Can you explain the M+Na and M+K mass peaks in MALDI spectra?

It is widespread to observe Na (sodium) and K (potassium) adducts in a MALDI spectrum. Sodium and potassium are ionized during the MALDI mass spec process and bind to free carboxyl groups of peptides. Sodium and potassium ions originate from the water and solvents used in the peptide solvents. Trace amounts of sodium and potassium ions are present in distilled and deionized water, and it can be challenging to remove these ions entirely.

Because it is challenging to remove every sodium or potassium ion from water, observing sodium and potassium adducts in a MALDI mass spectrum is very common. However, the presence of these ions is not an indication that the peptide is not pure.