Intact Cation Research Articles

Direct Analysis in Real Time Mass Spectrometry (DART-MS) of Ionic Liquids

Direct analysis in real time mass spectrometry (DART-MS) was used to analyze ionic liquids (ILs) containing either imidazolium or phosphonium cations combined with different types of inorganic and organic anions. Ionic liquids were directly inserted into the ionization source using a glass probe without dissolution into organic solvents. Mass spectra of the ILs were collected in both positive and negative mode with a linear ion-trap instrument. The intact cation of the compound was typically the dominant peak in positive mass spectra and cluster ion formation was present. Some individual anions were not readily observed in the negative mass spectra (based on the type of anion); however, the mass difference of adjacent cluster ions equal the mass of a complete IL and the anion mass could be verified by subtracting the known cation mass. The degree and intensity of the cluster ion formations was found to be dependent on the nature of the specific ILs as well as the DART temperature gas stream.

Imaging of a Tribolayer Formed from Ionic Liquids by Laser Desorption/Ionization-Reflectron Time-of-Flight Mass Spectrometry

For the first time, imaging using laser desorption/ionization (LDI) reflectron time-of-flight (RTOF) mass spectrometry (MS) was demonstrated to be a powerful tool for an offline monitoring of tribometrical experiments directly from disc specimen applying selected ammonium-, phosphonium-, and sulfonium-based ionic liquids (IL) with bis(trifluoromethylsulfonyl)imide as counterion for lubrication. The direct measurement of IL tribolayers by LDI-MS allowed the visualization of the lubricants in the form of the distribution of their intact cations and the anion in and outside the wear scar after the tribometrical experiment with a low degree of in-source generated fragmentation. Besides, also, an oxidation product formed during a tribometrical experiment was detected and located exclusively in the wear track. Comparative data of identical wear tracks were obtained by X-ray photoelectron spectroscopy (XPS) imaging not only enabling the determination of elemental distributions of the IL across the area imaged but also corroborating the mass spectrometry imaging (MSI) data, thus generating multimodal images. Merging data from MSI and XPS imaging exhibited that areas, where iron-fluorine bonds were detected in the wear track, are corresponding to data from LDI-MS imaging showing absence of IL cations and anions.

Simultaneous ion-pair photodissociation and dissociative ionization of an ionic liquid: velocity map imaging of vacuum-ultraviolet-excited 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

Isolated gas-phase ionic liquid (IL) molecules, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim+][Tf2N−]), are excited by laser-produced high-harmonic extreme ultraviolet light in the 17−26 eV range and the ions are probed by velocity map imaging. The velocities of the intact cation, Emim+, and intact anion, Tf2N−, are recorded in separate velocity map images. The kinetic energy distribution of the intact cation has both slow ions arising from previously reported dissociative ionization and fast ions from ion-pair dissociation of the neutral isolated ionic liquid (positive and negative ion pairs) where the branching ratio is approximately 0.18 ± 0.02:0.82 ± 0.02. The intact anion is also detected with a momentum that complements the higher velocity intact cation fragment, indicating that the process arises from two-body dissociation of the neutral ion-pair. Similar angular distributions for the anion and the higher velocity cation indicate that the electronic transition, which is above the ionization threshold and leads to ion-pair dissociation of the neutral, has a dipole moment perpendicular to the dissociating ion-pair bond. Observation of the ion-pair dissociation upon photoexcitation of the IL vapor not only reveals a previously unobserved photodissociation pathway for the ionic liquid, but it also provides direct evidence for the existence of the ion pairs in the isolated gas phase molecules.

