How Do You Read a Peptide Mass Spectrum?

Why one peptide produces a ladder of peaks

Electrospray ionisation adds or removes protons in solution and then ejects the charged droplets into the gas phase. A peptide with several basic sites can carry different numbers of protons, and each proton adds one unit of charge and one unit of mass. The instrument separates ions by mass-to-charge ratio, so a single molecular species appears at several m/z values, spaced according to its mass and charge.

A 1.3 kDa peptide with two or three basic residues commonly shows 2+, 3+, 4+ and 5+ ions. A peptide rich in arginine may extend to 6+ or higher; a neutral, acidic peptide may show only 2+ and 3+. That is normal and not a sign of degradation. It is why a raw ESI spectrum of a pure peptide looks busy, and why the deconvoluted mass, not the raw peak pattern, is the number to judge.

Calculating charge state from the spacing

Adjacent charge states differ by one proton, so the mass difference between them is the neutral mass divided by the product of the two charges. For a 1347.7 Da peptide the pattern is predictable.

To assign charge without knowing the mass, take two adjacent peaks and apply z = (lower m/z − 1.008) ÷ (higher m/z − lower m/z). Using 337.93 and 450.24 gives (337.93 − 1.008) ÷ 112.31 = 3.00, so the 450.24 peak is the 3+ ion. The same result from every pair is the internal check that the envelope has been read correctly. A spacing that does not produce a near-integer charge means the peaks are not adjacent charge states of one compound.

Deconvolution: turning the envelope into a neutral mass

Deconvolution is the software step that takes every observed charge state, strips the protons, and reports the mass of the neutral molecule. Good practice is to confirm that all major charge states converge on the same value, and to look at the width of the isotope envelope. At 1 kDa the isotopes of a peptide are resolved or nearly resolved; at 5 kDa they merge into a broad peak, and the centroid becomes the practical measurement.

The report should state the deconvoluted mass, the theoretical mass from the sequence, and the difference in Da and ppm. It should also state the ionisation mode and the charge states that were used. A deconvoluted mass with no visible envelope is a number without a derivation, and a buyer cannot tell whether it came from five consistent charge states or from a single noisy peak that the software over-interpreted.

MALDI-TOF: the singly charged [M+H]+ ion

Matrix-assisted laser desorption/ionisation usually produces singly charged ions, so a peptide appears as one dominant [M+H]+ peak with smaller sodium and potassium adducts at about +22 and +38 Da relative to the protonated ion. The spectrum is simpler to read than an ESI envelope, which makes MALDI attractive for rapid confirmation and for mixtures where you want to see several components at once.

The trade-offs are resolution and mass accuracy. A linear MALDI-TOF may not resolve a deamidation of 0.98 Da on a 3 kDa peptide, and adduct formation can broaden or split the main peak. Reflector mode improves resolution but can distort relative intensities. MALDI is therefore strong evidence of the nominal mass and weak evidence of fine structure, while ESI on a high-resolution instrument is better for confirming that a small mass difference is real.

Mass accuracy in ppm, and what a 1 Da error means

Mass error in ppm is the observed minus theoretical mass, divided by theoretical mass, multiplied by one million. At 1.2 kDa, 100 ppm is 0.12 Da, and 10 ppm is 0.012 Da. Modern ESI instruments on a clean peptide routinely reach 5 to 20 ppm; a report quoting whole-Dalton agreement is not demonstrating much on a molecule of that size.

A discrepancy of about 1 Da is far too large to be instrument error at this mass, so it is a structural signal. The usual explanations are deamidation of an asparagine or glutamine (+0.98 Da), a C-terminal free acid where an amide was expected (−0.98 Da), or, on low-resolution data, an unresolved truncation product that pulls the observed centroid. A missing residue would shift the mass by at least 57 Da for glycine and typically 71 to 186 Da for other residues, which is a different order of problem. Either way, ask for the annotated spectrum before accepting the batch.

Diagnostic mass shifts worth memorising

These increments explain most unexpected masses. A 2 Da increase on a cysteine-containing peptide usually means incomplete disulfide formation, not a synthesis error, and a 16 Da satellite peak on a methionine peptide is oxidation that may have occurred after release. Correlating the mass shift with an HPLC impurity peak is how the two techniques reinforce each other; the chromatogram side is covered in how to read an HPLC chromatogram.

What to check on a peptide mass spectrum report

• Method: ESI or MALDI, instrument type, and whether LC-MS or direct infusion was used.
• Raw spectrum: the actual peaks, not only a table. Charge states should be labelled.
• Deconvoluted mass: with the charge states used and the software algorithm.
• Theoretical mass: calculated from the stated sequence, with the convention identified as monoisotopic or average.
• Error: in Da and ppm, not a rounded nominal mass.
• Adduct and impurity annotation: sodium, potassium, TFA and oxidation peaks identified rather than ignored.

MOL Changes releases every batch with the mass spectrum alongside the chromatogram and COA, so the observed mass, the sequence and the purity can be checked as one package. If a spectrum is missing, the COA checklist explains what else is likely absent.