Ionisation bias undermines the use of matrix-assisted laser desorption/ionisation for estimating peptide deamidation: Synthetic peptide studies demonstrate electrospray ionisation gives more reliable response ratios.

MALDI may not be a suitable ionisation method for the determination of deamidation in peptide mixtures. However, ESI was successfully used to determine the ratio in known mixtures of Q- and E-containing peptides. These preliminary observations warrant further investigation into ionisation bias when measuring deamidation in other peptide sequences.