Mass spectrometry for peptide identity

Electrospray ionisation puts charge on a peptide by adding protons to its basic sites, and it generally adds more than one. The instrument measures mass divided by charge, so a single compound produces a series of peaks at different m/z values — none of which equals the molecular weight.

A certificate reporting an observed mass of 710.8 for this peptide has not made an error — it is reporting the doubly-charged ion, which is often the most intense. The charge state must be stated for the number to mean anything. Deconvolution converts the series back to a single neutral mass, and a good certificate reports that too.

Every element occurs as a mixture of isotopes, so a peptide does not have one mass. It has two conventional masses:

• Monoisotopic mass uses only the lightest isotope of each element — ¹²C, ¹H, ¹⁴N, ¹⁶O. This is the first peak of the isotope envelope, and what a high-resolution instrument reports.
• Average mass weights each element by its natural isotope abundance. This is the figure quoted in catalogues and on product pages.

For a peptide around 1400 Da these differ by roughly one dalton, and the gap widens with size. That would be a footnote, except for what else is worth about one dalton.

Deamidation — asparagine converting to aspartic acid, or glutamine to glutamic acid — is one of the most common degradation pathways in stored peptides. Because its shift is so close to the convention gap, a real degradation product can be waved away as a units mismatch, and a genuine units mismatch can be mistaken for degradation. The only defence is a certificate that states which convention it used — and a surprising number don't.

When an observed mass doesn't match, the size of the discrepancy usually names the cause. Monoisotopic shifts:

Deletion masses are simply the residue masses, so the shift identifies which residue failed to couple. This is the practical value of MS on a certificate: it doesn't just say pass or fail, it tells a synthesis chemist where the synthesis went wrong.

Three specific blind spots

Leucine and isoleucine are isobaric. Both have a residue mass of 113.084 Da. No mass spectrometer at any resolution can distinguish them, because they are not merely close — they are identical in elemental composition. A peptide specified with leucine that was synthesised with isoleucine passes MS identity perfectly.

Glutamine and lysine differ by 0.036 Da. That gap is resolvable on a high-resolution instrument and invisible on a unit-resolution quadrupole. Whether this substitution is detectable depends entirely on which instrument produced the certificate — which is a reason to want the instrument named.

Stereochemistry has no mass. Solid-phase synthesis can racemise residues during coupling, and a D-amino acid weighs exactly what its L counterpart weighs. Racemisation is completely invisible to mass spectrometry, and it is not visible on a standard purity chromatogram either. Detecting it requires chiral analysis, which does not appear on research-grade certificates at all.

Two different specifications get conflated. Resolution is the ability to separate two nearby masses. Accuracy is how close the reported value is to the true one, usually quoted in parts per million. A unit-resolution instrument reports nominal masses; a time-of-flight or Orbitrap instrument reports three or four decimal places with accuracy in the low ppm.

This matters when reading a certificate, because a reported figure implies a capability. A value quoted to two decimals from an instrument that resolves to the nearest dalton is a false precision — the digits are arithmetic, not measurement.

If mass alone cannot establish order, three methods can — in descending order of how often you will see them:

• Tandem MS (MS/MS). The peptide is fragmented and the resulting b- and y-ion series are read off. Consecutive differences give the residue at each position, walking the sequence from both ends. This is the standard method — and it still cannot separate leucine from isoleucine.
• Edman degradation. Removes and identifies one residue at a time from the N-terminus. Slow, needs more material, and blocked entirely by a modified N-terminus — but it does distinguish Leu from Ile.
• Amino acid analysis. Confirms composition rather than order, which is weaker. But it is genuinely orthogonal, and it catches a substitution MS would miss whenever that substitution changes the residue count.

None of these is standard on a research-grade certificate. That is not necessarily a failing — for most laboratory work, identity by mass alongside a purity chromatogram is a reasonable evidentiary standard. It is only a problem when a certificate implies more than it establishes.

Does mass spectrometry prove a peptide has the correct sequence?

No. It proves the mass is consistent with the claimed sequence. Any rearrangement that preserves total mass — residues in a different order, leucine swapped for isoleucine, a D-amino acid in place of an L — produces an identical result. Sequence confirmation requires tandem MS fragmentation or Edman degradation.

Why doesn't the observed mass on my certificate match the molecular weight?

Most likely because it is a multiply-charged ion. Electrospray adds several protons, and the instrument reports mass divided by charge, so a 1419 Da peptide commonly appears at around 710 as the doubly-charged species. Check whether a charge state is stated. If none is, ask.

What is the difference between monoisotopic and average mass?

Monoisotopic uses only the lightest isotope of each element and is what high-resolution instruments report. Average weights all isotopes by natural abundance and is what catalogues quote. Around 1400 Da they differ by roughly one dalton. A certificate comparing an observed monoisotopic value against an average theoretical value will appear to show an error that isn't there — and can equally hide a real one.

Can MS detect that a peptide has degraded in storage?

Often, yes. The two commonest storage-related changes both shift mass: deamidation at +0.98 Da and oxidation at +16.00 Da. Both are detectable on a high-resolution instrument. But a certificate reflects the material at the time of testing, not the material in your freezer — degradation after analysis is invisible by definition. See lyophilisation and peptide stability.

Is MALDI or ESI better for peptide identity?

They answer the same question differently. MALDI mostly produces singly-charged ions, so the spectrum is simpler to read and the mass is nearer the molecular weight. ESI produces a charge-state series that needs deconvolution but couples directly to HPLC, so purity and identity can be measured in one run. Either is acceptable; the certificate should say which was used.

• United States Pharmacopeia, general chapter <1736>, Applications of Mass Spectrometry — resolution, mass accuracy and ionisation methods.
• United States Pharmacopeia, general chapter <1052>, Biotechnology-Derived Articles — Amino Acid Analysis, for the orthogonal composition method.
• Standard monoisotopic residue masses and post-translational modification mass shifts, as tabulated by the Association of Biomolecular Resource Facilities and in the UniMod database.
• References on racemisation during solid-phase peptide synthesis, particularly at histidine and cysteine during coupling.