Analysis of the accuracy of determining average molecular weights of narrow polydispersity polymers by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (original) (raw)
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Analytical Chemistry, 1996
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. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has generated considerable interest as a new method for characterizing polymers. 1-10 The method is especially valuable for structural characterization of individual oligomers of low-mass polymers. 10-14 However, considerable attention has been focused on the ability of MALDI to provide accurate molecular weight distributions. 6-10,14 Because polymer chemists have developed property/function correlations based on measurable values such
Analytical Chemistry, 1997
We report a polymer characterization study by matrixassisted laser desorption/ionization (MALDI) on a linear time-of-flight instrument equipped with pulsed ion extraction for time-lag focusing. It is demonstrated that timelag focusing MALDI provides improved mass resolution and mass accuracy over continuous extraction instruments. Oligomer resolution is extended to a much higher mass range than that observed even by continuous extraction reflectron systems. This allows new opportunities to study the chemical composition and determine the molecular weights of individual components in a mixture of higher molecular weight polymers. It is shown that oligomer resolution can be obtained for poly(ethylene glycol) (repeat unit mass of 44) of mass up to 25 000 u and poly(styrene) (repeat unit mass of 104) up to 55 000 u. Mass measurement accuracy of 80 ppm or better is demonstrated, and the relevance to end-group analysis is shown for two derivatives of poly(ethylene glycol) used as slow-release drugs. The analysis of the molecular weight distribution was investigated at several extraction pulse potentials to determine if there was an effect on the relative peak area. We found that the values of the number-average molecular weight (M n ) and the weightaverage molecular weight (M w ) do not change significantly for a poly(styrene) blend with oligomer masses between 2000 and 15 000 u and a polydispersity of 1.155. The values are within the 1.6% standard deviation observed for repeat analyses at the same extraction pulse.
Journal of the American Society for Mass Spectrometry, 2002
We report quantitative MALDI-TOF measurements for polydimethylsiloxane (PDMS) of two different molecular weights using the relative ratio of the signal intensities of integrated oligomer distributions for these two molecular weight distributions. By reporting the ratio of intensities of the integrals of two oligomer distributions, we assume that the ionization and desorption efficiencies, crystallization conditions and other factors affecting intensity are similar. Poly(methyl methacrylate) (PMMA-33,000) was mixed with PDMS samples to show whether the presence of another material might affect the desorption efficiency. Quantitative values for the number-average molecular weight (M n ), weight-average molecular weight (M w ) and polydispersities (D) were calculated using the oligomer distributions. The results show a linear relationship between the analyte concentrations and the signal intensities in the range from 1,000 Da to 10,000 Da, and the desorption efficiency of these two PDMS materials was the same even in the presence of PMMA. (J Am Soc Mass Spectrom 2002, 13, 914 -920)