Molecular Resolution and Fragmentation of Fulvic Acid by Electrospray Ionization/Multistage Tandem Mass Spectrometry

Abstract

Molecular weight distributions of fulvic acid from the Suwannee River, Georgia, were investigated by electrospray ionization/quadrupole mass spectrometry (ESI/ QMS), and fragmentation pathways of specific fulvic acid masses were investigated by electrospray ionization/ion trap multistage tandem mass spectrometry (ESI/MST/ MS). ESI/QMS studies of the free acid form of low molecular weight poly(carboxylic acid) standards in 75% methanol/25% water mobile phase found that negative ion detection gave the optimum generation of parent ions that can be used for molecular weight determinations. However, experiments with poly(acrylic acid) mixtures and specific high molecular weight standards found multiply charged negative ions that gave a low bias to molecular mass distributions. The number of negative charges on a molecule is dependent on the distance between charges. ESI/MST/MS of model compounds found characteristic water loss from alcohol dehydration and anhydride formation, as well as CO2 loss from decarboxylation, and CO loss from ester structures. Application of these fragmentation pathways to specific masses of fulvic acid isolated and fragmented by ESI/MST/MS is indicative of specific structures that can serve as a basis for future structural confirmation after these hypothesized structures are synthesized.

Tandem mass spectrometry of poly(methacrylic acid) oligomers produced by negative mode electrospray ionization

“…As doubly deprotonated oligomers were detected from n ϭ 6 and since the chain length between the two furthest apart carbonyl groups includes 13 carbon atoms in the 6-mer, this result suggest that negative charges must be separated by at least 13 C atoms, i.e., four MAA monomers, to form stable multiply charged species. A very similar result has already been reported for PAA [41]. Singly and doubly deprotonated PMAA oligomers were formed with similar abundance but the m/z range of detected signals indicates that these two distributions are different and both less than the expected ones.…”

“…Signal-to-noise ratio of [PMAA -3H] 3Ϫ species was too low for precise average mass calculation but the center of this distribution, measured around m/z 372.5, also indicates a bias towards low mass oligomers. Such a discrepancy has already been reported for PMAA [35] in both positive and negative mode MALDI, as well as for other acrylic polymers produced in multiple charge states after electrospray ionization [41]. Variation in skimmer cone voltage can cause changes in the relative intensities of the multiple-charge distributions of polymers [42].…”

Contrasting Roles Of Natural Organic Matter On Colloidal Stabilization And Flocculation In Freshwaters

“…In general, HS aggregation is favoured by high HS concentrations, high electrolyte concentrations (typically > 100 mM), relatively low pH values (typically pH < 3-4 for FA, pH < 6-7 for HA) or by the presence of divalent cations (Conte and Piccolo 1999;Leenheer et al 2001;Leenheer and Croue 2003). These conditions are similar to those which support the aggregation of the mineral particles.…”

“…Because they are primarily of pedogenic origin, they may also be present in the coastal waters of oceans but are generally absent from the pelagic zones. HS are chemically heterogeneous polymers with small molar masses (typically ~1000 Da; (Leenheer et al 2001)), high charge densities at neutral or alkaline pH's ( Fig. 3A) and typical average lifetimes of several hundred years.…”

“…HS aggregate formation is favored at high concentrations of electrolyte, protons (i.e. low pH), HS or divalent cations (Conte and Piccolo 1999;Leenheer et al 2001;Leenheer and Croue 2003). Although HS are sometimes represented as linear polyelectrolytes, the linear representation of the HS does not fit with either the experimental measurements discussed above or the more modern models that consider HS as highly branched molecules (Schulten et al 1998).…”