Soft Ionization of Thermally Evaporated Hypergolic Ionic Liquid Aerosols

Isolated ion pairs of a conventional ionic liquid, 1-Ethyl-3-Methyl-Imidazolium Bis(trifluoromethylsulfonyl)imide ([Emim(+)][Tf(2)N(-)]), and a reactive hypergolic ionic liquid, 1-Butyl-3-Methyl-Imidazolium Dicyanamide ([Bmim(+)][Dca(-)]), are generated by vaporizing ionic liquid submicrometer aerosol particles for the first time; the vaporized species are investigated by dissociative ionization with tunable vacuum ultraviolet (VUV) light, exhibiting clear intact cations, Emim(+) and Bmim(+), presumably originating from intact ion pairs. Mass spectra of ion pair vapor from an effusive source of the hypergolic ionic liquid show substantial reactive decomposition due to the internal energy of the molecules emanating from the source. Photoionization efficiency curves in the near threshold ionization region of isolated ion pairs of [Emim(+)][Tf(2)N(-)] ionic liquid vapor are compared for an aerosol source and an effusive source, revealing changes in the appearance energy due to the amount of internal energy in the ion pairs. The aerosol source has a shift to higher threshold energy (∼0.3 eV), attributed to reduced internal energy of the isolated ion pairs. The method of ionic liquid submicrometer aerosol particle vaporization, for reactive ionic liquids such as hypergolic species, is a convenient, thermally "cooler" source of isolated intact ion pairs in the gas phase compared to effusive sources.

Tunable Wavelength Soft Photoionization of Ionic Liquid Vapors

Combined data of photoelectron spectra and photoionization efficiency curves in the near threshold ionization region of isolated ion pairs from [emim][Tf(2)N], [emim][Pf(2)N], and [dmpim][Tf(2)N] ionic liquid vapors reveal small shifts in the ionization energies of ion-pair systems due to cation and anion substitutions. Shifts toward higher binding energy following anion substitution are attributed to increased electronegativity of the anion itself, whereas shifts toward lower binding energies following cation substitution are attributed to an increase in the cation-anion distance that causes a lower Coulombic binding potential. The predominant ionization mechanism in the near threshold photon energy region is identified as dissociative ionization, involving the dissociation of the ion pair and the production of intact cations as the positively charged products.

In Situ SIMS Analysis and Reactions of Surfaces Prepared by Soft Landing of Mass-Selected Cations and Anions Using an Ion Trap Mass Spectrometer

Mass-selected polyatomic cations and anions, produced by electrosonic spray ionization (ESSI), were deposited onto polycrystalline Au or fluorinated self-assembled monolayer (FSAM) surfaces by soft landing (SL), using a rectilinear ion trap (RIT) mass spectrometer. Protonated and deprotonated molecules, as well as intact cations and anions generated from such molecules as peptides, inorganic catalysts, and fluorescent dyes, were soft-landed onto the surfaces. Analysis of the modified surfaces was performed in situ by Cs(+) secondary ion mass spectrometry (SIMS) using the same RIT mass analyzer to characterize the sputtered ions as that used to mass select the primary ions for SL. Soft-landing times as short as 30 s provided surfaces that yielded good quality SIMS spectra. Chemical reactions of the surfaces modified by SL were generated in an attached reaction chamber into which the surface was transferred under vacuum. For example, a surface on which protonated triethanolamine had been soft landed was silylated using vapor-phase chlorotrimethylsilane before being returned still under vacuum to the preparation chamber where SIMS analysis revealed the silyloxy functionalization. SL and vapor-phase reactions are complementary methods of surface modification and in situ surface analysis by SIMS is a simple way to characterize the products produced by either technique.

Mass spectrometric analysis of imidazolium-based ionic liquids by scanning atom probe

Scanning atom probe was used for studying several imidazolium-based ionic liquids of the form [R][Tf 2N], where [R] can be one of the three cations—[emim], [bmim] and [hmim]. High voltage pulsing caused the formation of the product ions via cations and anion decomposition. Laser pulsing (photon-assisted field evaporation) allowed generating the intact cation ions along with cluster ions composed of cations and fragments of anion.

