Anthony T. Jackson

The Application of Matrix‐assisted Laser Desorption/Ionization Combined with Collision‐induced Dissociation to the Analysis of Synthetic Polymers

Matrix-assisted laser desorption/ionization combined with collision-induced dissociation (CID) has been applied to the structural determination of synthetic polymers. Post-source decay/CID experiments on a time-of-flight (TOF) instrument have been compared with CID data from a hybrid sector-TOF mass spectrometer. Fragment-ation spectra of polymers, with molecular weights of up to 4500 Da, have been shown to aid structural and end-group determination. The polymers studied were poly(methyl methacrylate), poly(ethylene glycol) and poly(ethylene terephthalate).

Time-lag focusing and cation attachment in the analysis of synthetic polymers by matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry

Ultraviolet matrix-assisted laser desorption/ionization-mass spectrometry has been employed with time-lag focusing to explore its utility for the characterization of synthetic polymers with broad distributions. Mixtures of five polymer standards with narrow molecular weight distributions were analyzed. The spectra were found to be broadly those expected for three different types of polymer systems—poly(styrene), poly(methyl methacrylate), and poly(ethylene glycol)—when equimass mixtures were used. Large changes in the apparent molecular weight distribution of poly(ethylene terephthalate) were observed when the cation was varied. The shift in the envelope was found to be related to the size and the ability of the oligomers to solvate the cation.

Structural Analysis of Synthetic Polymer Mixtures Using Ion Mobility and Tandem Mass Spectrometry

Ion mobility (IM) combined with tandem mass spectrometry (MS/MS) has been employed to separate and differentiate between polyether oligomers with the same nominal molecular weights. Poly(ethylene glycol)s with the same nominal mass-to-charge ratio (m/z), but with differing structures, were separated using ion mobility. IM-MS/MS data were able to aid identification of the backbone and end groups of the four individual polyethers in the two sets of isobaric mixtures. The MS/MS data from the resolved oligomers enabled a detailed structural characterization of the polyether mixtures to be completed in one experiment.

Characterisation of synthetic polymer systems

Abstract Mass spectrometry has been used in the study of synthetic polymer systems since the 1960s. The application has been, for the most part, limited to the characterisation of polymer additive systems and polymers that had either been chemically or thermally degraded. The advent of newer ionisation approaches, coupled with the development of analyser technology, has led to the reappraisal of mass spectrometry for this work. Molecular weight distributions have been obtained and information on end groups and chemical variation with molecular weight has been measured. Polymer microstructure has been probed with information obtained on partial and, in some cases, complete sequence for oligomeric systems. Fundamental work to support these developments is needed and is being carried out. Information on gas-phase polymer conformations has been obtained and an important link with calculation established. The future of the approach, particularly when used in conjunction with other complimentary chromatographic and spectroscopic techniques, looks promising.

The effect of the variation of cation in the matrix-assisted laser desorption/ionisation-collision induced dissociation (MALDI-CID) spectra of oligomeric systems

Abstract The effect of the variation of the cation, on the matrix-assisted laser desorption/ionisation collision induced dissociation (MALDI-CID) spectra obtained, has been studied in a hybrid sector-orthogonal acceleration-time of flight tandem mass spectrometer for some synthetic oligomeric systems. Oligomers of poly(methyl methacrylate) and poly(styrene) in the molecular weight range of approximately 1000–3000 u were analysed. They were ionised by attachment of various alkali metal and copper and silver ions, respectively. The MALDI-CID spectra are shown to vary with different cations more from precursor ions of lower than those of higher mass-to-charge ratio.

Gas phase conformations of synthetic polymers: poly (methyl methacrylate) oligomers cationized by sodium ions

Gas phase conformations of a series of poly(methyl methacrylate) (PMMA) oligomers cationized by sodium ions were investigated experimentally and theoretically. Ion chromatography and molecular mechanics methods were used to measure the cross sections of Na 1 (PMMA)3 to Na 1 (PMMA)11 (i.e. from the 3-mer to the 11-mer). Collision induced dissociation (CID) of Na 1 (PMMA)10 is also reported. The dominant fragment peaks in the CID spectrum are solely in the low mass-to-charge range and correspond to the loss of 1‐2 monomer units from each end of the oligomer. The fragments also retain the Na 1 ion indicating that the ion interacts with both ends of PMMA. Molecular modeling of the Na 1 (PMMA) oligomers support this finding. The calculations show that the lowest energy structures are U-shaped with Na 1 bound to multiple carbonyl oxygen atoms near both ends of the oligomer. The cross sections of these structures agree with those obtained from the ion chromatography experiments. Ion chromatography and molecular dynamics simulations also show that the cross sections are independent of temperature between 300 and 600 K, further supporting the indication that the Na 1 ion strongly binds to both ends of the oligomer. A mechanism for the surprising CID results is suggested. (Int J Mass Spectrom 188 (1999) 121‐130)

