isotope peaks, the monoisotopic mass is the meaningful quantity, read from the all-light-isotope peak — which on larger species is not the most intense peak in the cluster. Where the analyser reports the centroid of an unresolved envelope, the figure is an average mass. An observed mass quoted without its basis cannot be checked.
Tolerance and what a deviation implies
A deviation is expressed two ways. The absolute deviation is the difference in daltons between observed and theoretical mass. The relative deviation is that difference divided by the theoretical mass and multiplied by one million, in parts per million. The ppm figure normalises across molecular size; the absolute figure matches against the discrete shifts below.
The acceptable window is a property of the analyser, its calibration and the acquisition, not of the peptide, and it belongs on the certificate beside the result. Unit-resolution analysers separate species roughly a dalton apart and cannot report a meaningful sub-dalton deviation; high-resolution analysers separate species far closer and resolve isotope patterns directly. An observed mass with no stated tolerance supports neither acceptance nor rejection.
Deviations sort into three classes. Within tolerance, the mass is consistent with the expected composition — and consistent is the whole of the claim. A deviation matching a known adduct or modification is an identification rather than a failure: the shift names the difference. A deviation matching neither means the species measured is not the species expected. An unexplained offset is information rather than noise, and is recorded as a number.
Composition, not sequence order
This is the central limitation and it is not a marginal one. Mass is the sum of the atomic masses present, so two molecules built from the same atoms weigh the same however those atoms are arranged. Every permutation of a residue set is isobaric with every other. A transposition of two adjacent residues, a swapped pair at distant positions, a fully scrambled sequence — each returns the theoretical mass and passes an intact-mass check with no flag raised.
Isobaric substitution compounds this, because coincidences occur between residues and between combinations of them. Leucine and isoleucine share a molecular formula and are identical in mass; no measurement distinguishes them and no increase in resolution ever will. Lysine and glutamine differ by about 0.036 Da, resolvable only at high mass accuracy. Combinations coincide exactly: two glycine residues carry the formula and mass of one asparagine, glycine plus alanine that of one glutamine. Glycine plus valine sits about 0.011 Da from arginine.
The population of molecules consistent with any given mass is therefore large. A mass match narrows the field sharply; it does not close it. A line reading that identity is confirmed by MS states that a measured mass agreed with a calculated one — a composition check, and recorded as one.
Sequence order is established by tandem mass spectrometry. A precursor ion of selected m/z is isolated, dissociated — commonly by collision with a neutral gas — and the fragment masses recorded 1. Cleavage along the backbone yields complementary series: b ions retaining the N-terminus, y ions the C-terminus. The difference between consecutive members of either series equals the residue at that position, so the series reads as a ladder from which order is inferred 1. Leucine and isoleucine remain indistinguishable; fragmentation resolves order, not that pair.
The reading follows directly. A certificate reporting only a parent mass has not established sequence, however tight the agreement. Establish whether the data behind it is a single deconvoluted mass or an annotated fragmentation spectrum with assigned b and y ions, and what fraction of the backbone those assignments cover.
| Question | Intact mass | Fragmentation | Note |
|---|---|---|---|
| Is the composition as expected | Yes, within stated tolerance | Yes | Mass is a function of formula alone |
| Are the residues in the intended order | No | Yes, across the region covered | All permutations of a residue set are isobaric |
| Leucine or isoleucine at a position | No | No | Identical formula; no mass method separates them |
| Are terminal modifications present | As an increment; not located | Yes, and located | Acetylation and amidation shift the mass |
| Is a disulfide bond formed | As −2 against the reduced form | Connectivity needs further work | Bond count follows from the shift |
| Is a deletion or truncation present | Only in the species measured | For the precursor selected | A parent mass reports nothing about minor species |
| How much of the sample is the target | No | No | Intensity is not proportion; ionisation efficiency differs |
Adducts and characteristic mass shifts
Electrospray ionises by attaching a charge carrier, ordinarily a proton. Where alkali metal ions are present — from glassware, solvents, buffers or the sample — the peptide ionises instead as a sodium or potassium adduct, about 21.98 Da and 37.96 Da above the protonated ion. Where deconvolution assumes protonation, those offsets pass into the reported mass.