Liquid Injection Field Desorption/Ionization-Mass Spectrometry of Ionic Liquids

Ten ionic liquids based on four types of organic cations, C(+) (imidazolium, pyrrolidinium, pyridinium, and phosphonium), combined with various types of anions, A-, were analyzed by liquid injection field desorption/ionization- (LIFDI) mass spectrometry. For the purpose of LIFDI analysis the ionic liquids were dissolved in methanol, acetonitrile or tetrahydrofuran at concentrations of 0.01-0.1 microl mL(-1). The measurements were performed on a double-focusing magnetic sector instrument. In all ionic liquid LIFDI spectra, the intact cation of the compound yielded the base peak accompanied by cluster ions of the general formula [C(2)A](+) and occasionally [C(3)A(2)](+). Tandem mass spectrometry and reconstructed ion chromatograms were employed to reveal the identity of the observed ions. Although limited to positive-ion mode, LIFDI also provided analytical information on the anions due to cluster ion formation. Depending on actual emitter condition and ionic liquid the limit of detection in survey scans was determined to 5-50 pg of ionic liquid.

S-SIMS and MetA-SIMS study of organic additives in thin polymer coatings

In the present study a methodology for TOF-S-SIMS measurements is developed to gain information on the distribution of molecules on and in polymer coatings (thickness ∼100 μm). Experiments were carried out on model systems consisting of one or more additive-containing polyvinylbutyral coatings. Several organic additives were selected: carbocyanine dyes, basonyl blue and the pharmaceutical risperidone. The additives have been measured as pure compounds on a Si substrate to obtain good reference spectra. After optimisation of the sample preparation method, the coatings were embedded in epoxy resin and stored in an oven (60 °C) for 24 h. Cross-sections were made by means of a microtome. S-SIMS spectra were taken on the prepared cross-sections before and after Au was deposited on the sample surface. Compared to the untreated samples, the Au covered samples give rise to more intense secondary ion signals. Generally, signals of the intact cations were more intense than those of the fragment ions. Apart from mass spectra, images of the additive distribution in the coatings could also be acquired by recording structural ion signals. It was possible to make secondary ion images of the additive molecule ions with a (sub)-micrometer lateral resolution.

Quantitative determination of alkylated quaternary amines and their n‐hydroxylated metabolites in an enzyme incubation matrix by liquid chromatography electrospray ionization mass spectrometry

A simple, rapid and sensitive reversed-phase liquid chromatography method coupled to electrospray ionization mass spectrometry has been developed for studying the in vitro metabolism of the long-chain quaternary ammonium compounds dodecyltrimethylamine, tetradecyltrimethylamine and hexadecyltrimethylamine. Samples were prepared from the biological matrix by a simple protein precipitation stage. The separation was performed using a BDS Hypersil C8 3 microm particle size (100 x 3 mm i.d.) column with a fast gradient separation (60% B to 100% B) using a mobile phase of 10 mm aqueous ammonium acetate (pH 4.0, with 0.06% triethylamine; (A)-acetonitrile (B) at 0.7 mL min(-1). To minimize contamination of the MS source a switching value was used to divert the solvent front to waste. Decylammonium bromide was used as the internal standard and analytes were identified and quantified by positive ion electrospray selected ion monitoring of their intact molecular cations. The assay had a limit of quantitation of 0.25 microm (6.25 pmol on column) and was linear over the range 0.25--100 microm assay concentration for this series of long-chain quaternary amines. The precision of intra- and inter-day assays was better than 19% and the accuracy was between 93 and 109%. The method was used to assess the in vitro metabolism of the quaternary amines by wild-type cytochrome P450 enzyme CYP 4 A 1 and mutants in an artifical membrane system.

Wavelength-dependent photofragmentation and photoionization of gaseous (eta4-cycloocta-1,5-diene)(eta5-cyclopentadienyl)cobalt.

