Atmospheric pressure photoionization mass spectrometry of polyisobutylene derivatives

Abstract

Atmospheric pressure photoionization mass spectrometric (APPI-MS) study on three types of polyisobutylene derivatives is reported. Two of the polyisobutylenes investigated were polyisobutylene with dihydroxy and diolefinic end-groups derived from aromatic moieties [dicumyl chloride, 1,4-bis(2-chloro-2-propyl)benzene], and the third contained no aromatic moieties with a monohydroxy end-group. All three polyisobutylene derivatives (PIBs) had an average molecular weight (M n ) of ϳ2000 g/mol, with a polydispersity lower than 1.2. In the positive ion APPI mode, protonated PIB molecules were formed, but the molecular weights obtained were considerably lower than those expected, indicating fragmentation of the PIB chains. In the negative APPI mode, using solvents such as tetrahydrofuran and toluene as dopants, no signal was obtained. However, in chlorinated solvents, such as CCl 4 , CHCl 3 , and CH 2 Cl 2 , in the presence of toluene dopant, PIB adducts with chloride ions were formed with relatively high signal intensity. In the case of CH 2 Cl 2 , no dopant (toluene) was necessary to generate chlorinated adduct ions, albeit increasing the toluene concentration in the flow increased the PIB signal intensity. The effect of the toluene concentration on PIB signal intensity was studied and models that include (1) photoionization of toluene, (2) formation of chloride ions from the chlorinated solvents by dissociative electron capture, (3) formation of chlorinated adduct ions and charge recombination reactions between the toluene radical cation, (4) chloride ions, and (5) olymer characterization, including the determination of molar mass, molar mass distribution, the mass of the repeat unit and end-groups, and functionality, is an important process that can be accomplished using several methods. Mass spectrometric methods, particularly those that utilize soft-ionization techniques, e.g., matrix-assisted laser desorption/ionization (MALDI) [1,2] or electrospray ionization (ESI) [3], are superior to other methods because they offer a unique capability to determine all of these parameters. Contrary to other spectroscopic methods, such as nuclear magnetic resonance (NMR), which can only determine an average value of some of the above parameters [e.g., the number-average molecular weight (M n ) and number average-functionality (F n )], mass spectrometric methods provide not only these average values, but the individual intact polymer molecules can also be detected. However, because soft-ionization is based on forming adduct ions with metal ions, e.g., alkali or silver ions or through protonation and/or deprotonation, mass spectrometric analysis of polymers having moieties that are unable to form ions presents difficulties. This is more pronounced as the polarity of the polymers decreases. For example, mass spectrometric analysis of polar polymers such as polyethylene glycol (PEG) and polypropylene glycol (PPG) can easily be achieved using . Nonpolar polymers such as polystyrenes are also amenable to MALDI and ESI through the f...

Dopant-assisted atmospheric pressure photoionization mass spectrometry of polyisobutylene derivatives initiated by mono- and bifunctional initiators

“…However, when these adducts were subjected to collision-induced dissociation, considerable losses in the precursor signals were observed without formation of any product ions. These observations are consistent with our previous studies obtained for other polyisobutylene derivatives [29]. An increase of the collision energy resulted in the dissociation of the adducts into a neutral fragment and a chloride ion.…”

“…This observation is consistent with our previous results obtained with other polyisobutylene derivatives [29]. Although intact polymer ions can hardly be achieved in positive mode, the APPI(ϩ)-MS spectra may offer information on the chain and the end-group structure, therefore it is worth considering them in more detail.…”

“…Observations of the formation of chlorinated adduct ions for these PIB derivatives when using a chlorinated solvent (CCl 4 ) and a dopant (toluene) are in good accordance with our earlier studies obtained with other polyisobutylene derivatives and polyethylene waxes [29,30].…”

“…This method has been successfully applied to the analysis of various classes of nonpolar compounds, such as polyaromatic hydrocarbons [20 -22], lipids [23,24], and steroids [25][26][27][28]. Very recently, we have shown that the negative ion mode of APPI-MS is also capable of the analysis of low molecular weight polyisobutylene [29] and polyethylene derivatives [30] by means of the formation of adducts of these polymers with chloride ions formed "in situ" from chlorinated solvents, e.g., CCl 4 , CH 2 Cl 2 , in the presence of toluene dopant. From these investigations, we have concluded that the chloride ion can attach to the partially positively charged H-atoms of the hydrocarbon chain and to those of the end groups.…”

Can Nonpolar Polyisobutylenes be Measured by Electrospray Ionization Mass Spectrometry? Anion-Attachment Proved to be an Appropriate Method

“…The development of new ionsources such as atmospheric pressure photoionization (APPI) [12,13] and direct analysis in real-time (DART) [14] has paved the way for the analysis of less polar compounds, including nonpolar hydrocarbon polymers. In previous reports, we have shown that APPI-MS is an appropriate method for the characterization of low molecular weight PIBs [15,16] and other nonpolar polymers [17] through the formation of [M + Cl] -adduct ions. In these experiments, the chloride ions were produced Bin situ^from chlorinated solvents.…”