Gas-phase conformations of cationized poly(styrene) oligomers

The gas-phase conformations of poly(styrene) oligomers cationized by Li+, Na+, Cu+, and Ag+ (M+PSn) were examined using ion mobility experiments and molecular mechanics/dynamics calculations. M+PSn ions were formed by MALDI and their ion-He collision cross-sections were measured by ion mobility methods. The experimental collision cross-sections of each M+PS n-mer were similar for all four metal cations and increased linearly with n. Molecular modeling of selected M+PS oligomers cationized by Li+ and Na+ yielded quasi-linear structures with the metal cation sandwiched between two phenyl groups. The relative energies of the structures were ∼2–3 kcal/mol more stable when the metal cation was sandwiched near the middle of the oligomer chain than when it was near the ends of the oligomer. The cross-sections of these theoretical structures agree well with the experimental values with deviations typically around 1–2%. The calculations also show that the metal cation tends to align the phenyl groups on the same side of the -CH2-CH- backbone. Calculations on neutral poly(styrene), on the other hand, showed structures in which the phenyl groups were more randomly positioned about the oligomer backbone. The conformations and metal-oligomer binding energies of M+PS are also used to help explain CID product distributions and fragmentation mechanisms of cationized PS oligomers.

Poly (ethylene terephthalate) oligomers cationized by alkali ions: Structures, energetics, and their effect on mass spectra and the matrix-assisted laser desorption/ionization process

In this article the folding dynamics and energetics for a set of poly(ethylene terephthalate) (PET) oligomers cationized by various alkali ions are studied: M+PETn for n=2 to 7 and M=Li, Na, and K. Experimental cross sections were determined for matrix-assisted laser desorption/ionization (MALDI) generated ions using the ion mobility based ion chromatography method. Very good agreement was obtained with cross sections generated by the amber 4.0 suite of molecular dynamics programs. For n=2 and 4 the benzene rings of the oligomers π stack with the metal ion coordinated to both terminal hydroxyl oxygen atoms and several of the nearby carbonyl oxygen atoms. For n=3, two isomers are both observed and predicted by theory: an open form where the third PET monomer attaches to the dimer and extends into space and a closed form where the third PET moiety bends back and coordinates its hydroxyl oxygen with the metal ion. For Na+PET3, equilibrium is observed between 100 and 180 K with an Arrhenius analysis yielding an open to closed form isomerization barrier of 1.6 kcal/mol. For this same system the two isomeric forms are frozen out at 80 K and coupling the observed isomeric distribution with an RRKM analysis indicates the closed form is more stable by 0.5 kcal/mol. For K+PET3 the barrier to isomerization is too low to observe (<1.0 kcal/mol), whereas for Li+PET3 a temperature independent isomer distribution is observed (80 to 55°K). Using methods developed for determining isomerization barriers in carbon clusters it was possible to obtain an open to closed form isomerization barrier of 7±2 kcal/mol for Li+PET3. In this system, the open and closed form isomer populations were observed to be strong functions of the laser power in the MALDI source. This allowed a detailed description of the formation mechanism to be formulated and indicated alkali ion attachment to the polymer during expansion of the plume emanating from the surface. Finally, the mass spectrum of a PET oligomer sample has been shown to strongly depend on the cationizing alkali metal ion. It is qualitatively shown that M+-PETn binding energies and structures are responsible.

Utilizing matrix-assisted laser desorption/ionization-collision induced dissociation for the generation of structural information from poly(alkyl methacrylate)s

Matrix-assisted laser desorption/ionization-collision induced dissociation (MALDI-CID) has been employed for the analysis of poly(alkyl methacrylate)s in a tandem hybrid sector-time of flight instrument. Spectra are shown for adducts of poly(ethyl methacrylate) and poly(butyl methacrylate) with sodium ions. It is proposed that the masses of the end groups may be inferred from the data for the polymers. Mechanisms are proposed for the formation of some of the series of ion peaks that are observed in the MALDI-CID spectra.

Generation of average mass values and end group information of polymers by means of a combination of matrix-assisted laser desorption/ionization-mass spectrometry and liquid secondary ion-tandem mass spectrometry

Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and liquid secondary ion-tandem mass spectrometry (LSI-MS/MS) have been applied to the analysis of synthetic polymers to generate values for the average mass and the mass of the end groups. The average mass values were calculated for polymethylmethacrylate and polystyrene standards from the MALDI-MS data. Abundant fragment ions of the polymers, generated by means of LSI-MS/MS, were consistent with the known structures of the end groups of the polymers. Furthermore, losses from the side chains of the polymers were also observed in the LSI-MS/MS spectra.