1Polymers
Division, National Institute of Standards and Technology, Gaithersburg,
MD 20899-8541, 2Department of Chemistry, American University,
Washington, D.C. 20016.
Abstract
Matrix-assisted
laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS)
has the potential to provide not only end group and branching information
for synthetic polymers, but also molecular mass data. But the characterization
of synthetic polymers by MALDI-TOF-MS has yielded inconsistent results,
indicating that the MALDI-MS determined molecular mass distribution (MMD)
is sensitive to instrumental and sample preparation parameters used to
obtain the polymer mass spectrum. The disparity of these results
produces questions as to the accuracy and repeatability of the MALDI-TOF-MS
method for characterizing synthetic polymers, therefore creating a need
to define the parameters that affect the molecular mass distribution of
the synthetic polymer as determined by MALDI-MS. We considered the effects
of laser energy and matrix on the molecular mass distribution of several
polymers of varying stability. The analysis of poly(ethylene glycol) (PEG)
reveals that the matrix used in the MALDI sample preparation has an effect
on the polymer mass distribution. The data analysis also reveals an influence
of laser energy on the molecular mass distribution. But the effect of laser
energy is matrix dependent. A more intense fragmentation pattern is seen
in the PEG spectrum when analyzed in 2,5-dihydroxybenzoic acid (DHB). Little
or no fragmentation is seen when PEG is analyzed with all-trans
retinoic acid (RA) or dithranol as the matrix.