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Neutral loss analysis in MALDI-MS using an ion-gate scanning mode.

Matrix-assisted laser desorption/ionization reflector time-of-flight (MALDI-reTOF) and electrospray ionization (ESI) mass spectrometry (MS) have become essential tools for the characterization of peptides and proteins. Whereas ESI in combination with a triple quadrupole analyzer allows product ion, precursor ion, and neutral loss analyses, MALDI-reTOF instruments can only be used to record product ion spectra based on the in-source or postsource decay (PSD). We describe a new method to perform neutral loss analyses in MALDI-reTOF instruments in a manner that identifies posttranslationally modified peptides and furthermore retrieves sequence information from peptides. The method is based on the selection of ions in a small time interval to record only signals within the corresponding mass interval. By stepping the time interval through the complete mass range, we obtained a spectrum of stable ions by combining the signals of all individually recorded time intervals. This method furthermore permits PSD fragment ions to be identified, since they reach the detector earlier than the stable ions transmitted in the chosen time interval. The neutral loss analysis were calculated by correlating the PSD fragment ions to the corresponding parent ion detected in this time interval. Moreover, this MALDI-MS mode increased the number of detectable signals in complex peptide mixtures and the signal-to-noise ratio.