A peptide can be detected in association with different ions. These adducts change the measured ion's mass and sometimes its charge assignment, even when the peptide's covalent sequence is unchanged. Reading the complete ion label helps prevent an associated ion from being mistaken for a new peptide or an unexplained sequence modification.
Read the added species as part of the assignment
IUPAC describes an adduct ion as containing a constituent's atoms together with additional atoms from an interacting species. Such associations can form in an ion source. In peptide spectra, labels such as [M+H]+ and [M+Na]+ distinguish proton-associated and sodium-associated forms of the same neutral molecular reference M.IUPAC — Adduct ion (opens in a new tab)
The brackets tell you which species contribute to the ion; the superscript charge tells you how that ion appears on the m/z axis. Do not interpret a sodium-containing assignment by changing the peptide's amino-acid sequence unless separate evidence supports such a covalent change.
Adduct terminology does not remove the need for precise chemical bookkeeping. A mixed proton-and-sodium ion, a neutral salt formula and a covalently modified molecule are different descriptions. The expected mass follows the actual proposed composition.
Calculate replacement shifts rather than adding a whole atom
For an original example, take a neutral monoisotopic mass of 1,000.0000 Da. Use rounded positive-ion masses of 1.0073 Da for a proton and 22.9892 Da for sodium. The sodium-ion value accounts for loss of an electron from the neutral atom; these rounded numbers are sufficient for this illustration.NIST — 2022 CODATA constants (opens in a new tab)NIST — Sodium isotope mass (opens in a new tab)
| Assignment | Calculated m/z |
|---|---|
| [M+H]+ | 1001.0073 |
| [M+Na]+ | 1022.9892 |
| [M+2H]2+ | 501.0073 |
| [M+H+Na]2+ | 511.9983 |
The shift from [M+H]+ to [M+Na]+ is approximately 21.9819, because sodium replaces a proton in this comparison. It is not an extra 23 added to the already protonated signal. At charge 2+, the corresponding replacement shift is halved, approximately 10.9910.
These calculations propose compatible ion assignments; they do not prove that real peaks with these gaps are related. The same charge, coherent isotope patterns and matching chromatographic behaviour strengthen an interpretation, while coincidental overlaps remain possible.
Look for support beyond one extra peak
A primary study by Voinov and colleagues examined sodium-adducted peptides using electron-capture dissociation. Sodium association affected fragmentation, but the tested spectra still contained useful sequence information. The study concerns specified peptides and analytical conditions, rather than a universal rule that every adduct behaves identically.Voinov and colleagues — Sodium-adducted peptide fragmentation (opens in a new tab)
For a routine interpretation, preserve both the proposed neutral mass and the supporting ion labels. If several assigned forms point back to the same neutral mass, explain that relationship explicitly. If one proposed adduct lacks supporting evidence, leave the assignment qualified.
Keep ion behaviour separate from vial composition
The intensity of a sodium-associated ion depends on how the sample and instrument produce and transmit that ion. It is not a direct gravimetric measurement of sodium in the supplied material. A strong adduct signal alone cannot establish a counterion ratio or a percentage salt content.
Similarly, disappearance of an adduct under different analytical conditions does not demonstrate removal of a covalent impurity. First establish whether the observed change concerns ion formation, separation, processing or the material itself. Questions about final salt composition need a measurement designed for that quantity.
Sources and further detail
- IUPAC — Adduct ion (opens in a new tab)
Terminology definition of associated ion composition.
- NIST — 2022 CODATA constants (opens in a new tab)
Proton mass in unified atomic mass units. Examples round it to 1.007276 Da or 1.0073 Da as stated; exact ion assignments require the appropriate mass convention.
- NIST — Sodium isotope mass (opens in a new tab)
Neutral sodium-23 mass. The example uses the positive-ion mass rounded after accounting for an electron.
- Voinov and colleagues — Sodium-adducted peptide fragmentation (opens in a new tab)
J Am Soc Mass Spectrom 26, 2096–2104 (2015), DOI 10.1007/s13361-015-1230-y. Primary electron-capture-dissociation study; no sample-preparation procedure reproduced.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.