Gas-phase photoreactions and photoproducts of the mixed-ligand compound (eta(4)-cycloocta-1,5-diene)(eta(5)-cyclopentadienyl)cobalt are reported. Significant amounts of the monoligated complexes CoCOD and CoCp are produced, and the relative amounts are wavelength dependent. The COD ligand (with the weakest metal-ligand bonds) is always preferentially labilized as expected, but the relative amounts of the CoCOD and CoCp fragments change by 1 order of magnitude as the excitation wavelength is changed. The gas-phase photoreactions exhibit other surprising features that are uncommon in the photoreactions of organometallic compounds in the gas phase. Significant amounts of the intact cation are formed, in contrast to most reported reactions where fragmentation of the weak metal-ligand bonds precedes ionization. Most surprisingly, fragmentation of the ligands occurs while the ligands are still coordinated. The resulting metal complexes contain ligand fragments that remain coordinated to the metal. Breaking several carbon-carbon bonds with retention of weaker metal-ligand bonds is unexpected. For example, C(5)H(5) undergoes fragmentation while still coordinated to the cobalt, yielding smaller compounds such as Co(CH)(+), Co(C(2)H(2))(+), Co(C(3)H(3))(+), and Co(C(4)H(6))(+). Correspondingly, coordinated COD yields Co(C(6)H(6))(+), Co(C(5)H(5))(+), Co(C(4)H(6))(+), Co(C(3)H(3))(+), Co(C(2)H(2))(+), and Co(CH)(+). The wavelength dependence of the ligand labilization is examined by utilizing mass-selected resonance enhanced multiphoton ionization spectroscopy. Both broad bands and sharp lines are observed in the mass-selected excitation spectra. The action spectra obtained in the gas phase while monitoring the cobalt ion follow the absorption onset found in solution. The changes in fragmentation pathways are interpreted in terms of the initially accessed excited state.

Dicopper(II) complexes of a phenolate-hinged ditopic ligand with solvent-derived ancillary ligands bound at the potentially dimetallic exogenous site

Copper complexes based on the [Cu 2(bpbp)] 3+ core (bpbp −=2,6-bis(( N,N′-bis-(2-picolyl)amino)methyl)-4-tertbutylphenolate) containing methoxide or water ancillary ligands, [Cu 2(bpbp)(OCH 3)](ClO 4) 2 ( 1), [Cu 2(bpbp)(OH 2) 2](ClO 4) 3 ( 2) and Cu 2(bpbpH)(OH 2) 4(ClO 4) 4 ( 3), were isolated from methanol–water solutions above pH 3, between pH 2–3, and below pH 2, respectively. ESR and magnetochemistry reveal that the copper ions are antiferromagnetically ( J=−156 cm −1) and weakly ferromagnetically ( J=+5.3 cm −1) coupled in the doubly-bridged 1 and its singly-bridged pseudo acid congener 2, respectively. They are uncoupled in 3, consistent with each copper atom being bound only by an amine and two pyridine donors of bpbpH and oxygen donors of water and/or perchlorate. Thus the phenol residue of the dinucleating ligand is protonated and uncoordinated. Further, the ESR spectra show that complexes 1, 2 and 3 retain their integrity in solution. ESI mass spectra demonstrate that, as expected, the cations are labile in solution. However, the patterns that emerge support the formulations; numerous ions containing the [Cu 2(bpbp)] 3+ core plus water, hydroxide and methoxide ligands are observed. The intensities of the more protonated species increase in the spectra going from complex 1 to 3. The only complex to give a molecular ion for the intact cation was 1, and only from methanol solutions. However, the dominant ion observed in the mass spectra of 1 is [Cu 2(bpbp)(OH)] 2+ from all solvents tried. Thus in solution, the methoxide complex, 1, is in equilibrium with the isoelectronic hydroxide complex, [Cu 2(bpbp)(OH)](ClO 4) 2, with the latter favored. Complex 1 was used to provide a labile preformed core for bridge replacement reactions, and the acetate and pyrazolate complexes, [Cu 2(bpbp)(CH 3CO 2)](ClO 4) 2 ( 4) and [Cu 2(bpbp)(C 3H 3N 2)](ClO 4) 2 ( 5), were prepared in this manner.

