Influence of the laser fluence in infrared matrix-assisted laser desorption/ionization with a 2.94 mu m Er : YAG laser and a flat-top beam profile

The dependence of the signal intensity of analyte and matrix ions on laser fluence was investigated for infrared matrix-assisted laser desorption/ioni zation (IR-MALDI) mass spectrometry using a hat-top laser beam profile, The beam of an Er:YAG laser (wavelength, 2.94 mum; pulse width, 90 ns) was cou pled into a sapphire fiber and the homogeneously illuminated end surface of the fiber imaged on to the sample by a telescope. Three different laser sp ot sizes of 175, 350 and 700 mum diameter were realized. Threshold fluences of common IR matrices were determined to range from about 1000 to a few th ousand J m(-2), depending on the matrix and the size of the irradiated area , In the MALDI-typical fluence range, above the detection threshold ion sig nals increase strongly with fluence for all matrices, with a dependence sim ilar to that for UV-MALDI, Despite the strongly different absorption coeffi cients of the tested matrices, varying by more than an order of magnitude a t the excitation laser wavelength, threshold fluences for equal spot sizes were found to be comparable within a factor of two. With the additional dep endence of fluence on spot size, the deposited energy per volume of matrix at threshold fluence ranged from about 1 kJ mol(-1) for succinic acid to ab out 100 kJ mol(-1) for glycerol, Copyright (C) 2000 John Wiley & Sons, Ltd.