Silver cluster interferences in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of nonpolar polymers

Potential difficulties associated with background silver salt clusters duri ng matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of nonpolar polymers are reported. Silver salt cluster ions were observed from m/z 1500 to 7000 when acidic, polar matrices, such as 2,5-dihydroxyben zoic acid (DHB), all-trans-retinoic acid (RTA) or 2-(4-hydroxyphenylazo)ben zoic acid (HABA), were used for the analysis of nonpolar polymers. These ba ckground signals could be greatly reduced or eliminated by the use of nonpo lar matrices such as anthracene or pyrene. Representative examples of these background interferences are demonstrated during the analysis of low molec ular weight nonpolar polymers including polybutadiene and polystyrene. Nonp olar polymers analyzed with acidic, polar matrices (e.g., RTA) and silver c ationization reagents can yield lower quality mass spectral results when in terferences due to silver clusters are present. Replacing the polar matrice s with nonpolar matrices or the silver salts with copper salts substantiall y improved the quality of the analytical results. In addition, it was found that silver contamination cannot be completely removed from standard stain less steel sample plates, although the presence of silver contamination was greatly reduced after thorough cleaning of the sample plate with aluminum oxide grit. Carry-over silver may cationize polymer samples and complicate the interpretation of data obtained using nonpolar matrices in the absence of added cationization reagents. (C) 2001 American Society for Mass Spectro metry.