Probing trapped ion energies via ion-molecule reaction kinetics: Fourier transform ion cyclotron resonance mass spectrometry

The kinetic energy-dependent Ar⁺+ N₂ ion-molecule reaction has been used as a chemical “thermometer” to determine the kinetic energy of ions produced by electron ionization and trapped by using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The rate constant for this reaction obtained on the FTICR mass spectrometer was compared to previous work, which allowed a kinetic energy estimate to be made. In addition, the effects of varying parameters such as trapping voltage and pressure on ion kinetic energy were investigated. No evidence of the differing reactivity of higher energy electronic states of Ar⁺, such as ²P_1/2, was observed and the results of a model of this system are presented that support this observation. Pressure studies revealed that with an average of as few as 13 ion-molecule collisions, Ar⁺ ions are collisionally relaxed to an extent unaffected by additional collisions. Based on recent variable temperature selected ion flow drift tube measurements, FTICR ion energies are estimated to be slightly above thermal.     

On the CO elimination from [M – CH3]+ ions in substituted phenyl methyl ethers

It was inferred from the collisional activation spectra that CO loss from [M – CH₃]⁺ ions generated from o-,m- and p-cyanoanisole yields a common ion, presumably the cyanocyclopentadienyl cation. A similar product ion is found to be generated in the three dimethoxybenzene isomers. In case of o-dimethoxybenzene loss of CO was also found to occur via an important additional route, which leads to the formation of protonated phenol.     

Improved ion optics for introduction of ions into a 9.4‐T Fourier transform ion cyclotron resonance mass spectrometer

Enhancements to the ion source and transfer optics of our 9.4 T Fourier transform ion cyclotron resonance (ICR) mass spectrometer have resulted in improved ion transmission efficiency for more sensitive mass measurement of complex mixtures at the MS and MS/MS levels. The tube lens/skimmer has been replaced by a dual ion funnel and the following octopole by a quadrupole for reduced ion cloud radial expansion before transmission into a mass‐selective quadrupole. The number of ions that reach the ICR cell is increased by an order of magnitude for the funnel/quadrupole relative to the tube lens/skimmer/octopole. Copyright © 2015 John Wiley & Sons, Ltd.     

Mass spectrometry of pyridine compounds–IV: The formation and decomposition of the [M – methyl]+ ion [C8H10N]+ from 4-t-butylpyridine in comparison with that of the [M – methyl]+ ion [C9H11]+ from t-butylbenzene,as studied by D- and 13C-labelling

A strong secondary isotope effect is observed in the preferred loss of methyl vs. trideutero-methyl from the molecular ions of appropriately labelled 4-t-butylpyridine and t-butylbenzene decomposing in the first and second field free regions of a double focusing mass spectrometer. This has been rationalised by invoking the theory of radiationless transitions², which can account for the higher population of activated states responsible for loss of methyl vs. that for trideuteromethyl. ¹³C-Labelling at the central carbon atom of the t-butyl group indicates that the [M – methyl]⁺ ions, decomposing further by elimination of ethylene, cannot be represented exclusively by a pyridylated (or phenylated) cyclopropane ion if present at all. It is concluded that ions with structures generated by 1,2-hydrogen-, 1,2-pyridyl- (or 1,2-phenyl-) and 1,2-methyl shifts must also play a role. D-labelling further shows an extensive randomisation of side-chain hydrogen atoms in the [M-methyl]⁺ ions of 4-t-butylbenzene; in this case, however, the expelled ethylene also contains ring hydrogen atoms (≤2). Presumably this is caused by exchange between the side-chain and ortho-hydrogen atoms in the initially generated phenyldimethylcarbinyl carbenium ion.     

Fourier transform ion cyclotron resonance: state of the art

This short review summarizes recent and projected advances in Fourier transform ion cyclotron resonance mass spectrometry instrumentation and applications, ranging from petroleomics to proteomics. More details are available from the cited primary literature and topical reviews.     

Fourier transform ion cyclotron resonance detection of multiphoton ionization spectroscopy

Fourier transform ion cyclotron resonance (FT-ICR) detection was tested for resonanceenhanced multiphoton ionization (REMPI) spectroscopy. The (2+1) REMPI spectra of acetaldehyde were obtained in the wavelength range 364–354 nm via a two-photon resonant 3s ← n Rydberg transition. The space-charge effects on the REMPI spectra were examined in the vicinity of the 0 ₀ ⁰ transition. The trapping efficiency measurement shows that all the ions produced from REMPI dissociation processes are arrested in the ion cyclotron resonance cell even in the presence of space-charge interactions. Axial kinetic energy release distributions of ions were extracted from the trapping efficiency data obtained under a new space-charge-free condition. FT-ICR peak heights were measured as a function of pressure at different laser powers, magnetic field strengths, and ion excitation methods to test for the detection linearity. The FT-ICR detection responds linearly to the number of ions in a low pressure limit. The product branching ratio was measured by using various ion excitation methods and was compared with the previous quadrupole mass spectrometric study. FT-ICR detection yields the mass-selected REMPI spectra and the product branching ratio in the absence of kinetic shifts.     

