Collision-induced oxygen addition to chloroaromatic compounds

The reactions of chloroaromatic radical anions with oxygen were studied with a triple quadrupole mass spectrometer. Two chlorobenzenes and eight polychlorinated biphenyls were analyzed by gas chromatography-tandem mass spectrometry under negative ion chemical ionization. The molecular radical anions were selected with the first quadrupole and reacted with oxygen in the collision cell. Under these conditions, [M+O ? Cl] ions were obtained with intensities similar to those of the transmitted precursor ions. This dechlorination reaction was not affected by a detectable chlorine isotope effect. The intensities of the [M+O ? Cl] ions vary with the nature of the chloroaromatic compounds and with the oxygen pressure and collision energy. Charge transfer reactions are also observed, and the relative amount of O ₂ ^?. produced is controlled by the relative electron affinity of the organochlorine. At high collision energies, collision-induced fragmentation of the molecular ion competes for the production of Cl^?.     

Negative gas‐phase ion chemistry of GeH4: a quadrupole ion trap study

Gas‐phase anion/molecule reactions of germanium hydride were studied by quadrupole ion trap (QIT) mass spectrometry. Under chemical ionization (CI) conditions and with a sample pressure of about 5.0 × 10^?5 Torr, clustering reactions proceed up to the formation of Ge₅H ion clusters. With increasing cluster size, the most abundant ion species are those with the lowest content of hydrogen. Reaction sequences obtained by ion isolation were determined for primary, secondary and tertiary germane ions, and reaction enthalpies were calculated for reactions of primary ions. Ion/neutral condensation processes followed by single and double dehydrogenation are by far the most important reactions; moreover, isotope scrambling is observed for almost every reactant ion. These results are in good agreement with previously published data and indicate that germane negative ions are quite efficient in formation and growth of ionic clusters, which can be considered suitable precursors of amorphous solid hydrogenated germanium used in the semiconductor field. Copyright © 2005 John Wiley & Sons, Ltd.     

Reactive collisions in quadrupole cells. Part I. Reaction of [CH3NH2]+˙ with the isomeric butenes and pentenes

The reactions of mass-selected [CH₃NH₂]⁺˙ ions with the isomeric butenes and pentenes were studied at low collision energies in the radiofrequency-only quadrupole collision cell of a hybrid BEqQ tandem mass spectrometer. Characteristic iminium ions arising by addition of the methylamine to the olefin followed by fragmentation are observed for but-1-ene pent-1-ene and 3-methylbut-1-ene. However, for but-2-ene pent-2-ene 2-methylpropene 2-methylbut-1-ene and 2-methylbut-2-ene the major reaction channel of [CH₃NH₂]⁺˙ is charge exchange to form the olefinic molecular ion. The isomeric olefins are characterized to a considerable extent by the characteristic ion–molecule reactions that these molecular ions undergo with the neutral olefin.     

Methylene substitution reactions in a quadrupole ion trap selectivity of ethylene,ethylene oxide,and dimethyl ether reactive ions

To elucidate the selectivity of methylene substitution reactions of monosubstituted and disubstituted oxyaromatic compounds in a low pressure quadrupole ion trap environment, the relative abundances of covalently bound and loosely bound adducts formed by ion/molecule reactions with ethylene (ET), ethylene oxide (ETOX), and dimethyl ether (DME) were compared. Adduct ions of all three reagent gases were formed in both a conventional ion source and a quadrupole ion trap and characterized by collisionally activated dissociation. For DME and ET, the covalently bound adducts formed at (M + 45)⁺ and (M + 41)⁺, respectively, are direct precursors to the methylene substitution product ions at (M + 13)⁺. ETOX and ET do not demonstrate the same functional group selectivity for methylene substitution as previously observed for DME. This is attributed to differences in reaction exothermicities and competing reactions.     

