IR-MALDI OF LOW-MOLECULAR-WEIGHT COMPOUNDS USING A FREE-ELECTRON LASER

Initial experiments on infrared matrix-assisted laser desorption/ioniz ation mass spectrometry (IR-MALDI) using a free electron laser in the analysis of low-molecular-weight compounds are reported. Mass spectra from samples of ethylenediaminetetraacetic acid (EDTA), nitrilotriacet ic acid (NTA) and a phosphate salt were obtained. Low molecular weight (LMW) compounds are of similar mass as the molecular weight of the ma trix compound and interference between matrix and analyte ions can deg rade MALDI sensitivity. Under optimal conditions, however, we find lit tle interference and that often the analyte signal exceeds the matrix signal, a result also observed in UV-MALDI of these compounds, In the IR-MALDI of EDTA near 3.0 mu m the FEL results are similar to those ob tained using a fixed frequency 2.94 mu m Er:YAG laser of 200 ns pulse duration. However. the tunable FEL laser can selectively excite partic ular vibrational modes of the matrix (e.g., an OH or CO stretch) or of residual water contained within the crystalline MALDI sample. This ca pability was used to explore possible vaporization and ionization mech anisms of IR-MALDI for EDTA. (C) 1998 Pacific Northwest National Labor atory. Published by Elsevier Science B.V.