Mass spectrometry of steroid systems. XXIV–The origin of [M – 28]+ and [M – 43]+ ions in equilenin and its 14β-isomer

The formation of the [M? 43]⁺ ion in equilenin is due mainly to elimination of Me radical from the [M? CO]⁺ ion and, to a lesser extent, to CO loss from the [M? Me]⁺ ion. In 14β-isoequilenin the [M? CO]⁺ ion is absent, and the formation of [M? 43]⁺ occurs via the [M? Me]⁺ ion. This makes the determination of the mode of junction of the rings C and D in the equilenin series possible, using high resolution mass spectra, even when only one stereoisomer is available.     

The structures of [C4H4]+˙ ions by photodissociation in an ion cyclotron resonance spectrometer

The photodissociation of [C₄H₄]⁺˙ fragment ions at the ion cyclotron resonance time-scale competes with relaxation of the internal energy by infrared emission. As a result the fraction of photodissociating ions increases with light intensity. The experiments indicate that [C₄H₄]⁺˙ from benzene and 1,5-hexadiyne consists of a mixture of 60% vinyl acetylene ions, 10% butatriene ions and 30% cyclic ions. This confirms previous conclusions from studies of the ion-molecule reactions of [C₄H₄]⁺˙ with benzene.     

Speciation of M+3-hydroxypyrone chelation complexes by electrospray ionization ion trap and Fourier transform ion cyclotron resonance mass spectrometry

Kojic acid (5-hydroxy-2-hydroxymethyl-4-pyrone) is known to have a high affinity for transition metals, and it and its derivatized cogeners are used both analytically and clinically. The interactions between kojic acid (KA) and eleven +3 metals (Al(+3), As(+3), Cr(+3), Ga(+3), Fe(+3), In(+3), Yb(+3), Y(+3), Gd(+3), Nd(+3), La(+3)) were examined by electrospray ionization mass spectrometry (ESI-MS) using an ion trap in an aqueous medium. For a subset of five ions, Fourier transform ion cyclotron resonance (FTICR)-MS was conducted to provide accurate mass confirmation of peak assignments for metals having clustering of abundant isotopes. KA readily formed complexes with all the metal ions tested. The most common complexes observed were ML(3)H(+) and M(2)L(5). Different behavior was seen for small and large ionic radius ions, with a relative cut-off between In(+3) ( approximately 80 pm) and Yb(+3) ( approximately 87 pm); a striking trend in % collision energy vs. cluster complexity was revealed. The KA-Cr(+3)complex shows a high affinity for H(2)O molecules in the gas phase, whilst In(+3) shows a preference for dimetal complexes and Y(+3) a deviant behavior when complexed to two neutrals.     

Solvation of acylium fragment ions in electrospray ionization quadrupole ion trap and Fourier transform ion cyclotron resonance mass spectrometry

In electrospray ionization (ESI) quadrupole ion trap and Fourier transform ion cyclotron resonance mass spectrometry, certain fragment ions (e.g. acylium ions) generated either during the ion transportation process (in the source interface region) or in the ion trap are found to undergo ion--molecule reactions with ESI solvent molecules (water, acetonitrile and aliphatic alcohols) to form adduct species. These unexpected solvated fragment ions severely complicate the interpretation of mass spectrometic data. High-resolution accurate mass measurements are important in establishing the elemental compositions of these adduct species and preventing erroneous data interpretation.     

Fourier transform ion cyclotron resonance mass spectral characterization of metal‐humic binding

The interaction between metals and naturally occurring humic substances and the thereby induced issues of bioavailability and hydrogeochemical turnover of metal ions in natural waters have been the subject of intense study for decades. Traditional bulk techniques to investigate metal‐humic binding (e.g. potentiometry and inductively coupled plasma mass spectrometry (ICP‐MS)) can provide quantitative results for the relative abundance and distribution of metal species in humic samples and/or overall binding constants. The shortcoming of these bulk techniques is the absence of structural detail. Ultra‐high‐resolution mass spectrometry, currently the only technique demonstrated to resolve individual humic ions, is not generally employed to provide the missing qualitative information primarily because the identification of metal complexes within the already complex mixtures of humic substances is non‐trivial and time‐consuming to the extent of eliminating any possibility for real‐time manipulation of chelated analytes. Here, it is demonstrated that with tailored selection of the metal ion, it is possible to visually identify large numbers of metal‐humic complexes (～500 for Be²⁺, ～1100 for Mn²⁺, and ～1500 for Cr³⁺) in real‐time as the spectra are being acquired. Metal ions are chosen so that they form primarily even‐m/z complexes with humic ions. These even‐m/z complexes stand out in the spectrum and can readily be characterized based on molecular formulae, which here revealed for example that Suwannee River fulvic acid (SRFA) complexes encompassed primarily highly oxygenated fulvic acids of relatively low double‐bond equivalence. Facile, real‐time identification of even‐m/z metal‐humic complexes additionally allows for the specific selection of metal‐humic complexes for MSⁿ analysis and in‐trap ion‐neutral reactions enabling investigation of metal‐humic complex structure. MS/MS data were collected to demonstrate the potential of the technique as well as highlight some of the remaining challenges. Copyright © 2009 John Wiley & Sons, Ltd.     