Experiments on size-selected metal cluster ions in a triple quadrupole arrangement

A high flux of positively and negatively charged metal cluster ions was produced in a sputtering arrangement, energy-analyzed and mass-filtered. The resulting mono-dispersed cluster ion beam was introduced into a quadrupole drift tube, where it interacted with a laser beam or reacted with an introduced gas. All inelastic scattering events were recorded with a subsequent quadrupole mass filter. The results exhibited a high sensitivity of positively and negatively charged silver clusters Ag _n ^± (n≤16) with respect to photofragmentation. Ion-molecule reactions of nickel clusters with carbon monoxide allowed to synthesize very interesting organometallic and carbonyl compounds, and the maximum number of ligands provided interesting structural indications.     

Negative- and positive-ion chemical ionization mass spectra of aromatic amines: Surface-assisted reactions involving oxygen

The O₂–N₂ and O₂–Ar negative-ion chemical ionization mass spectra of aromatic amines show a series of unusual ions dominated by an addition appearing at [M + 14]^?˙. Other ions are observed at [M – 12]^?˙, [M + 5]^?˙, [M + 12]^?˙, [M + 28]^?˙ and [M + 30]^?˙. Ion formation was studied using a quadrupole instrument equipped with a conventional chemical ionization source and a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. These studies, which included the examination of ion chromatograms, measurement of positive-ion chemical ionization mass spectra, variation of ion source temperature and pressure and experiments with ¹⁸O₂, indicate that the [M + 14]^?˙ ion is formed by the electron-capture ionization of analytes altered by surfaceassisted reactions involving oxygen. This conversion is also observed under low-pressure conditions following source pretreatment with O₂. Experiments with [¹⁵N]aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆]aniline show that the [M + 14]^?˙ ion corresponds to [M + O ? 2H]^?˙, resulting from conversion of the amino group to a nitroso group. Additional ions in the spectra of aromatic amines also result from surface-assisted oxidation reactions, including oxidation of the amino group to a nitro group, oxidation and cleavage of the aromatic ring and, at higher analyte concentrations, intermolecular oxidation reactions.     

Reaction of ammonia with accelerated benzoyl ions under multiple-collison conditions in a triple quadrupole instrument

The reaction of benzoyl ion with ammonia in multiple-collision conditions in the second quadrupole assembly of a triple quadrupole mass spectrometer at (laboratory) ion kinetic energies from 0 to 20 eV produced the even-electron ions [C₆H₅]⁺, [C₆H₅NH₃·(NH₃)_m]⁺ (m = 0, 1) and [C₆H₅CONH₃·(NH₃)_n]⁺ (n = 0, 1, 2, 3) and the odd-electron ions [C₆H₄NH₃·(NH₃)_p]⁺· (p = 0, 1). Thermochemical information could not be obtained under multiple-collision conditions: both exothermic and endothermic reactions were observed, with no translational-energy onset measurable for the endothermic processes, nor decrease in the yield of the exothermic processes at high energies. The behaviour of cluster-ion intensities as pressure varied was qualitatively as expected. There are pressure and energy regions where spectra change little; if this feature were to be general, it would point to some utility for these conditions in qualitative analysis.     

Detection mass bias in atmospheric pressure ionization mass spectrometry

A previously uncharacterized source of detection mass bias is shown to be associated with atmospheric pressure ionization mass spectrometry (APIMS), and is attributed to a mass dependence in the sampling of ions from the supersonic free jet expansion of gas emerging from the ion source. The halide ions Cl ^?, Br^?, and I^? are shown to be transported from the ion source aperture to a quadrupole mass filter with efficiencies that increase linearly with increasing mass of the ion. While the polyatomic anions SF ₆ ^- and C₇F ₁₄ ^- are detected with even greater efficiencies than would be expected for monatomic anions of the same mass, this additional sensitivity to the polyatomic anions is thought to be related to ion loss processes occurring within the ion source. The experimental conditions under which these mass bias effects can be minimized or enhanced in APIMS are described.     

