Can you explain the [M+Na] and [M+K] mass peaks observed in the MALDI-TOF or ESI mass spectra?
Peptide molecules can be ionized by the Na+ or/and K+, it is common to see their adducts from the MALDI-TOF or ESI mass spectra. Those ions come from the water used for the peptide purification or the buffer used for the mass analysis. They can never be completely removed, even distilled or deionized water is used. Instead of ionized by proton, some peptide molecules are ionized by Na+ or/and K+. It is common and normal to observe the [M+Na] and [M+K] peaks in MALDI-TOF mass spec.
