Radiative stabilization of trimethylsilyl adduct ions

Ketones and phenol react with trimethylsilyl ions to form adduct ions by radiatively or collisionally stabilized addition reactions, in contrast to aliphatic alcohols and ethers, which react with trimethylsilyl ions to form adduct ions by a rapid two-step process. Secondorder rate constants for the addition of trimethylsilyl ions to acetone were independent of pressure from 3×10^?7 to 50×10^?7 tort at room temperature; consequently, the adduct ions, [M+73]⁺, are formed primarily by radiatively stabilized addition in these ion cyclotron resonance experiments.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane

Mixtures of 50% tetramethylsilane (TMS) and methane have been found to give [M+73]⁺ adduct ions and structurally useful fragment ions for many oxygen- and nitrogen-containing organic compounds. All of the reagent ions in TMS react with polar compounds. The high-pressure TMS chemical ionization spectra of many simple oxygenated compounds are in agreement with predictions from ion chemistry of (CH₃)₃Si⁺ obtained by ion cyclotron resonance experiments at very low pressures, but differences are noted. Sensitivities for oxygen-, nitrogen-, and sulfur-containing compounds with TMS as the reagent gas appear to be approximately the same.     

Reactions of the trimethylsilyl ion with 1,2-cyclopentanediol isomers in the collision region of a triple quadrupole instrument

Ion-molecule reactions with the trimethylsilyl ion were used to distinguish between cis- and trans-1,2-cyclopentanediol isomers. The ion kinetic energy of [Si(CH₃)₃]⁺ was varied from 0 eV to 15 eV (center of mass frame of reference). At low ion kinetic energies (<4 eV), there are significant differences in the relative stabilities and decomposition behavior of the adduct ions [M + Si(CH₃)₃]⁺. The cis-1,2-cyclopentanediol isomer favors decomposition of [M + Si(CH₃)₃]⁺ to yield the hydrated trimethylsilyl ion [Si(CH₃)₃OH₂]⁺ at m/z 91. For the trans isomer, the formation of the hydrated trimethylsilyl ion is an endothermic process with a definite threshold ion kinetic energy.     

Chemical ionization mass spectrometry of halogenoalkanes with tetramethylsilane as reagent gas

Chemical ionization mass spectra of halogenoalkanes (RX) obtained using tetramethylsilane as reagent gas show two major peaks corresponding to the cluster ion RX⁺SiMe₃ and alkyl ion R⁺. Iodides exhibit the highest affinity toward the trimethylsilyl ion and produce the most stable silylated molecular ions, whereas bromides and especially chlorides are less reactive toward Me₃Si⁺ ions and form less stable [M + SiMe₃]⁺ ions.     

Enantiomeric differentiation of β‐amino alcohols under electrospray ionization mass spectrometric conditions

The enantiomeric differentiation of a series of chiral β‐amino alcohols (A) is attempted, for the first time, by applying the kinetic method using L‐proline, L‐tryptophan, 4‐iodo‐L‐phenylalanine or 3, 5‐diiodo‐L‐tyrosine as the chiral references (Ref) and Cu²⁺ or Ni²⁺ ion (M) as the central metal ion. The trimeric diastereomeric adduct ions, [M+(Ref)₂+A‐H]⁺, formed under electrospray ionization conditions, are subjected for collision‐induced dissociation (CID) experiments. The products ions, formed by the loss of either a reference or an analyte, detected in the CID spectra are evaluated for the enantiomeric differentiation. All the references showed enantiomeric differentiation and the R_chiral values are better for the aromatic alcohols than for aliphatic alcohols. Notably, the R_chiral values of the aliphatic amino alcohols enhanced when Ni²⁺ is used as the central metal ion. The experimental results are well supported by computational studies carried out on the diastereomeric dimeric complexes. The computational data of amino alcohols is correlated with that of amino acids to understand the structural interaction of amino alcohols with reference molecule and central metal ion and their role on the stabilization of the dimeric complexes. Application of flow injection MS/MS method is also demonstrated for the enantiomeric differentiation of the amino alcohols. Copyright © 2014 John Wiley & Sons, Ltd.     

