Nico M. M. Nibbering

Mass selection of ions in a Fourier transform ion cyclotron resonance trap using correlated harmonic excitation fields (CHEF)

Abstract A simple procedure has been developed to select ions in a Fourier transform ion cyclotron resonance (FT-ICR) ion trap. This procedure involves the use of correlated single- and swept-frequency excitation fields to eject unwanted ions from the FT-ICR ion trap. Correlation with the effective cyclotron frequency of the ions which are to be selected leads to optimization of the parameters controlling the single- and swept-frequency excitation fields, as a result of which accumulation of off-resonance excitation energy is prevented during the ion ejection event. Accordingly, it is demonstrated that the correlated harmonic excitation field (CHEF) procedure, which readily can be implemented in the data acquisition system of FT-ICR instruments, can conveniently be used for the selection of thermally labile ions. Hence, the CHEF procedure can be a generally applicable experimental tool for MS-MS studies, especially for studies requiring thermal ions such as ion/molecule reaction kinetic studies, and determinations of absolute threshold energies of ion decomposition reactions. The CHEF procedure can minimize the duration of the ion selection event, which especially is beneficial for ion selections which require high mass resolution. It is experimentally shown that ion selections which require a mass resolution beyond 60 000 do not suffer from ion loss.

Localization of Fatty Acyl and Double Bond Positions in Phosphatidylcholines Using a Dual Stage CID Fragmentation Coupled with Ion Mobility Mass Spectrometry

A high content molecular fragmentation for the analysis of phosphatidylcholines (PC) was achieved utilizing a two-stage [trap (first generation fragmentation) and transfer (second generation fragmentation)] collision-induced dissociation (CID) in combination with travelling-wave ion mobility spectrometry (TWIMS). The novel aspects of this work reside in the fact that a TWIMS arrangement was used to obtain a high level structural information including location of fatty acyl substituents and double bonds for PCs in plasma, and the presence of alkali metal adduct ions such as [M + Li]+ was not required to obtain double bond positions. Elemental compositions for fragment ions were confirmed by accurate mass measurements. A very specific first generation fragment ion m/z 577 (M-phosphoryl choline) from the PC [16:0/18:1 (9Z)] was produced, which by further CID generated acylium ions containing either the fatty acyl 16:0 (C15H31CO+, m/z 239) or 18:1 (9Z) (C17H33CO+, m/z 265) substituent. Subsequent water loss from these acylium ions was key in producing hydrocarbon fragment ions mainly from the α-proximal position of the carbonyl group such as the hydrocarbon ion m/z 67 (+H2C-HC = CH-CH = CH2). Formation of these ions was of important significance for determining double bonds in the fatty acyl chains. In addition to this, and with the aid of 13C labeled lyso-phosphatidylcholine (LPC) 18:1 (9Z) in the ω-position (methyl) TAP fragmentation produced the ion at m/z 57. And was proven to be derived from the α-proximal (carboxylate) or distant ω-position (methyl) in the LPC.

Gas-phase reactions of organic anions

Publisher Summary This chapter describes organic anion/molecule reactions that have been studied mostly with the methods of Fourier transform ion cyclotron resonance mass spectrometry and flowing afterglow. It discusses the formation of anions, the current theory of gas phase ion/ molecule reactions in a simple form and a variety of reactions of organic anions. The chapter explains that considerable progress has been made in the past ten years in studying reactions of organic anions in the gas phase. This has become possible largely because of the rapid development of the required and sophisticated instrumentation, which has allowed, in addition to the reactions described, the determination of proton affinities, electron affinities, and acidities of molecules in the absence of solvent molecules. The chemistry of mono-solvated anions is also increasingly becoming a topic of research.

Formation and chemistry of radical anions in the gas phase

The research concerned with the ion/molecule chemistry of radical anions is reviewed together with the thermochemical properties of these ions and the related neutral species. Attention has been given to the formation of radical anions in the gas phase by various pathways, such as (dissociative) electron attachment and bimolecular reactions. A brief discussion of the electron affinity of the neutral species related to a given radical anion has been included, because this parameter is important for the discussion of the finding that an HA˙ radical is often more acidic than the related H2A molecule. In addition, the thermochemical properties form the basis for a discussion of the various processes that may occur in reactions of radical anions with organic molecules. In particular, electron transfer, proton transfer, and hydrogen atom abstraction reactions have been described for selected radical anions. Other processes that have been reviewed include SN2 substitution and an attack on a carbonyl function in an organic molecule. The interplay between these latter two processes has been described for the reactions of some halogen-substituted carbene radical anions with the methyl ester of trifluoroacetic acid and dimethyl carbonate. The structural characterization of radical anions, distonic radical anions in particular, is summarized with an emphasis on methods such as isomer-specific ion/molecule reactions and collision-induced dissociation or charge-reversal processes.

