X. Michael Liu

Comparative studies of poly(dimethyl siloxanes) using automated GPC-MALDI-TOF MS and on-line GPC-ESI-TOF MS

In this study we compare on-line gel permeation chromatography (GPC) electrospray ionization (ESI) time-of-flight (TOF) mass spectrometry (MS) to automated GPC matrix assisted laser desorption ionization (MALDI) TOF MS for poly (dimethylsiloxane) (PDMS) analysis. Average mass values for a hydroxyl-terminated PDMS (OH-PDMS) sample were obtained and compared to traditional GPC that was calibrated with narrow polystyrene standards, by direct ESI and MALDI MS analysis, by a summation of mass spectra of all GPC fractions, and also by the recalibration method determined by both mass spectrometric methods. Quantitatively, the difference noted here between these hyphenated techniques is that GPC-ESI-TOF MS effectively reports the low-mass oligomers and underestimates the high-mass oligomers, while GPC-MALDI-TOF MS effectively reports the high-mass oligomers and underestimates the low-mass oligomers. In the GPC-ESI-TOF MS experiments, ion current suppression was observed in the high molecular weight region. The suppression effect was confirmed by repeatable sample runs and by injecting different PDMS samples. Higher chromatographic resolution was observed for GPC-ESI-TOF MS compared to GPC-MALDI-TOF MS. In fact, truly mono-disperse oligomers were observed in the low molecular weight range from GPC-ESI MS experiments.

Detailed analysis of α,ω-bis(4-hydroxybutyl) poly(dimethylsiloxane) using GPC-MALDI TOF mass spectrometry

In this study the prepolymer α,ω-bis(4-hydroxybutyl) poly(dimethylsiloxane), used in the formulation of oxygen permeable films, is evaluated by gel permeation chromatography (GPC) combined with matrix assisted laser desorption ionization (MALDI) time of flight (TOF) mass spectrometry (MS). Two unexpected mass distributions are observed in the mass spectra. Reaction schemes for the formation of these distributions are proposed. A solution phase trimethylsilane end group modification was performed on the prepolymer to determine whether the unexpected mass distributions occur as impurities from synthesis or as artifacts from the MS process. Evaluation of the TMS modified prepolymer indicates the unexpected mass distributions indeed occur as impurities from the synthetic procedure. Average molecular weight values are determined by traditional GPC, direct MALDI-TOF MS, and GPC-MALDI-TOF MS methods and the results are compared.

Characterization of silicone rubber extracts using gel- permeation chromatography, matrix-assisted laser desorption/ionization mass spectrometry, electrospray ionization mass spectrometry, Fourier transform infrared spectroscopy and gas chromatography/mass spectrometry

Extracts from a silicone rubber product were characterized by gel-permeation chromatography (GPC), matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry, Fourier transform infrared spectroscopy (FT-IR), electrospray time-of-flight (ESI-ToF) mass spectrometry and gas chromatography/mass spectrometry (GC/MS). The extracted sample was determined to be a complex mixture with a molecular weight ranging from 200 to 210,000 Da as determined from GPC measurements. GPC fractions of the extract sample were further analyzed by MALDI-ToF mass spectrometry, ESI-ToF mass spectrometry, FT-IR spectroscopy and GC/MS. The extracted products were determined to be macrocyclic poly(dimethylsiloxane) oligomers. Two additives known as bis(2-ethylhexyl) adipate and bis(2-ethylhexyl) phthalate were observed in the extract.

A modified thermal deposition unit for gel-permeation chromatography with matrix-assisted laser desorption/ionization and electrospray ionization time-of-flight analysis

We describe a modified commercially available thermal deposition unit that incorporates a post-column mixing cell for gel-permeation chromatography (GPC) effluent and matrix solution prior to continuous deposition onto a matrix-assisted laser desorption/ionization (MALDI) sample target. This modification overcomes the shortcomings of using a prefabricated matrix foil. This GPC MALDI method with time-of-flight (ToF) mass spectrometry is demonstrated by the analysis of a poly(dimethylsiloxane) (PDMS) sample used in the production of implantable device materials.