ANALYTICAL METHODS · PRACTICAL GUIDE
Extra Mass-Spectrum Peaks: Sodium and Potassium Adducts Explained
An extra peak may be another ion of the same molecule. Learn what an adduct is.
A mass spectrum has a second peak about 22 units above the first. Has the peptide gained an impurity? One possible explanation is a sodium adduct: the same molecule has formed a different ion. We will use a simple example to see the difference.
The instrument detects ions
In a positive-ion spectrum, a molecule may form an ion by gaining a proton. We write that as [M+H]+, where M represents the neutral molecule. It may also form [M+Na]+ or [M+K]+ with sodium or potassium.
These signals can represent different ions of the same molecule, rather than three different peptide sequences.
Calculate three single-charge signals
For a teaching neutral mass of 1,000.0000 Da, the approximate m/z values are 1,001.0073 for [M+H]+, 1,022.9892 for [M+Na]+ and 1,038.9632 for [M+K]+.
The sodium signal is about 21.9819 above the proton signal. The potassium signal is about 37.9559 above it. Each difference compares two matching single-charge ions.
Why charge changes the apparent gap
The horizontal axis is m/z: mass divided by charge. If sodium replaces one proton in otherwise matching double-charge ions, the gap is about 21.9819 ÷ 2 = 10.9910 m/z.
Before using an expected gap, check the charge and the actual ion formulas. Do not compare unrelated peaks simply because their spacing looks familiar.
A plausible gap is a clue, not a final answer
Check the full pattern, isotope clusters and the chromatographic region in which the signals occur. Background signals or another molecule can complicate the picture.
Adding the heights of these ions does not give an HPLC purity percentage. They are different measurements.
Check the charge before calculating
A useful report names the proposed ions instead of leaving the reader to guess from peak spacing alone.
See the answer
About 10.9910 m/z. This calculation assumes that specific replacement and a charge of two.
Reading KPV or MOTS-c spectra
For KPV or MOTS-c spectra, ask which peaks were assigned to the target molecule and which adducts were considered. An extra peak needs an explanation, but it is not automatically a new impurity.
Discuss material and document requirements with NHD · Continue with the analytical guide
MOTS-c (Human)
SS-31 (Elamipretide)