Novel Procedure for Molecular Weight Averages Measurement of Polydisperse Polymers Directly from Matrix-assisted Laser Desorption/Ionization Time-of-flight Mass Spectra

Abstract

A new method for the determination of reliable molecular weight averages (MW) and polydispersity index (MWD) values for polydisperse polymers, directly analyzing their matrix-assisted laser/desorption ionization time-of-flight (MALDI-TOF) mass spectra, is reported. Our success is due to the concerted effect of an off-line procedure of MALDI-TOF spectrum processing that includes: (i) signal smoothing to improve the recorded traces of the spectra up to high m/z values; (ii) a subtraction of the misleading contribution to the signals introduced by the offset used in spectrum recording; (iii) examination of the spectrqup to the highest possible m/z value for a better accuracy in estimating the MW values. To illustrate this method, three polymers with a wide polydispersity index, a polydimethylsiloxane, a polymethylmethacrylate, and a bisphenol-A copolyether, have been analyzed. Contrary to the general opinion that MW values can be measured directly by MALDI-TOF only for polymers with a very narrow mass distribution, the data reported here show that our off-line procedure is also able to measure reliable MW values for polydisperse materials.The soft ionization which occurs in matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry allows the detection of very large molecules, even if they are in a complex mixture.'-'' In polymer analysis, the MALDI technique may be used for the direct determination of the polymer structure by the identification of mass-resolved macromolecules including oligomers and terminal groups."-'4 In this field, the prospect of a simultaneous measurement of molecular weight averages (MW) and molecular weight dispersion (MWD) has attracted the interest of polymer scientists. However, for this purpose, the MALDI technique, originally developed for proteins,', has shown serious limitations up to the present time.Several authors have found that MW values provided by MALDI-TOF agree with those obtained by conventional tools (viscosity, gel-permeation chromatography (GPC), laser light scattering) only in the case of polymer samples with narrow MWD (< 1.2>,'. '*-I7 whereas with polydisperse polymers MALDI fails to yield reliable MW estimate^.'^-^' It has been suggested that MALDI underestimates the higher molecular weight species so that, as the polydispersity index of the sample increases, the measured MW values become much lower than those obtained by other methods2 ' Recently, in an attempt to solve this problem, MW and MWD values of polydisperse polymers such as dextrans, polyesters and polysiloxanes, were determined using MALDI as a detector for GPC.'2.'7.2' According to this procedure, during the GPC experiment, several fractions of the polymer solution eluted from the columns were Author for correspondence. collected and the contained materials separately analyzed off-line by MALDI-TOF MS. Since these polymer fractions have a narrow MWD, they give good MALDI spectra with MW values in agreement with those obtained by other techniques. The eluted volumes of the o...

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