ABSTRACTThe invention relates to measurement methods for time-of-flight mass spectrometers which operate with an ionizationof analyte subst ances adsorbed at the surface of a samplesupport and an improvement in mass resolution throughdelayed ion acceleration (or "delayed extraction,T) in front ofthe sample support. It particularly relates to velocity focus-ing for good mass resolution simultaneously for wide rangesof masses within the spectrum.The invention consists of focusing the flight-times of theions simultaneously for all masses in wide ranges of interestrelative to their initial velocity, by allowing the accelerationin the first accelerating region to increase in time after beingswitched on. Thus a good resolution cannot only be set forone mass on the spectrum but for all masses in wide rangessimultaneously. In computer simulations, provided there is acorrelation of space and velocity distribution, focusing of aleast first order is obtained simultaneously for all ions.