Quantitative analysis of broad molecular weight distributions obtained by matrix-assisted laser desorption ionisation time-of-flight mass spectrometry

In order to quantify the error of matrix-assisted laser desorption ionisation (MALDI) time-of-flight (TOF) mass spectrometry in the determination of broad molecular weight distributions, different mixtures by weight of two poly(methyl methacrylate) standards were prepared. These mixtures, with well-defined bimodal molecular weight distributions were analysed by MALDI-TOF mass spectrometry using different matrices (2,4,6-trihydroxyacetophenone and 2,5-dihydroxybenzoic acid) and different cations (Li+, Na+, K+, Rb+ and Cs+) for doping the analyte. From the MALDI-TOF mass spectrometric data, the weight fractions of the two polymers of all mixtures were determined and compared to the values measured by size-exclusion chromatography. The results show that it is possible to obtain the correct M(n) and M(w) values with 2,5-dihydroxybenzoic acid as matrix and K+ as cationisation metal.