Derivatization of Low Molecular Weight Polymers for Characterization by Matrix-assisted Laser Desorption/Ionization Time-of-flight Mass Spectrometry

Abstract

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is shown to give inaccurate results for the analysis of low molecular weight ethoxylate polymers. It is demonstrated that when the degree of ethoxylation is low (n < 9), MALDI-TOFMS produces substantially higher values for the number-average molecular weight (M n ) than obtained with more classical methods such as NMR spectrometry and a wet chemical method to determine the hydroxyl number. Apparently, this is the result of discrimination of the former technique against lower molecular weight ethoxylates in a polymer distribution. The results presented in this paper demonstrate this discrimination through the analysis of a series of ethoxylates with progressively lower M n values. It was also found that by derivatization of the polymer with phthalic anhydride, to produce the carboxylate derivative, the discrimination is reduced and data obtained with MALDI-TOFMS match more precisely the results obtained with the other methods.

A Comparison of Electrospray Ionization and Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry with Nuclear Magnetic Resonance Spectroscopy for the Characterization of Synthetic Co-Polymers

“…Reports in the literature suggest that MALDI is biased toward high molecular weight (MW) oligomers. 26,27 This was confirmed in the present study by examining the ratio of oligomer peak intensities summed between m/z 500-1200 with that for m/z 1200-1800 for four different reaction times. ESI gave ratios of 1.73, 2.06, 2.12 and 2.07 whereas MALDI gave 1.12, 0.99, 0.91 and 1.02, showing that MALDI consistently gave results skewed to higher m/z value.…”

“…One suggested reason for this bias is that low MW oligomers volatilize off the target prior to analysis. 26 To test this, the mole fraction of BA for a single sample was determined at three different residence times in the ion source: 19 min, 45 min and 57 min. The same ratio was obtained each time, and, thus, the low BA mole fraction results are not due to the higher vapor pressure of low MW oligomers.…”

Small molecule analysis by MALDI mass spectrometry

“…Good resolution of repeat units and cationized molecular ion series was observed up to 1500 Da. [47] However, other studies have shown MALDI to be less accurate at low molecular weights than techniques such as NMR or hydroxyl number determinations for PEGs [48]. Because polymer samples contain a molecular weight distribution of oligomers rather than a single analyte, matrix mass discrimination effects have created problems in analysis.…”

“…PEGs have been derivatized with phthalic anhydride to reduce MALDI mass discrimination effects [48]. Derivatization of polyesters with trifluoroacetic acid was applied as a means of solubilizing otherwise insoluble aromatic polyesters [52].…”

Differential analysis on precise determination of molecular weight of triblock copolymer using SEC/MALS and MALDI-TOF MS

“…The origins of this problem are probably different and may be instrumental in nature [24,25]. We had to disregard this phenomenon for the moment.…”