Comparison of most probable peak values as measured for polymer distributions by MALDI mass spectrometry and by size exclusion chromatography

Matrix-assisted laser desorption/ionization mass spectrometry (MS) has been shown to provide most probable peak (M(p)) values for poly(methyl methacrylate) polymers that are low relative to manufacturers, M(p) values measured by size exclusion chromatography (SEC). Comparison of theoretical M(p) values determined by MS or SEC is shown to be a function of how the data are displayed. For narrow polymer distributions, the theoretical M(p) value determined by MS will be 2 monomer units smaller than the M(p) determined by SEC. For wide polydispersity, the M(p) value determined by MS will be considerably lower than those obtained from SEC. The M(p) value reported should be reserved for weight fraction vs log mass plots as in SEC. The modal molecular mass, M(m), is recommended for data represented as number fraction vs linear mass as with MS data.