We use 2D UV-MS cold-ion spectroscopy for the identification of l-Asp, d-Asp, l-isoAsp and d-isoAsp residues in a fragment peptide derived from the hormone protein amylin. Relative solution concentrations of all four isoforms in an equimolar quaternary mixture have been determined within 4% error. This method demonstrates that for binary mixtures of the peptides an accuracy of 2.5% can be reached in few-second measurements.