Coordination of sodium cation to an oxygen function and olefinic double bond to form molecular adduct ion in fast atom bombardment mass spectrometry

Steroidal allylic alcohols formed Na+ adduct ion peaks [M+Na]+ by the addition of NaCl in FAB mass spectrometry. A comparison of the intensities of the adduct ion peaks of allylic alcohols with those of the corresponding saturated alcohols and olefin suggested that the olefinic double bond and the proximal hydroxyl group had coordinated to Na+. The adduct ion was stable and did not undergo dehydroxylation. We suggest that the Na+ adduction will be useful for the molecular weight determination of allylic alcohols which are susceptible to dehydroxylation under FAB mass spectrometric conditions. Na+ adduct ions of alpha,beta-unsaturated carbonyl compounds were also investigated.     

Tetramethylsilane chemical ionization mass spectrometry of some isomeric unsaturated dicarboxylic acids and esterst

The tetramethylsilane (TMS) chemical ionization (CI) mass spectra of some geometrical isomers of unsaturated dicarboxylic acids, esters and isomeric phthalic acids reveal explicit differences. The (E)-acids show an abundant [M + 73 ? CH₄]⁺Ion whereas the (Z)-acids exhibit a strong [M + 73 ? H₂O]⁺ ion in their TMS CI spectra. The loss of CH₄ from the adduct of fumaric acid has been confirmed by the study of fumaric acid-d₂ and B/E linked scan studies. In the case of esters, the TMS CI spectra of (E)-isomers contain abundant [M + 73]⁺ adduct ions, whereas these are weakly abundant in the TMS CI of the (Z)-isomers.     

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.     

Soft or hard ionization of molecules in helium nanodroplets? An electron impact investigation of alcohols and ethers

Electron impact (70 eV) mass spectra of a series of C1-C6 alcohols encased in large superfluid liquid helium nanodroplets (approximately 60,000 helium atoms) have been recorded. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others, most notably cleavage of the C(alpha)-H bond in the parent ion to form the corresponding oxonium ion. Parent ion intensities are also enhanced by the helium, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. To demonstrate that these findings are not unique to alcohols, we have also investigated several ethers. The results obtained for both alcohols and ethers are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels. The set of molecules explored in this work suggest that electron impact ionization of doped helium nanodroplets does not provide a sufficiently large softening effect to be useful in analytical mass spectrometry.     

OH stretching force constants in complexes of water with F−, Cl−, Li+ ions and LiF,LiCl ion pairs by 6-31G ab initio MO calculations

Ab initio MO calculations using 6-31G and 6-31 + G (for complexes with F⁻ and LiF) basis sets have been carried out for complexes of H₂O (monomer and dimer) with F⁻, Cl⁻, Li⁺ ions as well as with LiF and LiCl ion pairs for the evaluation of the OH stretching force constants. The changes in force constants are discussed in terms of molecular interactions, cooperativity effect and interionic electrostatic interactions. It is noticed that the cooperativity effect also operates through ionic bonds in symmetrically hydrated ion pairs and that OH stretching force constants are found to increase in the case of solvent bound ion pairs and symmetrically hydrated halide ions showing anticooperativity effect.     

Direct analysis in real time mass spectrometry (DART‐MS) of highly non‐polar low molecular weight polyisobutylenes

Low molecular weight polyisobutylenes (PIB) with chlorine, olefin and succinic acid end‐groups were studied using direct analysis in real time mass spectrometry (DART‐MS). To facilitate the adduct ion formation under DART conditions, NH₄Cl as an auxiliary reagent was deposited onto the PIB surface. It was found that chlorinated adduct ions of olefin and chlorine telechelic PIBs, i.e. [M + Cl]^? up to m/z 1100, and the deprotonated polyisobutylene succinic acid [M? H]^? were formed as observed in the negative ion mode. In the positive ion mode formation of [M + NH₄]⁺, adduct ions were detected. In the tandem mass (MS/MS) spectra of [M + Cl]^?, product ions were absent, suggesting a simple dissociation of the precursor [M + Cl]^? into a Cl^? ion and a neutral M without fragmentation of the PIB backbones. However, structurally important product ions were produced from the corresponding [M + NH₄]⁺ ions, allowing us to obtain valuable information on the arm‐length distributions of the PIBs containing aromatic initiator moiety. In addition, a model was developed to interpret the oligomer distributions and the number average molecular weights observed in DART‐MS for PIBs and other polymers of low molecular weight. Copyright © 2015 John Wiley & Sons, Ltd.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with ethers

