Electrospray Ag (I) cationization mass spectrometry

Abstract

Metal cationization mass spectrometry is an important tool for studying gas-phase organometallic chemistry because it can provide an ideal environment for studying the intrinsic physiochemical and reactivities of organometallic complexes, in the absence of solvent effects. The main theme of this project is to investigate the chemical binding of Ag(I) ion to organic compounds by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Specifically, Ag(I)-ligand complexes ([Ag.(Ligand)n], n = 1 and 2) were generated by ESI, then characterized by their unimolecular (metastable) and collision-induced dissociation (CID) inside a tandem mass spectrometer. The associated analytical applications of these studies were also explored. In this study, electrospray Ag(I) cationization was successfully used to synthesize a wide variety of Ag(I)-ligand complexes in the gas phase. The ligands included benzene, alkylbenzenes, polyaromatic hydrocarbons (PAHs), unsaturated fatty acids (FAs) and their methyl esters (FAMEs). Experimentally, Ag(I) ion was found to show a strong affinity toward aromatic compounds. A theoretical study of a model Ag(I) cationized benzene complex, Ag+-benzene, showed that the Ag(I) ion binds to the π-face of the aromatic ring with C6V geometry. The binding was shown to be mainly electrostatic in nature. For the first time, the absolute Ag(I) ion affinities of benzene (Bz), 14 alkylbenzenes (RBz, R = alkyl group) and 14 PAHs were determined using the mass spectrometric kinetic method. Alkyl substituents were found to have more pronounced effects on Ag(I) ion affinities of alkylbenzenes than proton affinities. An increase in Ag(I) ion affinity was observed with increasing number of methyl (alkyl) substitutions, chain length of n-alkyl substituents, and the extent of branching of the alkyl substituents. The results were attributed to the positive inductive effect and polarizability effect of alkyl substituents, leading to enhanced ion-quadrupole and ion-induced dipole interaction between the Ag(I) ion and the aromatic ring. The polarizability or ion-induced dipole effect was the major factor accounting for the increasing trend within a series of homologous n-alkylbenzenes or branched-chain alkylbenzenes. Similarly, the Ag(I) ion affinities of PAHs were found to increase with the quadrupole moments and polarizabilities of PAHs, i.e., with increasing number of fused benzene rings in the PAHs. However, the ion-quadrupole and ion-induced dipole interactions were noticeably weakened for larger PAHs containing four or more aromatic rings. This could be due to the spread of aromatic π electrons over a wide molecular surface area, and/or intermolecular electronic repulsion between the Ag(I) ion and the aromatic systems. High-energy CID of Ag(I) cationized alkylbenzenes, [Ag.(RBz)]+, produced four types of fragmentation ions derived from simple ligand detachment, dissociative charge transfer, hydride and methanide abstraction, and charge-remote fragmentations. The relative ion abundances of these fragment ions were found to vary considerably with the length and branching position of the alkyl chains, as well as the number of methyl substitutions on the benzene ring. Isomeric alkylbenzenes could therefore be differentiated based on the different fragment ions and/or different relative ion abundances found in the high-energy CID mass spectra. On the other hand, high- and low-energy collision-induced dissociation of Ag(I) cationized polyaromatic hydrocarbons (i.e., [Ag(PAH)n+, n = 1,2) yielded mainly ions resulting from simple cleavage of the relatively weak Ag+-PAH bond. Silver has an ionization potential (I.P.) of 7.58 eV, which lies within the range of I.P.s (6.9 - 7.95 eV) of the isomeric PAHs. The [PAH]+/[Ag]+ and [PAH]+[Ag.PAH]+ ion abundance ratios for the respective [Ag.PAH]+ and [Ag(PAH)2]+ complexes were found to decrease with increasing I.P.s of PAHs. Four series of isomeric PAHs with different I.P.s, i.e., the C14H10, C16H10, C18H12 and C20H12 isomers, were completely differentiated based on observed significant differences of these two ion abundance ratios. Based on these findings, an analytical method for differentiation of isomeric PAHs that could be carried out with a relative simple single quadrupole mass spectrometer equipped with an electrospray ionization source was developed. The electrospray Ag(I) cationization efficiency of monounsaturated and polyunsaturated FAs and FAMEs was found to be about 8 times more sensitive than that of lithium ion. The high-energy CID of Ag(I) cationized FAMEs and FAs yielded four series of fragment ions derived from charge-remote fragmentations, i.e., loss of alkane, alkyl radical, alkene and carboxylic acid functional group from the Ag(I) cationized complexes. The fragment ions were derived from [Ag.M]+ complexes with the Ag(I) ion bound either to the carboxylate functional group or to the carbon-carbon double bond. Fragment ions derived from C-C bond cleavage allylic to the C=C bond were noticeably abundant. Location of double bond(s) in mono- and polyunsaturated FAs and FAMEs could be determined from these regular patterns of structure-diagnostic fragment ions. Based on these findings, a set of artificial intelligence algorithms was developed to identify the functionality (acid or ester) and the position of double bond(s) of methylene-interrupted unsaturated FAs and FAMEs containing one to four double bonds.