A purchasing analyst compares the mass reported on a peptide’s analytical document with the theoretical value she calculated from the sequence. They agree to within a fraction of a dalton. Reassured, she files the document. Then a colleague points out that the raw electrospray spectrum shows no peak anywhere near that mass. The peaks sit at around half and a third of it. Nobody has made an error. The reported figure was never read directly from the spectrum; it was calculated from a pattern of peaks by a process called deconvolution.
Understanding that process explains both why the reported mass is usually trustworthy and why it can occasionally be wrong without any visible warning.
Electrospray produces charge states, not one peak
Electrospray ionization (ESI) adds protons to molecules. In a peptide, the basic sites accept them: the N-terminal amine and the side chains of lysine, arginine and histidine. Across the many molecules entering the source, different numbers of protons are picked up, so a single compound appears as a family of ions with different charges.
A mass spectrometer does not measure mass directly. It separates ions by mass-to-charge ratio, written m/z. An ion of neutral mass M carrying n protons appears at (M + n x 1.00728) / n, where 1.00728 is the mass of a proton. Each charge state gives its own peak, and the whole set is called the charge envelope. That is why the colleague saw peaks at fractions of the expected mass.
Working out the mass by hand
Any two neighboring peaks in the envelope are enough to solve for the mass, because their charges differ by exactly one. Consider an envelope with peaks at m/z 1201.0 and 801.0:
- Let the peak at 1201.0 carry charge n, so the peak at 801.0 carries charge n + 1.
- The neutral mass from the first peak is n x (1201.0 – 1.00728).
- The neutral mass from the second is (n + 1) x (801.0 – 1.00728).
- Setting these equal gives 1199.99n = 799.99n + 799.99, so n = 2.
- The neutral mass is therefore about 2 x 1199.99, or roughly 2400.0 Da.
The check comes next. A 4+ ion of a 2400.0 Da molecule should appear near m/z 601.0. If a peak is there, the assignment holds together. Deconvolution software does the same thing across the whole envelope at once and produces a reconstructed spectrum plotted against neutral mass. The number on an analytical document usually comes from that reconstruction.
The assumptions built into the answer
Because the mass is derived, it rests on assumptions: that the peaks belong to one species, that the spacing between them reflects protons, and that enough of the envelope was recorded to fix the solution. When those hold, the result is better than any single peak, because it averages several measurements. When they fail, the software still produces a confident number that looks identical to a correct one.
How deconvolution goes wrong
| Situation | What happens | How it might show |
|---|---|---|
| A closely related second species | Two envelopes interleave; software may average them into one wrong mass or split them incorrectly | Broad or shouldered peaks in the raw spectrum |
| Sodium or potassium adducts | Ions carry sodium or potassium in place of a proton, adding about 22 or 38 Da to the neutral mass | Satellite peaks at consistent offsets; may be reported as real species |
| Part of the envelope outside the scan range | Fewer charge states constrain the solution, so uncertainty rises | Envelope visibly cut off at one end |
| Heavy smoothing | Minor components disappear from the reconstruction | Reconstruction cleaner than the raw data |
Reading charge from the isotope pattern
There is a second, more direct way to find the charge of an ion. Naturally occurring carbon-13 gives every organic molecule a cluster of isotope peaks about 1.0033 Da apart in mass. On the m/z axis that spacing is divided by the charge: about 1.0 for a 1+ ion, 0.5 for 2+, 0.33 for 3+, 0.25 for 4+. Reading the spacing gives the charge without any assumption about neighboring peaks.
The catch is resolving power. The instrument must separate the individual isotope peaks. Lower-resolution instruments cannot, and have to rely on the spacing between charge states instead. More on this in isotope patterns in peptide mass spectra.
Monoisotopic or average mass?
Whether the isotope cluster was resolved also decides what kind of mass is reported. With resolved isotopes, the monoisotopic mass, built from the lightest isotope of each element, can be determined. Without resolution, the reconstruction gives an average mass weighted across the natural isotope distribution. For a peptide of around 3,000 Da the two differ by roughly two daltons. Comparing a reported value against the wrong kind of calculated value produces a discrepancy of exactly that size, which can look like a problem with the material when it is only a mismatch of conventions.
What a useful mass report includes
When reviewing a reported peptide mass, look for:
- The raw spectrum alongside the deconvoluted result, so the envelope and charge spacing can be checked
- A statement of whether the value is monoisotopic or average
- Any visible adducts or secondary envelopes, rather than a reconstruction alone
A deconvoluted mass without the raw data is usually correct, but it cannot be checked. For how mass evidence fits with other identity data, see mass spectrometry and peptide identity, and for reading analytical paperwork in general, the peptide certificate of analysis. Battle Born’s own published testing for each product is independent reverse-phase HPLC; when a mass result from another source is part of a research file, these are the points to verify.
Questions
Why does an ESI spectrum show peaks at fractions of the expected mass?
Because each peak is an ion with multiple charges, and the instrument measures mass-to-charge. A 3+ ion appears at about one third of the neutral mass plus a proton.
Can deconvolution invent peaks?
It can report artifacts, such as unmodeled salt adducts or interleaved envelopes, as if they were real species. The raw spectrum is the check.
Which is more reliable, isotope spacing or charge-state spacing?
Isotope spacing, when the instrument resolves it, because it gives the charge directly without assumptions about neighboring peaks.
Why might a reported mass be about two daltons off for a 3,000 Da peptide?
Most likely because the reported value is average mass and the comparison value is monoisotopic, or the reverse.
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