Explanation of the M+Na and M+K mass peaks in MALDI spectra?

Routinely sodium (Na) and potassium (K) adducts are observed in a MALDI spectrum. Sodium and potassium salts become ionized during the MALDI mass spec process and bind to the peptide's negatively charged groups. Sodium and potassium ions are present in water and solvents used during peptide synthesis and purification. Even distilled and deionized water may have trace amounts of sodium and potassium ions.

Because it is hard to remove sodium and potassium ions quantitatively, these ions are prevalent and unavoidable in MALDI mass spectrums.

The presence of these ions does not indicate that the peptide is not pure, nor should it be confused with an incorrect molecular weight.