Chemical ionization mass spectra of several ethers obtained with He/(CH₃)₄Si mixtures as the reagent gases contain abundant [M + 73]⁺ adduct ions which identify the relative molecular mass. For the di-n-alkyl ethers, these [M + 73]⁺ ions are formed by sample ion/sample molecule reactions of the fragment ions, [M + 73 ? C_nH_2n]⁺ and [M + 73 ? 2CnH_2n]⁺. Small amounts of [M + H]⁺ ions are also formed, predominantly by proton transfer reactions of the [M + 73 ? 2CnH_2n]⁺ or [(CH₃)₃SiOH₂]⁺ ions with the ethers. The di-s-alkyl ethers give no [M + 73] ⁺ ions, but do give [M + H]⁺ ions, which allow the determination of the relative molecular mass. These [M + H]⁺ ions result primarily from proton transfer reactions from the dominant fragment ion, [(CH₃)₃SiOH₂]⁺ with the ether. Methyl phenyl ether gives only [M + 73]⁺ adduct ions, by a bimolecular addition of the trimethylsilyl ion to the ether, not by the two-step process found for the di-n-alkyl ethers. Ethyl phenyl ether gives [M + 73]⁺ by both the two-step process and the bimolecular addition. Although the mass spectra of the alkyl etherr are temperature-dependent, the sensitivities of the di-alkyl ethers and ethyl phenyl ether are independent of temperature. However, the sensitivity for methyl phenyl ether decreases significantly with increasing temperature.     

Stereospecific adduct ion formation in the chemical ionization mass spectra of E- and Z-1,2,3-triaryl-2-propen-1-ones

Stereospecific adduct ion formation has been observed in the chemical ionization mass spectra (positive and negative) of certain E- and Z-1,2,3-triaryl-2-propen-1-ones. The Z isomers are found to give higher relative abundances of adduct ions than the E isomers. This has been interpreted in terms of the differences in the proton affinities of the isomers originating from their different degrees of enone resonance. Halide ion (CI^? and Br^?) attachment spectra of these compounds also show stereochemical differences in the relative abundances of [M]^?˙ and [M+halide]^? ions, though the effect is not as pronounced as in the case of the positive ion spectra.     

Direct exposure chemical ionization mass spectra of explosives

Mass spectra of explosives, including TNT, tetryl, nitroglycerin, PETN and RDX have been recorded by direct exposure chemical ionization with isobutane as reagent at source temperatures of 50–100°C. The mass spectra contain major [MH]⁺ ions, adduct ions and some fragment ions. The configuration of the relative abundances of these ions has been found to be a function of temperature and source pressure. Maximum [MH]⁺ ion abundance has been obtained at source pressures much lower than normal chemical ionization pressures.     

Influence of weakly bound adduct ions on breath trace gas analysis by selected ion flow tube mass spectrometry (SIFT-MS)

This paper describes how weakly bound adduct ions form when the precursor ions used in selected ion flow mass spectrometry, SIFT-MS, analyses, viz. H₃O⁺, NO⁺ and O₂⁺, associate with the major components of air and exhaled breath, N₂, O₂ and CO₂. These adduct ions, which include H₃O⁺N₂, H₃O⁺CO₂, NO⁺CO₂, O₂⁺O₂ and O₂⁺CO₂, are clearly seen when dry air containing 5% CO₂ (typical of that in exhaled breath) is analysed using SIFT-MS. These adduct ions must not be misinterpreted as characteristic product ions of trace gases; if so, serious analytical errors can result. However, when exhaled breath is analysed these adduct ions are partly removed by ligand switching reactions with the abundant water molecules and the problems they represent are alleviated. But the small fractions of the adduct ions that remain in the SIFT-MS spectra, and especially when they are isobaric with genuine characteristic product ion of breath trace gases, can result in erroneous quantifications; such is the case for H₃O⁺N₂ interfering with breath ethanol analysis and H₃O⁺CO₂ with breath acetaldehyde analysis. However, these difficulties can be overcome when the isobaric adduct ions are properly recognised and excluded from the analyses; then these two important compounds can be properly quantified in breath. The presence of O₂⁺CO₂ in the product ion spectra interferes with the analysis of CS₂ present at low levels in exhaled breath. It is likely that similar problems will occur as other trace compounds are detected in exhaled breath when consideration will have to be given to the possibility of overlapping between their characteristic product ions and ions produced by hitherto unknown reactions. Similar problems are evident in other systems; for example, H₃O⁺CH₄ adduct ions are observed in both SIFT-MS analyses of methane rich mixtures like biologically generated waste gases and in model planetary atmospheres.     

