MASS-SPECTROMETRY OF LOW-MOLECULAR-MASS SOLIDS BY MATRIX-ASSISTED LASER DESORPTION IONIZATION/

Matrix-assisted laser desorption/ionization combined with time-of-flig ht mass spectrometry (MALDI/TOF-MS) was used for the analysis of low m olecular mass compounds. Three classes of molecules were studied: orga nic acids, salts of oxyanions and amine-based chelating compounds. Mas s spectra from samples of citric, propionic, butyric, oxalic and stear ic acid; ethylenediaminetetraacetic acid (EDTA), N-(2-hydroxyethyl) et hylenediaminetriacetic acid (HEDTA), ethylenediamine N,N'-diacetic aci d (EDDA) and nitrilotriacetic acid (NTA); and sulfate, nitrate, nitrit e and phosphate salts were obtained. These species were analyzed alone and as mixtures in both the positive and negative ion modes. The orga nic acids and oxyanion salts displayed much stronger signals in the ne gative ion detection mode whereas chelating compounds, which contain b asic amine functional groups, yielded stronger signals in the positive ion mode. This implies that detection sensitivity is often better for a particular ion mode in the analysis of small molecules containing l imited classes of functional groups. In all analyses, the presence of high concentrations of sodium was found to quench the MALDI signals. T o increase the detection sensitivity, some samples were processed thro ugh an ion-exchange column to remove sodium ions, This step was found to enhance the signal by two orders of magnitude over untreated sample s. (C) 1997 by John Wiley & Sons, Ltd.