The matrix-assisted laser desorption/ionisation in-source decay of peptides using ion mobility enabled quadrupole-time-of-flight mass spectrometry

Rationale In-source decay (ISD) matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry with a 1,5-diaminonaphthalene (1,5-DAN) matrix is used for the structural characterisation of peptides. However, MALDI spectra are intrinsically complicated by the presence of matrix ions, which interfere with the peptide fragments. This may cause false-positive results or reduced sequence coverage. This paper reports investigations of ISD processes in an intermediate pressure MALDI ion source and a protocol for the removal of interfering ions using ion mobility separation (IMS). Methods An intermediate pressure MALDI source of a Q-IMS-Q-TOF instrument (Synapt G2) has been employed for the ISD of selected peptides using a 1,5-DAN matrix. Results Successful coupling of the MALDI source tuned for ISD experiments using IMS is demonstrated. The IMS made it possible to remove interfering matrix ions effectively from the spectra and thus to increase the confidence of spectral interpretation. Extensive fragment series corresponding to N-C-alpha bond cleavages were observed under optimised conditions; on the other hand, weaker series of ions caused by peptide bond cleavages were prevalent for default conditions and/or the alpha-hydroxycinnamic acid matrix. Conclusions Ion mobility has been used for the elimination of matrix ions. The technique has been applied to top-down sequencing of non-tryptic peptides, such as the human palmitoylated analogue of prolactin-releasing peptide used in recent obesity studies, and human and insect antimicrobial peptides.