Ion source emittance influence on the transmission of a quadrupole operated in the second stability region

The variation of transmission (T) with resolution (R) has been calculated for a quadrupole mass filter operated in the second stability region with Mathieu parameters q=7.547 and a=0 to 0.02995. The fringing fields at the entrance to the quadrupole, which can be strongly defocusing, and which can dramatically reduce the acceptance, have been included in the calculation. Even in the absence of fringing fields, at a resolution of 10,000 the acceptance in the x and y directions is less than 2 × 10^?4 πr ₀ ⁴ f ², which is about 10^?3 of the acceptance in the first stability region at low resolution. Because the source emittance can strongly effect the T(R) behavior, the calculation of ion transmission has been done for two source emittances that correspond to different degrees of focusing of ions into the quadrupole. The T(R) characteristics for two realistic source emittances give behavior markedly different from the decrease in acceptance with increasing resolution which previously has been used as a measure of the quadrupole transmission. Comparisons of the calculated transmission losses with increasing resolution to earlier experimental results obtained with an inductively coupled plasma source show good agreement provided an emittance which corresponds to ions being tightly focused into the quadrupole is used. The calculation demonstrates that up to a resolution at half height of 7000 in the experiment, the transmission losses were a result of the decreasing quadrupole acceptance. At higher resolution the experimental transmission was limited by either the residence time of the ions in the quadrupole or the rod quality. It is also shown that the strong defocusing effects of the fringing fields at the entrance of the quadrupole can be largely overcome by accelerating the ions through these fields and then decelerating the ions in the quadrupole.     

Zero kinetic energy ions in dissociative attachment on triatomic molecules: S−/OCS,O−/CO2

The zero kinetic energy yield of S^? ions in OCS and O^? ions in CO₂, produced by dissociative electron attachment through the lowest shape resonance, has been measured in a quadrupole mass spectrometer with electrostatic filter for translational energy analysis. The relative cross sections for S^? and O^? ion formation associated with CO fragments excited up to the vibrational level υ = 4 are obtained.     

A segmented radiofrequency-only quadrupole collision cell for measurements of ion collision cross section on a triple quadrupole mass spectrometer

The gas collision cell of a triple quadrupole mass spectrometer has been modified to consist of ten short quadrupole rod segments that allow an axial field to be applied to the cell in order to make measurements of ion mobility. The radiofrequency (rf)-quadrupole field provides effective radial confinement that greatly reduces diffusional losses at low pressure. The mobilities of mass-selected ions from an ionspray source have been measured at a pressure of 8 × 10^?3 torr at electric fields of 0. 1 to 3 V/cm, and used to calculate the collision cross sections of the ions. The measured cross sections compare well with those measured by other techniques.     

Negative chemical ionization in quadrupole ion trap mass spectrometry: Effects of applied voltages and reaction times

The effects of applied voltages and reaction times on negative ion chemical ionization in the quadrupole ion trap are investigated. Mass-selected ejection of undesired reagent ions and selective mass storage of only negative ions are required for practical negative ion chemical ionization. This is achieved by application of rf and dc voltages to the ring electrode to control the mass-to-charge ratios one polarity) of ions stored, as well as by application of a supplemental rf voltage applied across the endcap electrodes to selectively eject ions of a particular mass-to-charge ratio. Even with careful control of these parameters, negative chemical ionization is not as sensitive as electron ionization and positive chemical ionization because of the lack of thermal electrons in the ion trap. Mass selection of the hydroxide anion as a reagent ion and exclusion of all positive ions provide [M ? H]^? ions with little or no fragmentation for a wide variety of compounds.     

Ion manipulation and enrichment mass spectrometer for cleavage of disulfide bond via ion/ion reaction

A novel mass spectrometer with the capability of ion manipulation and enrichment was developed to perform gas‐phase ion/ion reactions followed by product ions accumulation. The development of this apparatus opens opportunities for more complex sequences of ion manipulations, thus offers the potential on extensive application involving ion/ion reaction. Here, cleavage of disulfide bond in peptide was demonstrated based upon this ion manipulation and enrichment mass spectrometer. Two typical peptides including S‐glutathionylated ARACAKA with an intermolecular disulfide bond, and oxytocin with an intramolecular disulfide bond were chosen as typical samples to demonstrate the ability of the apparatus. After ion/ion reaction between selected peptide cations and periodate ions (IO₄^?), two kinds of product ions (eg, [M + O + H]⁺ and [M + H + Na + IO₄]⁺) were enriched with a number of accumulation events. Afterwards, the enriched ions were subjected to activation, and the disulfide bond cleavage was clearly observed from the tandem mass spectra. These results illustrate the potential of this apparatus for ion manipulation performing ion/ion reaction, and low abundance product ion enrichment.     