Oxirane as a chemical ionization gas: Reactivity towards different classes of compounds

Oxirane chemical ionization (CI) gives numerous ions, including C₂H₃O⁺ and C₂H₅O⁺. These ions react with organic molecules through various specific ion–molecule reactions such as hydride abstraction, protonation, additions or cycloadditions. Oxirane CI allows discrimination between unsaturated compounds with [M + 43]⁺ and [M + 57]⁺ adduct ions and heteroatom functions with [M + 45]⁺ adduct ion. All are diagnostic ions. Oxirane CI permits selectivity during the ionization process of a mixture and discrimination of isomers.     

Studies on primary hydrated complexes of Li+, Na+ and K+ ions by the extended Hückel method

The potential curves for aquacomplexes of Li⁺, Na⁺ K⁺ ions with the coordination numbers, n, equal to 4, 6 and 8 have been calculated by the extended Hückel method. The equilibrium values of the hydrated shell radius and the binding energy have been determined. The complexes of Li⁺ with n = 6 and Na⁺ and K⁺ with n = 8 were found to be the most advantageous energetically. As could be expected the contribution of the 3d-orbitals to the binding for the K⁺ion is much more considerable than for the Na⁺ion. The character of the potential curves for aquacomplexes of sodium and potassium ions is quite different. In the case of the K⁺ion the curves are found to be very smooth and slowly decreasing with distance, which can be attributed to the poor hydratability of this ion and the “loosening” of water structure by it.     

Collision-Induced decomposition of peptides. An investigation into the effect of collision gas pressure on translational energy losses

The dependence on the gas pressure of translational energy losses, ΔE, suffered by incident ions in collision with helium target gas atoms was investigated for the [M + Na]⁺ ion of valinomycin. ΔE was measured from the fragment ion peak positions in the mass-analysed ion kinetic energy (MIKE) spectra. The variation of ΔE with target gas pressure was found to be different for different fragment ions. The implications of the results for the number of collisions which may occur under normal collision-induced decomposition conditions (30–40% transmission) are discussed.     

Radiochemical Study of Reactions of Diethylsilylium Ions with Dibutyl Ether in the Gas and Liquid Phases

Reactions of diethylsilylium ions with dibutyl ether in the gas and liquid phases were studied radiochemically. These reactions, as those with benzene and alcohols, are accompanied by partial rearrangement of diethylsilylium ions into monoethylsilylium and dimethylsilylium ions. The extent of transformations of the (C₂H₅)₂SiT⁺ ions decreases in going from benzene to dibutyl ether, which is due to the higher energy of adduct formation with the ether, compared to benzene.     

Mechanisms for negative reactant ion formation in an atmospheric pressure corona discharge

In an effort to better understand the formation of negative reactant ions in air produced by an atmospheric pressure corona discharge source, the neutral vapors generated by the corona were introduced in varying amounts into the ionization region of an ion mobility spectrometer/mass spectrometer containing a ⁶³Ni ionization source. With no discharge gas the predominant ions were O₂ ⁻, however, upon the introduction of low levels of discharge gas the NO₂ ⁻ ion quickly became the dominant species. As the amount of discharge gas increased the appearance of CO₃ ⁻ was observed followed by the appearance of NO₃ ⁻. At very high levels, NO₃ ⁻ species became effectively the only ion present and appeared as two peaks in the IMS spectrum, NO₃ ⁻ and the NO₃ ⁻·HNO₃ adduct, with separate mobilities. Since explosive compounds typically ionize in the presence of negative reactant ions, the ionization of an explosive, RDX, was examined in order to investigate the ionization properties with these three primary ions. It was found that RDX forms a strong adduct with both NO₂ ⁻ and NO₃ ⁻ with reduced mobility values of 1.49 and 1.44 cm²V⁻¹ s⁻¹, respectively. No adduct was observed for RDX with CO₃ ⁻ although this adduct has been observed with a corona discharge mass spectrometer. It is believed that this adduct, although formed, does not have a sufficiently long lifetime (greater than 10 ms) to be observed in an ion mobility spectrometer.