Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) allows detection of large molecules such as those present in synthetic and natural macromolecules. Until recently, it was reported that MALDI-TOF measurements can provide correct molecular weight (MW) averages only for samples with a narrow MW distribution (MJMn < 1.20). We have now developed a methodology for polydisperse samples. We recorded the GPC trace of two polydisperse polymeric samples, namely, poly(butylene adipate) (PBA) and poly(butylene adipate-co-butylene succinate) (PBAS), collecting about 40-50 fractions per run. Selected fractions were analyzed by MALDI-TOF, and the average MW of each fraction was determined, allowing calibration of the GPC curves against absolute MW. The two calibrated GPC traces were then used to compute average MW and molecular weight distributions (MWD) of the unfractionated samples. End group analysis from MALDI-TOF spectra revealed that the PBA sample is composed of seven different types of chains. For the copolymer sample, PBAS, analysis of the MALDI spectra established a random sequence distribution of comonomer units. The succinate/adipate molar ratio calculated from the MALDI spectra is in good agreement with the molar ratio found by NMR.
The structural characterization and composition of five random copolyesters, originating
from 1,4-butanediol and mixtures of succinic, adipic, sebacic, and terephthalic acids, were obtained by
analysis of their fast atom bombardment (FAB) and their matrix-assisted laser desorption ionization
(MALDI) mass spectra. Multicomponent condensation copolymers may sometimes prove difficult to
characterize by conventional techniques, whereas mass spectrometry is able to handle them. Once the
choice between Bernoullian or Markoffian models has been made, the determination of copolymer
composition, number-average sequence length, and related quantities can be achieved by applying well-defined analytical equations. The theoretical mass spectra of multicomponent copolymers are remarkably
simple, and the number of peaks appearing in each copolymer mass spectrum is easily predictable.
Different kinds of spectral fitting algorithms may help in the actual computations, and it has been shown
that the apparent complexity of mass spectra of copolymers is due to the presence of mass series bearing
different end groups. By selecting a single mass series, one obtains an experimental spectrum immediately
comparable to the theoretical one. Detailed examples, together with a discussion on the reliability of
results, are given to apply the computation procedures and to gain proper understanding of the concepts
involved.
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