Identification of the primary chemi-ion in hydrocarbon oxidations

The ion CHO⁺ has been identified as the primary chemi-ion in the reaction of oxygen atoms with acetylene, ethylene and isobutane. Low concentrations of reactants in helium were mixed in the annular region between two concentric stainless steel cylinders. The chemi-ions were sampled through a pinhole in the outer cylinder and analyzed by a quadrupole mass spectrometer. By applying an electric field between the two cylinders, the residence time of the ions could be decreased, thereby reducing the importance of secondary ion—molecule reactions. CHO⁺ was the only ion that increased in importance as the electric field was applied, becoming more than 90% of the total ions at high fields. This observation provides strong support for the proposal of Calcote that the reaction, O + CH → CHO⁺ + e^?1 is the source of chemi-ionization in hydrocarbon flames.     

Mobilities of H+3 and HeH+ ions in helium gas and rate coefficient for HeH+ + H2 → H+3 + He

The mobilities of mass-identified H⁺₃ and HeH⁺ ions in helium and the reaction rate coefficient for HeH⁺ + H₂ → H⁺₃ + He have been measured by a drift-tube quadrupole mass spectrometer at 300 K. The zero-field reduced mobilities of H⁺₃ and HeH⁺ ions, corrected to 273 K, are 31.0 ± 0.8 and 23.4 ± 0.6 cm² V^?1 s^?1 respectively. The reaction rate coefficient was found to be (1.26 + 0.16) × 10^?9 cm³s^?1 and was observed to be independent of the mean ion kinetic energy in the range from 0.04 to 0.3 eV.     

Are liquid chromatography/electrospray tandem quadrupole fragmentation ratios unequivocal confirmation criteria?

Multiple reaction monitoring (MRM) ratios as provided by tandem mass spectrometers are used to confirm positive residue findings (e.g. veterinary drugs or pesticides). The Commission Decision 2002/657/EEC defines tolerance levels for MRM ratios, which are intended to prevent the reporting of false positives. This paper reports findings where blank sample extracts have been spiked by a drug (difloxacin) and the corresponding measured MRM ratios significantly deviated from MRM ratios observed in matrix‐free solution. The observation was explained by the formation of two different [M+H]⁺ analyte ions within the electrospray ionization (ESI) interface. These two ions vary only by the site of analyte protonation. Since they are isobaric, they are equally transmitted through the first quadrupole, but are differently fragmented in the collision chamber. The existence of two isobaric ions was deduced by statistical data and the observation of a doubly charged analyte ion. It was hypothesized that the combined presence of [M+H]⁺ and [M+2H]²⁺ implies the existence of two different singly charged ion species differing only by the site of protonation. Low‐ and high‐energy interface‐induced fragmentation was performed on the samples. The surviving precursor ion population was mass selected and again fragmented in the collision chamber. Equal product ion spectra would be expected. However, very different product ion spectra were observed for the two interface regimes. This is consistent with the assumption that the two postulated isobaric precursor ions show different stability in the interface. Hence the abundance ratio among the two types of surviving precursor ions will shift and change the resulting product ion spectra. The existence of the postulated singly charged ions with multiple chargeable sites was finally confirmed by successful ion mobility separation. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass resolution of 11,000 to 22,000 with a multiple pass quadrupole mass analyzer

CO⁺ and N₂⁺ are separated with resolution of 11,000 [full width half maximum (FWHM)] using a conventional quadrupole mass spectrometer by applying square wave voltages to the entrance and exit lenses to trap or reflect the ions for multiple passes. A resolution of 22,000 (FWHM) with 63% of the total signal remaining is attained using multiple passes when ions are stored between injection pulses. Gated ion extraction also reduces the mass shift and number and intensity of artifact peaks and permits better resolution compared to the performance obtained when the ions are injected continuously.     

