The Influence of Matrix and the Laser Energy of MALDI on the Molecular Mass Distribution of Poly(ethylene glycol)
Stephanie J. Wetzel1,2, Charles M. Guttman1 and James E. Girard2

1Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8541, 2Department of Chemistry, American University, Washington, D.C. 20016.


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.