High front- and back-end resolution MS/MS in Fourier transform ion cyclotron resonance mass spectrometry☆

Abstract It is demonstrated that each of the molecular ions of cyclopropylbenzene and di- n -propyl sulphide, having the same nominal mass m/z 118 and generated from a nearly 1:1 mixture by 70 eV electron impact in a Fourier transform ion cyclotron resonance mass spectrometer, can be selectively isolated in the cell by ejection of all other ions from the cell. This corresponds to a so-called front-end resolution of primary ion selection for MS/MS experiments in excess of 35 000 at m/z 118. Subsequent collisionally induced dissociation (CID) experiments on each of the mass-selected molecular ions have been performed using helium, introduced via a pulsed valve, as collision gas. A so-called back-end resolution of 205 000–475 000 has been obtained for the CID product ions over the mass range m/z 61–117, while the accuracy of the mass measurements on the corresponding ions is shown to be better than 1 ppm.

Hydrogen/deuterium exchange in gas phase reactions of anions with some alkyl phenyl ethers

Abstract The gas phase reactions of anions with methyl and ethyl phenyl ether have been studied by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. 18O-Labelling has shown that part of the reactions of OH- with methyl phenyl ether proceed via ipso-substitution, the main reaction channel being SN2 substitution. Deuterium labelling has shown that extensive inter- and intramolecular hydrogen/deuterium exchange can precede the final substitution reaction. Hydrogen atoms originating from the methoxy substituent are involved in this exchange process. The reactions of anions with ethyl phenyl ether proceed mainly via an elimination mechanism. Deuterium labelling has shown that in some cases hydrogen/deuterium exchange takes place prior to elimination.

Characterization of Hydrogen‐Bonded Supramolecular Assemblies by MALDI‐TOF Mass Spectrometry after Ag+ Labeling

The high affinity of Ag+ ions for aromatic π donors and cyano groups is exploited in a novel MALDI-TOF mass spectrometric method for the identification of hydrogen-bonded assemblies. The interaction with the Ag+ ions-which, for example, can be complexed by two phenyl groups in a sandwich-type manner (see drawing on the right)-provides positively charged assemblies in a nondestructive way.

Bis- and oligo(trifluoromethyl)benzenes: hydrogen/metal exchange rates and gas-phase acidities

The proton mobilities (kinetic acidities) of bis- and tris(trifluoromethyl)benzene are dictated to a large extent by steric factors; the trifluoromethyl group is a fairly bulky substituent that can seriously impede the approach of the metalating reagent. Most satisfactory results in terms of yields and selectivities have been achieved with lithium 2,2,6,6-tetramethylpiperidide or with methyllithium in the presence of potassium tert-butoxide, a slim version of the std. superbase. The rates of deprotonation under irreversible conditions do not parallel the thermodn. (equil.) acidities. Substituent effects on the deprotonation energies in the gas phase appear to be additive: each trifluoromethyl group lowers it by 13 kcal mol-1 when located ortho with respect to the carbanion, and by 10 kcal mol-1 when located in a meta or para position. [on SciFinder (R)]

Additivity of substituent effects in the fluoroarene series: equilibrium acidity in the gas phase and deprotonation rates in ethereal solution

Benzene and all mono-, di-, tri, tetra-, and pentafluoro-substituted derivs. thereof were equilibrated with the corresponding aryl anions in the gas phase. Perfect additivity of the substituent effects on the acidity was obsd. The basicities were diminished by 12, 6, and 4 kcal/mol, depending on whether the supplementary fluorine atom occupied the ortho, meta, or para position with respect to the deprotonation site. On the other hand, substituent effects on the rates of hydrogen/metal interconversion processes level off with increasing nos. of halogens. For example, while the free energy of activation of sec-butyllithium-promoted lithiation of fluorobenzene is at least 4 kcal/mol smaller than that of benzene, the difference was found to shrink to less than 2.5 kcal/mol when pentafluorobenzene and 1,2,3,4-tetraflorobenzene were compared. [on SciFinder (R)]

Proton affinities and heats of formation of the imines CH2NH, CH2NMe and PhCHNH

The proton affinities of the imines CH2NH, CH2NMe and PhCHNH have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass Spectrometry to be 854 ± 8 kJ mol–1, 880 ± 8 kJ mol–1 and 908 ± 8 kJ mol–1, respectively. These results lead, in conjunction with known ΔHf0 values for the protonated species, to ΔHf0(CH2NH)= 69 ± 8 kJ mol–1, which is significantly lower than previously reported in the literature, and to ΔHf0(CH2NMe)= 44 ± 8 kJ mol–1. The proton affinities of the small imines are between the values for the corresponding amines and nitriles. The decrease in proton affinity with the change in hybridization of the nitrogen atom from sp3 to sp2 to sp is discussed together with the increase in proton affinity upon introduction of a methyl group on either the carbon atom or the nitrogen atom in CH2NH.