Poly(2-hydroxyethyl methacrylate) (poly(HEMA)) has been characterised by means of electrospray ionisation-mass spectrometry/mass spectrometry (ESI-MS/MS), in order to evaluate this technique for the generation of end group information. Low energy collision-induced dissociation (CID) data from poly(HEMA) enabled information on both end groups of the polymer chain to be gleaned, in a similar fashion to that proposed previously for other methacrylate polymer systems. Exact-mass CID information was employed to aid the understanding of the dissociation mechanism of the polymer. Some additional fragmentation pathways, compared to other methacrylate polymer systems, are proposed. An example of how software can aid the interpretation of the MS/MS data is also shown.

This chapter contains sections titled: Introduction to MALDI-MS/MS Techniques of MALDI-MS/MS Experimental Details Using MS/MS for Polymer Structure and Composition Analysis Examples of Applications Conclusions References

The characterization of end group fictionalization from synthetic polymers is important for a number of reasons and is described in this chapter with particular reference to industrial polymeric systems. End group functionality can be important to the application for the polymeric system, for example, the end groups are very important in prepolymers for polyurethanes, so that efficient formation of soft blocks and hard blocks, plus cross-linking, can occur. Knowledge of the end groups of a polymer often gives evidence for the polymerization methodology employed to make the material. Information on the initiators and end-cappers, plus chain transfer agents, can be obtained. An example reaction scheme for polymerization of a free radical polymer system is given in the chapter.

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.

The end-group functionalisation of a series of poly(propylene glycol)s has been characterised by means of electrospray ionisation-tandem mass spectrometry (ESI-MS/MS). A series of peaks with mass-to-charge ratios that are close to that of the precursor ion were used to generate information on the end-group functionalities of the poly(propylene glycol)s. Fragment ions resulting from losses of both of the end groups were noted from some of the samples. An example is presented of how software can be used to significantly reduce the length of time involved in data interpretation (which is typically the most time-consuming part of the analysis).

End-group characterisation of three functionalised poly(methyl methacrylate) (PMMA) samples was performed by means of matrix-assisted laser desorption/ionisation-mass spectrometry (MALDI-MS). MALDI-time of flight (MALDI-TOF) data were employed to generate information about the initiators and chain-transfer agents used for the polymerisations. Further confirmation of the structural assignments was gleaned from MALDI-collision-induced dissociation (MALDI-CID) experiments. This latter technique is a powerful method for generating information on the masses of individual end groups.

Novel software has been developed to aid the interpretation of tandem mass spectrometry (MS/MS) data from synthetic polymers. The software is particularly focused toward aiding the end-group determination of these materials by significantly speeding up the interpretation process. This allows information on the initiator and/or chain transfer agents, used to generate the polymer, and the mechanism of termination to be inferred from the data much more rapidly. The software allows the validity of hypothesized structures to be rapidly tested by automatically annotating the data file using previously proposed fragmentation rules for synthetic polymers. Low-energy collision-induced dissociation (CID) data from methacrylate, styrene, and polyether oligomers are used as example data for the software. Exact-mass CID information was used to aid the understanding of the dissociation mechanism of the polymers. The software can use exact-mass data to provide more confidence in the results. The MS/MS results indicate that the fragmentation pathways are those previously proposed for these polymers.

A rapid method for the characterisation of polyglycol esters and ethers is described which uses accurate mass desorption electrospray ionisation (DESI) quadrupole time-of-flight mass spectrometry (Q-ToFMS). The results are combined with newly developed software which aids the interpretation of product ions produced using collision-induced dissociation (CID) of selected precursor ions. The poly(ethylene glycol) (PEG) samples analysed were PEG dibenzoate, PEG monooleate, PEG butyl ether, PEG bis(2-ethyl hexanoate) and PEG diacrylate. Lithium metal was used for cationisation of the PEG oligomers since it yielded the most useful structural information compared with other group I metals. The full scan mass spectra and product ion mass spectra were all obtained in <5 s. Interpretation of the MS/MS product ion spectra, using the product ion interpretation software which incorporates previously developed fragmentation rules, was carried out in <1 s.