Ruthenocenyl ruthenium bimetallic complexes: electrospray mass spectrometric study of [RuX( η5-C 5H 5)( η2-dppr)] n+ (dppr=1,1′-bis(diphenylphosphino)ruthenocene) (X=Cl, n=0; X=CO, CH 3CN, CCHPh, n=1) and the X-ray crystal and molecular structure of [Ru( η5-C 5H 5)(CO)( η2-dppr)]PF 6

[RuCl(Cp)(PPh 3) 2] (Cp= η 5-C 5H 5) reacts with dppr in refluxing benzene to give [RuCl(Cp)(dppr)], 1, {dppr=[Ru( η 5-C 5H 4PPh 2) 2]} in 91% yield. Complex 1 ionizes in boiling acetonitrile in the presence of excess NH 4PF 6 to give [Ru(Cp)(CH 3CN)(dppr)]PF 6, 2, in 76% yield. Under CO at 60°C, 1 converts to [Ru(Cp)(CO)(dppr)]Cl, 3a, whose derivative [Ru(Cp)(CO)(dppr)]PF 6, 3b, can also be obtained from 2 in 86% with CO. With HCCPh, 2 instantaneously gives a vinylidene complex, [Ru(CCHPh)(Cp)(dppr)]PF 6, 4, quantitatively (98%). The kinetic stability of the η 2-coordinated dppr ring is evident in these reactions. The X-ray molecular structure of 3b [space group P2 1/ c, a=9.990(2), b=19.498(4), c=19.113(4) Å and β=96.21(3)°] reveals a pseudo-octahedral Ru(II) structure with a η 5-Cp, a chelated dppr, a terminal CO and an uncoordinated PF 6 − anion. It is the first piano-stool dppr structure characterized by X-ray single-crystal diffractometry. The dppr chelate has a large bite (100.5(1)°) and there is no direct interaction between the two Ru(II) centers (Ru(1)⋯Ru(2) 4.389 Å). The electrospray mass spectra (ESMS) of 2–4 generally give peaks due to the intact cations at low cone voltages. As the cone voltage increases, fragmentation commences which inevitably gives [Ru(Cp)(dppr)] + as the primary fragment ion. In-situ doping of dppr with AgNO 3 gives [Ag(dppr)] + as the major species plus [Ag(dppr) 2] + ( m/z 1307) and other oxidized by-products. Similar treatment of 4 gives an acetylide complex [Ag{Ru(Cp)(CCPh)(dppr)} 2] + ( m/z 1843) at 20 V which ejects one Ru metalloligand to give [Ag{Ru(Cp)(CCPh)(dppr)}] + ( m/z 975) at higher voltages. Complex 4 is hydrogenated with H 2 gas to give ethylbenzene in 55% yield after 4 h in refluxing THF. It also catalyzes the hydrogenation of HCCPh to give 52% of ethylbenzene in 5 h at 5 mol.% catalyst level.

Charge-remote fragmentation in dialkylaminostyryl dyes

Dialkylaminostyryl probes with two long alkyl chains were used to investigate the energetics of charge-remote fragmentation (CRF). These compounds provide high-quality liquid secondary ion (LSI) and electrospray ionization (ESI) mass spectra in which the intact cation predominates. The intact cation undergoes CRF in LSI but not in ESI, in which the cation is of lower internal energy. CRF is initiated in collision-induced dissociation (CID) and surface-induced dissociation (SID) for intact cations generated by either LSI or ESI. Single bond cleavage to lose one entire alkyl chain followed by CRF within the remaining alkyl chain seems to be the lowest energy CID and SID process. © 1998 John Wiley & Sons, Ltd.

Probing the energetics of charge-remote fragmentation in carbocyanine dyes using collision- and surface-induced dissociation mass spectrometry

Carbocyanine dyes used as molecular probes are symmetric ring compounds with long alkyl chains on each ring proximate to a resonance-stabilized charge site. They provide high-quality positive-ion liquid second (LSI) and electrospray ionization mass spectra in which the intact cation is the dominant ion in the mass spectrum. The intact cation of a carbocyanine dye undergoes charge-remote fragmentation (CRF), evident in the LSI mass spectra. CRF is also observed in collision-induced dissociation and surface-induced dissociation of the mass-selected intact cation. Differential CRF processes from the two chains in the molecule are observed with increasing internal energy deposition.

