DELAYED EXTRACTION MATRIX-ASSISTED LASER-DESORPTION TIME-OF-FLIGHT MASS-SPECTROMETRY

A new delayed extraction matrix-assisted laser desorption/ionization ( MALDI) time-of-flight (TOF) mass spectrometer system is described whic h provides dramatically improved performance over that obtained with i dentical TOF analyzers operated with constant electrical fields. In th is system the ions formed by MALDI are produced in a weak electrical f ield, and subsequently extracted by application of a high voltage puls e after a predetermined time delay. Three parameters can be independen tly adjusted to optimize the performance. These include the field magn itude and direction during and immediately following ion production, t he magnitude of the extraction field, and the time delay between the l aser pulse and application of the extraction field. Theoretical equati ons are presented which guide the selection of these parameters to opt imize 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 substanti ally improved resolution over that obtained from static operation. In the smallest linear analyzer resolution of M/Delta M (full width at ha lf maximum) of 4000 is demonstrated for angiotensin I, and with the la rgest reflecting analyzer, the isotope peaks of bovine insulin are res olved nearly to baseline. With larger proteins, the isotopic peaks are not resolved, but resolution approaching the width of the isotopic en velope is obtained. Resolution improvement is demonstrated for protein s 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.