A series of polyethers, namely poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG), poly(butylene glycol) (PBG) and poly(tetramethylene glycol) (PTMeG), has been characterised by means of matrix-assisted laser desorption/ionisation collision-induced dissociation (MALDI-CID) using a hybrid sector orthogonal-time-of-flight (TOF) instrument. The data indicate that this technique can be used to generate information about the end-group functionality of these polymers, including in some cases information about branching of the alkyl chains of the initiating groups. Proposals are made for the fragmentation pathways for these polymers.

A range of low molecular weight synthetic polymers has been characterised by means of desorption electrospray ionisation (DESI) combined with both mass spectrometry (MS) and tandem mass spectrometry (MS/MS). Accurate mass experiments were used to aid the structural determination of some of the oligomeric materials. The polymers analysed were poly(ethylene glycol) (PEG), polypropylene glycol (PPG), poly(methyl methacrylate) (PMMA) and poly(a-methyl styrene). An application of the technique for characterisation of a polymer used as part of an active ingredient in a pharmaceutical tablet is described. The mass spectra and tandem mass spectra of all of the polymers were obtained in seconds, indicating the sensitivity of the technique. Copyright (c) 2006 John Wiley & Sons, Ltd.

Electrospray ionisation–tandem mass spectrometry (ESI–MS/MS) has been employed for the characterisation of two poly(alkyl methacrylate) polymers, namely poly(methyl methacrylate) (PMMA) and poly(n-butyl methacrylate) (PBMA). Collision-induced dissociation (CID) experiments were performed in a quadrupole orthogonal time-of-flight (ToF) tandem mass spectrometer fitted with a nanospray source. Tandem mass spectra from singly, doubly and triply charged precursor ions (with alkali metals used for cationisation of the oligomers) are shown and the data are compared to those previously generated by means of matrix-assisted laser desorption/ionisation-collision-induced dissociation (MALDI-CID). These data indicate that cations with greater ionic radii may yield the most useful structural information as the mass-to-charge ratio of the precursor ion increases, whereas lithium or sodium ions are proposed to be ideal for obtaining spectra from lower molecular weight oligomers. Fragment ions at low mass-to-charge ratios dominate the spectra. Two series of peaks may be used to calculate the masses of the initiating and terminating end groups of the polymer. Ion peaks of greater mass-to-charge ratios form series that may be used to infer sequence information from the polymers.

A combined structural/conformational study of ethylene oxide/propylene oxide (EO/PO) copolymers has been undertaken. Electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI) methods have been utilised and ESI-tandem mass spectrometry (MS/MS) product ion spectra, including accurate mass measurements, utilised to establish fragmentation pathways. This has enabled end group and sequence information to be obtained. Ion mobility mass spectrometry experimental, along with theoretical, approaches has been used in tandem to probe the gas-phase conformation of selected cationised species from the block and random copolymers. The cross-sections established from these measurements and calculations have been shown to be dependent on molecular weight of the oligomer and radii of the cation but largely independent of the sequence of the ion in the gas-phase. The ion mobility results have been used to aid the understanding of the fragmentation of these copolymers by means of ESI-MS/MS.

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.

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.

Matrix-assisted laser desorption/ionization-collision induced dissociation (MALDI-CID) has been employed for the analysis of poly(styrene) in a tandem hybrid sector-time of flight instrument. Spectra are shown for adducts of poly(styrene) with copper and silver ions. The distributions of fragment ion peaks were found to be consistent from precursor ions containing both metal ions. It is shown how the masses of the end groups of the polymer may be inferred from the mass-to-charge ratios of two of the series of ion peaks that are seen in the MALDI-CID spectra. Mechanisms are proposed for the formation of some of the other series of ion peaks that are observed in the spectra.

Time-lag focusing matrix-assisted laser desorption/ionisation-mass spectrometry (MALDI-MS) has been employed to analyse synthetic polymers with average molecular weights between approximately 3500 and 12500 Da. Spectra are shown that demonstrate the improvement in resolution and sensitivity when time-lag focusing is used instead of prompt extraction techniques. A resolution of almost 9000 (FWHM) was obtained, in the reflectron mode of operation, for a poly(methyl methacrylate) (PMMA) sample with a relatively narrow dispersion. The data for PMMA polymers of industrial relevance easily enables the differentiation of many end groups as a consequence of the improved signal-to-noise and good mass accuracy when time-lag focusing is employed.