Synchrotron UV photoactivation of trapped sodiated ions produced from poly(ethylene glycol) by electrospray ionization.

RATIONALE: By taking advantage of the gas-phase decompositions of polymer ions, tandem mass spectrometry of polymers allows us to obtain more accurate structural information than from a simple mass measurement. Applied to a model polymer, the goal of this work was to evaluate the performances of an activation technique based on ultraviolet (UV) irradiation, as an alternative to conventional collisional activation. METHODS: Sodiated poly(ethylene glycol) produced by electrospray ionization was isolated in a linear ion trap, then submitted to synchrotron UV irradiation over a range of wavelengths (52 to 248 nm). Fragmentation pathways resulting from UV photoactivation were investigated. The proposed mechanisms take into account: (i) the comparison with collision-induced dissociation (CID) product ions, (ii) the effect of wavelength-tunable UV activation, and (iii) deuterium-labeling and various other complementary experiments. For the highest molecular weight compounds, ion mobility spectrometry was used before UV photoactivation. RESULTS: Synchrotron UV irradiation can induce dissociation of poly(ethylene glycol) sodiated ions without the requirement of the presence of a specific chromophore, if the photon energy is above 10 eV. UV photoactivation of poly(ethylene glycol) ions can yield fragmentations that differ from those in classical low-energy CID, especially from higher masses (>4000 g mol-1 ). A successful coupling of UV photoactivation with ion mobility pre-filtering was presented. CONCLUSIONS: UV activation combined or not with pre-filtering ion mobility is a promising alternative approach for the structural characterization of polymers. UV synchrotron radiation with a tunable wavelength was a great opportunity to study the effect of the photon energy, and to probe the mechanisms of ion decomposition from poly(ethylene glycol).

Ultraviolet Photoactivation Using Synchrotron Radiation for Tandem Mass Spectrometry of Polysiloxanes.

Gas-phase decompositions of polymer ions play an important role in mass spectrometry to obtain accurate structural information. In this work, UV photoactivation experiments were performed from two poly(dimethylsiloxane)s bearing different end groups (two trimethylsilyl, or α-sec-butyl and ω- trimethylsilyl). Precursor ions, such as [Polysiloxane+Cation]+ produced by an electrospray source, were stored in a linear ion trap and then submitted to synchrotron UV irradiation during different activation times and over a range of wavelengths (52 to 248 nm) from extreme UV (XUV) to deep UV. Upon photoactivation of a precursor ion from poly(dimethylsiloxane) (PDMS; with two trimethylsilyl end groups, [PDMS25+Na]+), important fragmentations were observed, including the loss of a methyl radical followed by various heterolytic cleavages along the polymer backbone, for photon energies typically >9.5-10 eV (ionization threshold of the neutral oligomer). This report focuses on different aspects: (i) the identification of the UV photodissociation (UV-PD) products of PDMS, (ii) the influence of the irradiation time for two photon energies (10 or 20 eV), (iii) the influence of the energy of the photon for two activation times (100 or 5000 ms), (iv) the influence of the nature of the cation, and (v) the influence of the end groups of PDMS. Synchrotron UV irradiation with a tunable wavelength was a great opportunity to study the effect of the photon energy and to probe the original mechanisms of ion decomposition from poly(dimethylsiloxane).

Study of the gas-phase decomposition of multiply lithiated polycaprolactone, polytetrahydrofurane and their copolymer by two different activation methods: Collision-induced dissociation and electron transfer dissociation.

Collision-Induced Dissociation and Electron Transfer Dissociation experiments were carried out from various lithium adducts ([M+2Li]2+, [M+3Li]3+) from polycaprolactone diol (pCL), polytetrahydrofurane (pTHF), and one triblock copolymer (pCL-pTHF-pCL). In both cases (pCL and pTHF), CID of triply lithiated precursors led to complex mass spectra compared to corresponding ETD spectra, which remained relatively simple because CID product ions exhibit multiple charge states whereas ETD mainly led to singly charged fragment ions. CID of pCL involves charge-remote rearrangements over the ester groups and intramolecular transesterification reactions, whereas ETD leads to radical and charge induced cleavages leading globally to structurally different product ions but accounting for the same bond cleavages: (CO)O-C and (CO)-O respectively. Both CID and ETD can produce a low amount of undesirable reactions such as consecutive fragmentations especially from 3+ precursors but these fragmentations are absent in ETD from 2+ species. CID of a triply lithiated pTHF involved charge-induced and charge-remote fragmentations. In contrast, under ETD conditions, in the absence of suitable chemical functionality, pTHF did not undergo any backbone fragmentations at all. Nevertheless, proton abstraction by the fluoranthene reagent anion allowed the formation of species that could further be collisionally activated, leading to a depolymerization process from the ends. This strategy combining sequentially ETD and CID led to dramatically simplified product ion spectra. Concerning the supposed triblock copolymer, which was commercially purchased, both CID and ETD led to the same conclusion that at least a part of the copolymer was a diblock rather a triblock.