The limitations of MALDI-TOF mass spectrometry in the analysis of wide polydisperse polymers

Average molecular weight determination of polymers with polydispersities greater than 1.2 is an ongoing challenge in the field of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Mass discrimination effects observed in the analysis of these polymers have been attributed to sample preparation, desorption/ionization, and instrumental factors. In an effort to separate these factors, we studied poly(methyl methacrylate) (PMMA) standards using two different ion detection systems installed on the same time-of-flight mass analyzer. Equimass blends of narrow PMMA standards were used to simulate a polymer with a wide polydispersity. MALDI-MS analysis was also performed on a PMMA standard with a polydispersity of 1.7. All samples were analyzed by size exclusion chromatography for comparison. Although sample preparation and ionization/desorption factors were found to influence the spectral appearance of the MMA distributions, we demonstrate that, under similar sample preparation and instrument conditions, different ion detection systems produce different results for synthetic polymer blends. The differences in the detector responses for the blends and wide polydisperse standard arise from several factors related to the ion detection system: (1) detection mechanisms, (2) saturation effects, and (3) signal-to-noise limitations.