In-source H/D exchange and ion-molecule reactions using matrix assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry with pulsed collision and reaction gases

Controlled in-source ion-molecule reactions are performed for the first time in an external matrix assisted laser desorption ionization (MALDI) source of a Fourier transform ion cyclotron resonance mass spectrometer. The MALDI source with a hexapole ion guide that was originally designed to incorporate pulsed gas to collisionally cool ions (Baykut, G.; Jertz, R.; Witt, M. Rapid Commun. Mass Spectrom. 2000, 14, 1238-1247) has been modified to allow the study of in-source ion-molecule reactions. Upon laser desorption, a reaction gas was introduced through a second inlet and allowed to interact with the MALDI-generated ions trapped in the hexapole ion guide. Performing ion-molecule reactions in the high pressure range of the ion source prior to analysis in the ion cyclotron resonance (ICR) cell allows to maintain the ultra high vacuum in the cell which is crucial for high mass resolution measurements. In addition, due to the reaction gas pressure in the hexapole product ion formation is much faster than would be otherwise possible in the ICR cell. H/D exchange reactions with different peptides are investigated, as are proton-bound complex formations. A typical experimental sequence would be ion accumulation in the hexapole ion guide from multiple laser shots, addition of cooling gas during ion formation, addition of reaction gas, varied time delays for the ion-molecule reactions, and transmission of the product ions into the ICR cell for mass analysis. In this MALDI source H/D exchange reactions for different protonated peptides are investigated, as well as proton-bound complex formations with the reaction gas triethylamine. Amino acid sequence, structural flexibility and folding state of the peptides can be seen to play a part in the reactivity of such ions.     

Obtaining more accurate Fourier transform ion cyclotron resonance mass measurements without internal standards using multiply charged ions

Space-charge effects produce frequency shifts in Fourier transform ion cyclotron resonance (FTICR) mass spectrometry and correction for these shifts is necessary for obtaining accurate mass measurements. We report a novel method for obtaining accurate mass calibration to correct for space-charge induced mass shifts without the requirement for internal calibrants. The new approach is particularly well suited for electrospray ionization-FTICR mass spectra that contain multiple charge states of the same molecular species. This method, deconvolution of Coulombic affected linearity (DeCAL), is described and presented with several examples demonstrating the increased mass measurement accuracy obtained. DeCAL provides the basis for more routinely obtaining higher mass accuracy measurements in conjunction with chromatographic separations for complex mixture analysis, and obviates the need for internal calibration in many applications.     

Determination of ion—molecule collision frequencies by Fourier transform ion cyclotron resonance spectroscopy

It is shown that nonreactive ion—molecule collision frequencies may be determined either by analysis of the transient signal resulting from excited ion cyclotron motion or by linewidth measurement of the Fourier transform of the transient signal.     

Theory of peak coalescence in Fourier transform ion cyclotron resonance mass spectrometry

Peak coalescence, i.e. the merging of two close peaks in a Fourier transform ion cyclotron resonance (FTICR) mass spectrum at a high number of ions, plays an important role in various FTICR experiments. In order to describe the coalescence phenomenon we would like to propose a new theory of motion for ion clouds with close mass‐to‐charge ratios, driven by a uniform magnetic field and Coulomb interactions between the clouds. We describe the motion of the ion clouds in terms of their averaged drift motion in crossed magnetic and electric fields. The ion clouds are considered to be of constant size and their motion is studied in two dimensions. The theory deals with the first‐order approximation of the equations of motion in relation to dm/m, where dm is the mass difference and m is the mass of a single ion. The analysis was done for an arbitrary inter‐cloud interaction potential, which makes it possible to analyze finite‐size ion clouds of any shape. The final analytical expression for the condition of the onset of coalescence is found for the case of uniformly charged spheres. An algorithm for finding this condition for an arbitrary interaction potential is proposed. The critical number of ions for the peak coalescence to take place is shown to depend quadratically on the magnetic field strength and to be proportional to the cyclotron radius and inversely proportional to the ion masses. Copyright © 2009 John Wiley & Sons, Ltd.