Tandem mass spectrometry and ion-molecule interactions: Application to structural characterization of polymers and other compounds
Abstract
Tandem mass spectrometry (MS/MS), used in conjunction with electron ionization (EI), chemical ionization (CI) and desorption ionization (DCI), has been applied to structural characterization of polysiloxanes and analogs of the anti-malarial artemisinin. In addition, hydrogen-deuterium exchange occurring in the collision cell is applied to elucidate structures of polyfunctional aromatic compounds. Two novel polysiloxanes, with N-methylpyridyl substituents, form abundant cyclic oligomeric ions when heated under DCI conditions. MS/MS product spectra of the cyclic ions reveal characteristic loss of $-$CH$\sb2$(CH$\sb3$)NC$\sb5$H$\sb4$ group. The lower oligomers are cyclic but oligomers higher than the pentamer are mainly comprised of proton bound dimers of smaller oligomers. Marcovian statistical calculations show that the monomers in the copolymers are non-randomly distributed. Artemisinin, an effective antimalarial agent from Artemisia annua, and its congeners produce abundant ammonium adducts, (M + NH$\sb4)\sp+$, using ammonia desorption chemical ionization (DCI). Dissociation of the mass-selected adducts yields the protonated molecules, (M + H)$\sp+$, which subsequently eliminate neutral molecules such as H$\sb2$O, CO, HCOOH, CH$\sb3$COOH. A rapid screening method utilizing MS/MS and ammonia DCI is described for artemisinin-analogs. MS/MS neutral loss scans which are selective for different elimination reactions are used to suggest structural features of the analogs present in Artemisia annua L. H/D exchange occurring between mass-selected polyfunctional aromatic ions and deuterated reagents is investigated as a function of interaction time, reagent gas and collision energy. Some reagents such as CH$\sb3$OD exchange specific types of active hydrogens, such as phenolic and carboxylic hydrogens, without exchanging amino hydrogens, while others such as ND$\sb3$ exchange all active hydrogens. The proximity of the functional groups, site of protonation and proton affinity difference between the analyte and the reagent gas are important factors in site-specific H/D exchange in polyfunctional compounds. Cluster ion formation plays a major role in the exchange mechanism operating in the triple quadrupole.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
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