Why Mass-Spectrum Peaks Form a Cluster: Isotope Spacing and Charge

Several closely spaced peaks appear in the spectrum. Are they several impurities? They may instead be an isotope cluster from one type of molecule. The spacing can help explain the ion’s charge.

The same element can have slightly different masses

Atoms of the same element can have different numbers of neutrons. These versions are called isotopes. A molecule containing a heavier isotope has a slightly different mass, even when its peptide sequence has not changed.

Peptides contain many atoms, so a sample can give a group of nearby isotope signals rather than a single line. Actual patterns depend on the elements and isotope combinations present.

Why more charge squeezes the gaps

The axis is m/z: ion mass divided by charge. Using a common carbon-isotope spacing approximation of 1.00335 Da, a single-charge group has gaps near 1.00335 m/z. With two charges, the gaps are near 0.50168; with three, near 0.33445.

The mass difference is divided by a larger number. That is why the peaks move closer together on this axis.

Read the scale, not just the shape

Zoom into a clearly resolved cluster and subtract the positions of adjacent peaks. A gap near 0.50 m/z suggests two charges in this model. A gap near 0.33 suggests three.

This only works if you are comparing peaks from the same cluster. Overlapping clusters or insufficient resolution can make the estimate misleading.

Turn m/z back into molecular mass

Suppose the ion is confirmed as [M+2H]2+ and its m/z is 501.0073. Multiply by two, then subtract the mass of the two added protons: 2 × 501.0073 − 2 × 1.007276 ≈ 1,000.0000 Da.

Spacing helps estimate charge, but does not by itself establish the adduct type. Check the ion formula before using this calculation.

Try reading a three-charge cluster

A convincing cluster can help interpret the mass signal. It does not measure peptide activity or establish every position in its sequence.

Checking MOTS-c and SS-31 plots

For MOTS-c or SS-31 spectra, ask which isotope clusters and charge states support the reported molecular mass. Keep the annotated spectrum with the calculation.

MOTS-c / SS-31

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