The influence of laser fluence on ion yield in matrix-assisted laser desorption ionization mass spectrometry

Ion yields in matrix-assisted laser desorption ionization have been measured as a function of laser fluence using pulse-counting and analog detection methods in the same TOF instrument configured in the usual axial geometry. In addition, measurements have been made using pulse-counting methods in an orthogonal-injection TOF instrument, which uses collisional cooling to spread the plume along the injection axis. This allows pulse-counting methods to be used without ambiguity as to the number of ions striking the detector, and extends the dynamic range over which pulse-counting can be used reliably without changing the instrument transmission. All measurements indicate a steep power-law dependence of the ion yield on fluence with an exponent in the range of 7-9. The same power-law dependence extends below the fluence usually associated with a threshold in analog measurements by at least a factor of two, and the data show no indication of physical fluence threshold. (C) 2002 Elsevier Science B.V. All rights reserved.