Generation of highly charged peptide and protein ions by atmospheric pressure matrix-assisted infrared laser desorption/ionization ion trap mass spectrometry

We show that highly charged ions can be generated if a pulsed infrared laser and a glycerol matrix are employed for atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry with a quadrupole ion trap. Already for small peptides like bradykinin, doubly protonated ions form the most abundant analyte signal in the mass spectra. The center of the charge-state distribution increases with the size of the analyte. For example, insulin is detected with a most abundant ion signal corresponding to a charge state of four, whereas for cytochrome c, the 10 times protonated ion species produces the most intense signal. Myoglobin is observed with up to 13 charges. The high m/z ratios allow us to use the Paul trap for the detection of MALDI-generated protein ions that are, owing to their high molecular weight, not amenable in their singly protonated charge state. Formation of multiple charges critically depends on the addition of diluted acid to the analyte-matrix solution. Tandem mass spectra generated by collision-induced dissociation of doubly charged peptides are also presented. The findings allow speculations about the involvement of electrospray ionization processes in these MALDI experiments.