Electrohydrodynamic Ionization Mass Spectrometry of Poly(ethylene glycols)

Abstract

Electrohydrodynamic ionization mass spectrometry was applied to the analysis of poly(ethy1ene glycols) of number-average molecular weights 406,605, 1041, and 1396 (determined by end-group titration). Spectra were obtained from glycerol solutions of each polymer, using NaI as supporting electrolyte. Singly, doubly, and triply charged ion series were detected, corresponding to attachment of one, two, or three sodium ions to oligomer molecules. Ions of low intensity attributable to protonated oligomers were also detected in the lighter samples. No evidence of significant fragmentation was obtained. Molecular weight distributions calculated from mass spectra could be used to obtain estimates of number-average molecular weights in excellent agreement with end-group-titration determinations. In addition to molecular weight information, the spectra also provided estimates of the level of vinyl, methylene, and propylene impurities in the sample.Both average molecular weight and molecular weight distribution are among the physical properties which determine whether a given polymer is suitable for a certain application. Sensitivity of bulk behavior to these parameters, and also to the presence of low-level impurities, can be extreme. The resultant need for characterization of the degree of polymerization in many systems has promoted the development of a variety of analytical techniques for determining polymer molecular weights.End-group titration is probably the most reliable method for absolute determination of number-average molecular weight, MnIn eq 1 Ni is the number of moles of molecules of molecular weight M , contained in a polymer sample. Titrations give no information about molecular weight distributions. Furthermore, the measured average can be spuriously lowered for branched polymers with multiple end groups.The only common technique which routinely provides both weight-average and weight-distribution information is gel permeation chromatography (GPC). Recent developments in "high-performance'' GPC (see, for example, ref 1 and 2) have extended its applicability to relatively low-molecular-weight samples. However, resolution falls quickly with increasing molecular weight, requiring multiple-column, time-consuming experiments for characterization of a broadly distributed sample. In addition, molecular weight standards of near monodispersity are usually required for good quantitation. Even when such standards are available, morphological differences between samples and standards (e.g., different branching or conformation) may shift molecular weight calibration. The need for calibration standards can be relieved by use of low-angle laser light scattering (LALLS) detection of the GPC effluenta3 However, this technique measures instantaneous average effluent molecular weight and requires efficient chromatographic separation for accurate determination of molecular weight distributions. Furthermore, calculation of molecular weights from scattering data requires prior knowledge of sample virial coefficients and "refracti...

Alkali-cation affinities of polyoxyethylene dodecylethers and helical conformations of their cationized molecules studied by electrospray mass spectrometry

“…The potassiated C 12 EO:n is the most stable in gas-phase, and one helical turn of the POE chain must be comprised of six consecutive oxyethylene units. The critical EO unit numbers introduced by this study can well explain the similar results from electrohydrodynamic ionization of PEGs by Cook et al [13].…”

“…Such inconsistency may come from the fact that the molecular mechanics/dynamics calculation can simulate chemical interactions without charge migration but cannot demonstrate chemical reactions such as protonation forming a new covalent bond. Therefore, it seems difficult to rationalize thermochemically our experimental findings that the alkali-cationized POE surfactants are more stable than their protonated molecules, which is consistent with the earlier Cook's results [13]. Contrary to this, the molecular modeling simulation has proved that the stability in helical conformations of cationized POE surfactants can arise from the highly size-dependent host-guest interac- …”

“…Although many articles describing cation affinity of PEGs have been published for the last few decades [13][14][15][16][17][18][19][20][21][22], we have first presented the differences in cation selectivity between cyclic and helical POEs, by using …”

“…In addition to this, PEGs are often used as the m/z calibration materials in the atmospheric pressure ionization mass spectrometers such as ESI-MS. Therefore, the mass spectral characteristics of their alkali-cation adducts have been well studied by many workers [13][14][15][16][17][18]. The formation of multiply charged molecules cationized by alkali-metal ions is widely recognized but the differences in alkalication affinity have been still unclear.…”

Basic surface-active properties in the homologous series of β-alkyl (C12H25/C18H37) polyethyleneoxy (n = 0-20) propionamides

“…As the EO unit number increased, the ESI (Electrospray ionization) mass spectra could exhibit the multiply charged C 12 H 25 (EO) n H (average) cationized by alkali metal ions in addition to their singly cationized species [14,37,38]. …”