Use of MALDI-TOF to measure molecular weight distributions of polydisperse poly(methyl methacrylate)

The main problem encountered in the MALDI-TOF analysis of polydisperse polymers is mass discrimination against high-mass oligomers. This work investigated some of the causes of this problem by using PMMAs as the polymer analytes. It was found that both instrumental and matrix factors could lead to this problem. Among the instrumental factors, detector saturation resulting from strong signals of matrix-related and low-mass oligomer ions can be a potential major cause of this problem. Since most of the ion detectors do not have an adequate dynamic range to avoid saturation, detection saturation could be a fundamental limitation, especially when the molar ratio of high- to low-mass oligomers is small. A quantitative analysis was also performed to examine the matrix effect. IAA and HABA were selected for this study. It was found that mass discrimination occurred in both cases, but the use of HABA led to more profound mass discrimination. This shows that the use of improper matrixes could be another source causing mass discrimination. Hence, unless new approaches are developed, one must be cautious in using MALDI-TOF for directly measuring MWDs of polydisperse polymers, especially those highly polydisperse polymers.