Equilibrium space charge distribution in a quadrupole ion trap

A simple model provides a basis for evaluating the ion spatial distribution in a uadrupole (Paul) ion trap and its effect on the total potential (trap potential plus space charge 3 acting on ions in the trap. By combining the pseudopotential approximation introduced by Dehmelt 25 years ago with the assumption of thermal equilibrium (leading to a Boltzmann ion energy distribution), the resulting ion spatial distribution (for ions of a single mass-to-charge ratio) depends only on total number of ions, trap pseudopotential, and temperature. (The equilibrium assumption is justified by the high helium bath gas pressure at which analytical quadrupole ion traps are typically operated.) The electric potential generated by the ion space charge may be generated from Poisson’s equation subject to a Boltzmann ion energy distribution. However, because the ion distribution depends in turn on the total potential, solving for the potential and the ion distribution is no longer a simple boundary condition differential equation problem; an iterative procedure is required to obtain a self-consistent result. For the particularly convenient operating condition, (a _z = -8qU/m? ₀ ² Ω², and q _z =-4qV m? ₀ ² Ω², where U and V are direct current and radiofrequency (frequency, ω) voltages applied to the trap, m/q is ion mass-to-charge ratio, and ?₀ is the radius of the ring electrode at the z=0 midplane], both the pseudopotential and the ion distribution become spherically symmetric. The resulting one-dimensional problem may be solved by a simple optimization procedure. The present model accounts qualitatively for the dependence of total potential and ion distribution on number of ions (higher ion density or lower temperature flattens the total potential and widens the spatial distribution of ions) and pseudopotential (higher pseudopotential increases ion density near the center of the trap without widening the ion spatial distribution).     

Substitution reactions of gaseous ions in a three‐dimensional quadrupole ion trap

Substitution reactions between gaseous ions and neutral substrate molecules are of ongoing high interest. To investigate these processes in a qualitative and quantitative manner, we have constructed a device, with which a defined amount of a volatile substrate can be mixed with a defined amount of helium gas and added into a three‐dimensional quadrupole ion trap. From the known inner volume of the device, the known ratio n_substrate:n_He of the mixture, and the determined absolute partial pressure of helium in the ion trap, we can derive the partial pressure of the substrate in the ion trap and, thus, convert the directly observable pseudo–first‐order rate constants of the substitution reactions into absolute bimolecular rate constants. We have tested the device by investigating a series of S_N2 reactions of Br ^? and CF₃CH₂O ^? anions as well as ligand exchange reactions of ligated Na⁺ cations. As the obtained results suggest, the described device makes it possible to determine the bimolecular rate constants of substitution reactions as well as other ion‐molecule reactions with satisfactory accuracy and reliability.     

Negative ions generated by reactions with oxygen in the chemical ionization source. I. Characterization of gas-phase and wall-catalyzed reactions of fluorene,anthracene and fluoranthene

The methane negative ion chemical ionization (NICI) mass spectra of polycyclic aromatic hydrocarbons are usually dominated by molecular, M^{? ˙} or M ? H^? ions; however, ions resulting from additions to M have also been reported. Some of these ions have been observed at [M + 14]^{? ˙}, [M + 15]^?, [M + 30]^? and [M + 32]^?˙ and have been attributed to reactions with either oxygen-containing impurities in the buffer gas or alkyl radical species generated by ionization of a hydrocarbon buffer gas. In this study, the NICI spectra of fluorene, anthracene and fluoranthene were studied in detail using quadrupole and Fourier transform mass spectrometers. Spectra were acquired when reactive species such as oxygen, water, nitrous oxide and carbon dioxide were added to the nitrogen buffer gas. Experiments with deuterated methane were also carried out. These studies indicated that buffer gas impurities affect the NICI spectra; however, gas-phase ion-molecule reactions were not responsible for all of the observed products. In addition to electron- and ion-molecule reactions, ions were observed that resulted from wall-catalyzed oxidation reactions followed by electron capture. These reactions were enhanced by the addition of oxygen and elevated ion source temperatures. Depending upon the parent PAH structure, oxidation products such as ketones, quinones and anhydrides were formed.