Delayed extraction matrix‐assisted laser desorption time‐of‐flight mass spectrometry

Abstract

A new delayed extraction matrix‐assisted laser desorption/ionization (MALDI) time‐of‐flight (TOF) mass spectrometer system is described which provides dramatically improved performance over that obtained with identical TOF analyzers operated with constant electrical fields. In this system the ions formed by MALDI are produced in a weak electrical field, and subsequently extracted by application of a high voltage pulse after a predetermined time delay. Three parameters can be independently adjusted to optimize the performance. These include the field magnitude and direction during and immediately following ion production, the magnitude of the extraction field, and the time delay between the laser pulse and application of the extraction field. Theoretical equations are presented which guide the selection of these parameters to optimize the performance. Results are presented from three TOF analyzers of similar design, but ranging in size from a 1.3 m linear analyzer to a 6.4 m reflector. In all cases delayed extraction provides substantially improved resolution over that obtained from static operation. In the smallest linear analyzer resolution of M/ΔM (full width at half maximum) of 4000 is demonstrated for angiotensin I, and with the largest reflecting analyzer, the isotope peaks of bovine insulin are resolved nearly to baseline. With larger proteins, the isotopic peaks are not resolved, but resolution approaching the width of the isotopic envelope is obtained. Resolution improvement is demonstrated for proteins up to 30 kDa. In addition to improved resolution, delayed extraction is shown to improve the quality of MALDI mass spectra by suppressing matrix background, reducing chemical noise, and minimizing the effect of laser intensity on performance.