Trifluoroacetic acid is the other routine source of an offset. It serves as an ion-pairing modifier in reversed-phase purification and is frequently the counterion of the isolated salt, so adducts near 114 Da, and multiples of that spacing, are common on material purified that way.
Adducts are recognised by their behaviour in the spectrum, not by the offset alone: they appear alongside the protonated species rather than instead of it, at fixed spacing, and they repeat — two sodiums give twice the single offset. An offset that is a whole multiple of a known adduct mass and co-occurs with the unadducted ion is an artefact of ionisation. An offset matching a modification mass is a chemical difference in the material, and the routes concerned are established degradation chemistry 4.
| Shift (Da) | Assignment | Origin | How to recognise it |
|---|---|---|---|
| +15.995 (≈ +16) | Oxidation | Methionine to the sulfoxide; also cysteine, tryptophan | Multiples for further sites; grows with air and light |
| +0.984 (≈ +1) | Deamidation | Asparagine or glutamine to the acid, via a cyclic imide | Merges with the isotope pattern below high resolution |
| −18.011 (≈ −18) | Dehydration | Aspartimide formation, side-chain condensation, in-source loss | In-source loss varies with source conditions; a modification does not |
| −2.016 (≈ −2) | Disulfide formation | Two cysteine thiols oxidised, losing two hydrogens | Compare against the reduced form; each further bond subtracts 2.016 |
| +21.982 (≈ +22) | Sodium adduct | Sodium replacing the ionising proton | Co-occurs with the protonated ion; doubles for two |
| +37.956 (≈ +38) | Potassium adduct | Potassium replacing the ionising proton | As sodium; from glass, solvents and buffers |
| +113.993 (≈ +114) | Trifluoroacetate adduct | Trifluoroacetic acid from purification or as counterion | Multiples common; absent after conversion to acetate |
What mass spectrometry cannot see
A spectrum reports on species that ionise under the conditions used and fall within the scanned m/z range. Everything else is absent from it, and absence from a spectrum is not absence from the vial.
- Inorganic content. Salts and inorganic residue are not reported by a peptide identity method.
- Counterion load. Acetate or trifluoroacetate adds to gross weighed mass, but the salt dissociates in the source and the measured mass is that of the free peptide.
- Water content. Lyophilised solids are hygroscopic; retained water adds to gross mass without appearing in the spectrum.
- Bacterial endotoxin. A separate assay entirely, with no relation to any mass result.
- Species that ionise poorly, that are suppressed by co-eluting components, or that fall outside the scanned range.
- Quantity. Relative peak intensity is not proportion, because ionisation efficiency differs between species.
Published analyses of unregulated material show what analysis exists to detect. A 2015 study in Talanta on illegal peptide biopharmaceuticals encountered by controlling agencies reported samples in which the declared peptide could not be detected 2. A 2018 Talanta study profiling impurities in falsified polypeptide drugs on the Belgian market reported synthesis-related impurities, including truncated and deletion sequences 3. Those findings came from analysis of the samples, not from the paperwork. Neither extrapolates to other supply.
Checking a reported mass against a theoretical value
- Record the sequence as stated, with terminal modifications and any declared disulfide bridges.
- Derive the molecular formula from it; residue masses sum, with one water added for the free termini.
- Calculate both the monoisotopic and the average mass, and keep both.
- Read which basis the observed mass is quoted on. Where it is not stated, record the comparison as indeterminate.
- Compare like with like: monoisotopic against monoisotopic, average against average.
- Compute the deviation in daltons and in parts per million, and record both.
- Inside the stated tolerance, record the mass as consistent with the expected composition — and no more.
- Where no tolerance is stated, record that the certificate supports neither acceptance nor rejection.
- Outside tolerance, test the absolute figure against the shift table, including whole multiples.
- Where the offset matches an adduct, confirm the unadducted species co-occurs at the expected spacing.
- Where it matches a modification, record it as a finding about the material, not an error in the document.
- Where it matches nothing, record the measured species as unidentified. That is a result.
- Establish separately whether the identity claim rests on an intact mass or on an annotated fragmentation spectrum.
- Where fragmentation data is supplied, check that b and y assignments are given and note the backbone coverage.
- Bind every entry to the batch identifier on the container.
Recorded this way, an identity check states what it has shown: the mass agreed, on a stated basis, within a stated tolerance, on a named batch — and the order of the residues stays unaddressed unless fragmentation data was supplied and read.