Peptide Identity by Mass Spectrometry: Monoisotopic Mass, Adducts and Deconvolution

The method answers one question: does the dominant ionisable species in this sample have a mass consistent with the mass calculated for the proposed sequence? For a peptide of known composition this is a strong constraint. A chain of thirty residues has a theoretical mass that few other plausible synthesis products will match within a few daltons.

It does not confirm the residue arrangement. Two sequences of identical composition, differing only in the placement of two residues, have identical mass and cannot be separated by a single MS measurement. Nor does peptide identity mass spectrometry report chirality: a chain built with one D residue where L was intended carries exactly the same mass as the intended molecule.

Peptide identity mass spectrometry also cannot quantify how much of the sample carries that mass. Ion signal depends on ionisation efficiency, which varies substantially between sequences and between coexisting species. A strong peak at the expected mass is evidence of presence, not of proportion; proportion belongs to chromatographic purity measurement with a stated method.

• Confirms: observed mass agrees with the mass calculated for a proposed composition.
• Does not confirm: residue arrangement along the chain.
• Does not confirm: chirality at individual residues.
• Does not confirm: the fraction of the sample carrying that mass.

MALDI-TOF embeds the sample in a crystalline matrix, desorbs it with a laser pulse and measures flight time. It produces mostly singly and doubly charged ions, tolerates salts better than electrospray, and gives a spectrum that is quick to read. Signal reproducibility between spots is poor, which limits any reading of relative intensity.

Electrospray introduces the sample in solution at atmospheric pressure and produces a distribution of charge states. A peptide with several basic sites accepts multiple protons, so a molecule of three thousand daltons may appear as a series of peaks between m/z 500 and 1500. That suits most analysers, which perform best at lower m/z, and it is why a deconvolution step is needed.

In practice the two routes are complementary. MALDI-TOF suits a rapid check. Electrospray, particularly when coupled to liquid chromatography, attaches a mass to a chromatographic peak, which is what we want when the question is which peak in a purity trace carries the expected mass.

Monoisotopic mass is calculated from the most abundant isotope of each element: carbon-12, hydrogen-1, nitrogen-14 and oxygen-16. Average mass is the abundance-weighted mean across the natural isotopic distribution. For a small molecule the two differ little; above roughly two thousand daltons the difference reaches one dalton or more and cannot be ignored.

Which peak is being quoted is the practical issue. At low resolution the isotopic envelope is unresolved and the reported value is its centroid, nearer the average mass. At high resolution the isotope peaks separate and the monoisotopic peak can be picked directly. Quoting a monoisotopic theoretical value against a centroided average observation manufactures an error of about one dalton that has nothing to do with the sample.

We record both the theoretical figure and the convention it uses. A certificate stating observed 3558.2 against calculated 3558.1, without saying monoisotopic or average, leaves the reader to guess, and the guess changes the whole error band.

• State monoisotopic or average on the same line as the number.
• Check that observed and calculated values use the same convention.
• Above 2 kDa the two conventions differ by a dalton or more.
• A centroided low-resolution peak is not a monoisotopic value.

In peptide identity mass spectrometry a peptide rarely appears as a single peak. Sodium and potassium adducts sit 21.98 and 37.91 daltons above the protonated species and appear readily when glassware, salts or buffer residues are present. Trifluoroacetate adducts carried over from reversed-phase purification add roughly 114 daltons per counter-ion and can dominate a spectrum even when the peptide is well behaved.

Under electrospray, multiple charging produces an envelope: adjacent peaks separated by a mass-to-charge increment corresponding to one additional proton. Recognising the envelope is straightforward once the spacing is checked, but assigning a sodium adduct of one charge state as the protonated species of another introduces an error of tens of daltons.

• Protonated species: M plus nH, divided by n.
• Sodium adduct: 21.98 Da above the protonated form.
• Potassium adduct: 37.91 Da above the protonated form.
• Trifluoroacetate adduct: roughly 114 Da per counter-ion.
• Multiple charging compresses large peptides into a low m/z range.

Deconvolution of the envelope

Deconvolution is the arithmetic that collapses a charge envelope to one neutral mass. The algorithm tests candidate charge states for adjacent peaks, solves for the neutral mass, and requires the series to be self-consistent across several peaks. Software does this routinely, but it can mis-assign a noisy envelope, so we check charge states against the raw trace.

A deconvoluted spectrum is a derived result. When we archive identity evidence we keep the raw profile and the deconvoluted value together, because the transformation is where an adduct can quietly be promoted to a main species.

• Keep the raw spectrum beside the deconvoluted result.
• Verify that assigned charge states cover the whole envelope.
• Read a single-peak deconvolution with caution.

Mass accuracy is reported as an error in parts per million: the difference between observed and calculated mass in daltons, divided by the calculated mass, multiplied by one million. Five parts per million on a three thousand dalton peptide is fifteen millidaltons; on a five hundred dalton fragment the same ratio is under three millidaltons and far harder to deliver.

For peptide identity mass spectrometry an error band is meaningless without instrument context. A low parts-per-million figure from a time-of-flight or Orbitrap analyser, calibrated internally and read from a resolved isotopic peak, is a different claim from the same number produced by a low-resolution quadrupole reading a centroided envelope. We note analyser type, calibration mode and whether the value came from a raw or a processed spectrum.

Accuracy is also bounded by composition. Within a modest error band several compositions may fit one mass, particularly for larger peptides where the candidate set grows quickly. A tight error narrows that set; it does not close it. This is the point at which fragmentation evidence becomes necessary rather than optional.

Tandem mass spectrometry isolates a precursor, fragments it and reads the fragment series. A b-ion and y-ion ladder walking across the full sequence, with coverage at every residue junction, is the evidence that turns a mass match into a sequence assignment. Partial coverage narrows the field but leaves gaps, and a gap at a junction is exactly where an isomeric swap would sit.

We record three things together: a mass measurement with stated convention and accuracy, a chromatographic trace from a stated method, and the extent of any fragmentation coverage. Peptide identity mass spectrometry is reported this way in the literature, as a package rather than as a single figure.

• Mass match: observed against calculated, with convention stated.
• Fragmentation coverage: which residue junctions are evidenced.
• Chromatographic trace: which peak carries the reported mass.
• Instrument and calibration noted beside the accuracy figure.

Limits we state explicitly

Even with full fragmentation coverage, standard tandem experiments do not distinguish leucine from isoleucine, and they do not resolve a D residue from an L residue without a chiral separation or a specific derivatisation. We record these as open items rather than failures, because the analytical claim should match the claim the evidence supports.

This page forms part of the editorial desk notebook on analytical documentation. Where a documentation set is incomplete we say so in the record rather than infer the missing measurement, and we keep the raw material quality overview as the place where these separate lines of evidence are brought together.

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