El noticiero: la difícil tarea de informar

Interest in using MS and MS/MS for the study of synthetic polymers has grown sharply in the past decade due to development of raw polymeric products and advances in mass spectrometers. In practice, it can be difficult to ionize or fragment many industrial polymers, and salt doping is becoming one of the most effective solutions, which has been widely investigated.259-266 Additionally, inducing metallic adducts to improve the fragmentation efficiency in MS/MS has been frequently applied for many types of molecules, such as olaigosaccharides,165,267-269 and peptides/proteins.270-274

The choice of adducts, and metallic ions in particular, can have a great impact on the ionization259-261 and fragmentation.275-277 By facilitating the ionization, silver ions are frequently used as a cationizing agent for polymers like polystyrene because of a possible binding with the phenyl ring.258,259,276 Ag+ cationization is also very effective for generating MS spectra of polyglycols and can improve fragmentation in CAD.258 The most important class of adducts in polymer research is the alkali metal because of the 2_{This chapter has been partially adapted from Wei, J.; Bristow, A.W.T.; O’Connor, P. The competitive influence} of Li+, Na+, K+, Ag+ and H+ on the fragmentation of a PEGylated polymeric excipient, accepted. Copyright 2014 Springer. Bristow, A.W.T. provided the TPGS samples. Wei, J. did all of the experiments and drafted the manuscript under the supervision of O'Connor P. B..

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ubiquitous usage of sodium/potassium salt in polymer synthesis278,279 and the evidence of improving the ionization and fragmentation efficiency of some polymers.258,280 In the presence of binary mixtures of NaCl and another salt (LiCl, KCl, CsCl, or NH4Cl), the selectivity between poly(propylene glycol) and the cations were found in the order of NH4

+_≈

K+˃Na+≈ Cs+˃ Li+, though the solvent, counter ions, and the conformation are also significant.261 In terms of the fragmentation, most literature studies were conducted by CAD, and some trends were monitored. The binding strength of polyethylene glycol (PEG) with alkali metals has been investigated experimentally and theoretically. 253,260,281

Using a quadrupole ion trap mass spectrometer, the relative affinity of Na+, K+, and Cs+ to PEG was characterized by CAD experiments, and it was found that the larger adduct was lost more easily,253 which was complemented by other studies carried out on different instruments and polymers.143,145,260 Furthermore, the binding energy between alkali cations and PEG was determined to be decreasing in the order of Li+˃Na+˃ K+˃Cs+ using the electronic structure calculation based on density functional theory, and this order is in agreement with experimental results.253,281 The calculation also indicates that the binding energy of the cation is not significantly affected by the polymerization degree (n) when n ˃10. Additionally, it is believed that smaller alkali ions with stronger interaction with the oligomer provide more structural information in CAD.145,258,282

Comparatively, there are fewer studies focused on the influence of the adduct on the fragmentation of polymers using electron-based fragmentation methods. Because electron-based fragmentation methods are not as widely available as CAD, rules for fragmentation of synthetic polymers in electron-

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based fragmentation methods are not as well understood as they are for biopolymers. In the past decade, it was demonstrated that ECD or ETD can provide complementary structural information of different polymers,81,149,283 though there are also some exceptional cases.145,284 Understanding of the influence of different adducts on electron-based fragmentation process can be, therefore, useful in extraction structural information from ECD spectra and valuable in selecting adducts in practice. In Smith and Mosely’s recent work,143 they studied the CAD, ECD, and hot-ECD behaviours of doubly- charged PEG with the presence of two different charge carriers among Na+, K+, and Li+. A dissociation order, Na+ ˃ K+ ˃ Li+ (loss of Na ispreferred), different from the binding strength deduced from CAD experiments, was observed in ECD and hot-ECD experiments and suggests that the reduction potential (also known as the redox potential, the standard reduction potential of a cation is measured in solution using a hydrogen electrode as a reference and reflects the tendency of a species to capture an electron) of the ion may play an important role in the electron-based dissociation process, such that the ions with higher reduction potential are lost more easily.143 In addition to reduction potential, second ionization energy,270 charge density,271 recombination energy165,274 and the electronic configuration285 of the cationizing metal ion were also suggested as factors that might be relevant to the ECD process. In general, for most of the molecules investigated by electron-based fragmentation techniques, introducing the smaller alkali metal ion usually produces more abundant fragments.78,143,163,165 The role of the metal ion in electron-based fragmentation process, however, is still far from clear.

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5.1.2 Content of the chapter

In Chapter 4, the fragmentation of TPGS was investigated by both CAD and ECD, and TPGS with three silver cations showed different ECD behaviour in contrast to the precursors with 3Li+ or 3Na+ ions. The principal thrust of the research in this chapter was to discuss the influence of metal ion adducts in CAD and, particularly, in ECD processes according to the observations from the fragmentation of TPGS. Several widely used metallic ions (Li+, Na+, K+, Ag+) were selected and also compared with proton adduction. To estimate the impact of each adduct, doubly-charged TPGS27 (27 is the degree of polymerization, structure shown in Figure 5.1) ions in the presence of two different charge carriers (Li+, Na+, K+, Ag+, H+) are used in most of the experiments.

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5.2 Experimental section 5.2.1 Chemicals

TPGS samples (ISOCHEM, Vert-Le-Petit, France) were dissolved in 50:50 methanol:water to a concentration of ~1 µM before use. For different experiments, sodium nitrate, lithium nitrate, potassium nitrate, and silver nitrate, (Fisher Scientific UK limited, UK) were added to a final concentration of 1 mM or 0.1 mM depending on the experiment. 0.2% of formic acid (Sigma- Aldrich Co., St. Louis, MO, USA) was used for the protonation. Water was purified by a Millipore Direct-Q purification system.

5.2.2 Mass spectrometry experiments.

All mass spectrometry experiments were carried out on a Bruker 12 T solariX FTICR mass spectrometer (Bruker Daltonik, GmbH, Bremen, Germany) with a homemade nano-electrospray ion source. For CAD, the parent ions were isolated in the quadrupole and then transferred to the collision cell for fragmentation using a collision energy from 9 to 46 V (in general, 11 ± 2 V for precursors with one/two protons, and 42 ± 4 V for all of the other precursors) which was optimized for each sample to provide an even spread of fragments, and fragments were finally detected in the ICR infinity cell.110 In the ECD experiment, the isolated ions were accumulated externally in the collision cell and then transferred to the ICR cell. Trapped ions were then irradiated with electrons from a 1.5 A heated hollow cathode; the bias voltage used for ECD experiments was ~ 2 V and the pulse length was around 0.1 s. To achieve a desirable S/N, up to 20 scans were averaged.

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5.3 Results and discussions

5.3.1 The influence of the metallic adduct on the fragmentation of