POSITIVE ION LIQUID SECONDARY ION MASS SPECTRA OF AROMATIC SULFONIUM SALTS

Organic sulfonium salts exist as pit-charged species in the matrices normally used for liquid secondary ionization mass spectrometry. Positive ion LSIMS mass spectra of the sulfonium salts therefore exhibit the intact cation (C+) as the dominant ion species. Generally, the fragment ions are of 30–40% relative intensity and are formed through simple cleavages and rearrangements. Cluster ions such as CAC+ and C3A2 + are also present in the mass spectra (A is the counter ion of the salt). These cluster ions provided confirmation of the molecular weight. Adduct ions are formed between organic sulfonium salts and matrices used in LSIMS such as diethanolamine and triethanolamine. Beam-induced reactions are identified by comparison of mass spectra obtained with electrospray ionization, and by changes in ion products with changes in solvent matrix.

Comparisons between electron impact and fast atom bombardment mass spectra of some triphenylphosphonium and triphenylarsonium salts

The electron impact (EI) mass spectra of alkyltriphenylphosphonium and alkyltriphenylarsonium salts were compared with fast atom bombardment (FAB) mass spectra in a 3-nitrobenzyl alcohol matrix. It is clearly demonstrated that FAB is an effective ionization method compared with EI for analysis of these non-volatile and thermally labile compounds, particularly triphenylarsonium salts. In FABMS, the spectra generally show the intact cations (C+) as the base peaks, in addition to the singly charged cluster ions (C2A)+. High-resolution mass spectrometry was used to determine the elemental composition of the ions at m/z 277 for some phosphonium salts in the EI mode and the results show that the formation of (O(double bond)PPh3 − 1)+ (m/z 277) involves an oxygen rearrangement as a possible reaction pathway. B/E linked scans of the intact cations of triphenylarsonium salts generated by FABMS were studied in detail both to illustrate the dissociation pathways and to compare the reaction mechanism with that occurring for triphenylphosphonium salts.

Investigation of cationic organic dyes used as molecular probes by liquid secondary ion mass spectrometry

AbstractThe positive‐ion mass spectra of twelve organic dyes used as molecular probes were measured using liquid secondary ion mass spectrometry (LSIMS). Nine of the twelve dyes were singly charged cations and the other three were doubly charged cations. The mass spectra of each of the dyes in m‐nitrobenzyl alcohol contain abundant signals for the intact cation, C+ (singly charged cation dyes), or for singly‐charged forms of the doubly charged cation formed by proton loss, [C2+ H+]+, or halogen counter ion attachment, [C2+ + X−]+. Fragmentation is usually minimal under the conditions used. However, the cations of five of the singly charged compounds appear to undergo charge‐remote fragmentation. Collision‐induced dissociation experiments on a hybrid mass spectrometer of EBqQ geometry at collision energies up to 300 eV failed to access this fragmentation pathway. In contrast to the LSIMS of many other doubly charged organic compounds, two of the dicationic dyes produced a doubly charged ion of reasonable abundance (2–20%) in the mass spectrum. When glycerol was used as a matrix solvent, the addition of the matrix modifier trifluoroacetic acid increased the abundance of C2+.

Probing onium ions and carbocations by desorption/ionization mass spectrometry: Crossing the threshold between fragile salts and gaseous intact cations and subsequent guest–host and cation–molecule cluster chemistry

AbstractThe utility of desorption/ionization mass spectrometric (DI/MS) techniques in dealing with physical organic problems involving fragile onium salts and carbocation salts has been explored. The efficacy of these methods, especially when coupled to tandem mass spectrometry, goes much beyond the analytical aspects dealing with identity/purity determinations. For example, insight can be gained regarding the role of the counterion in subsequent cation decomposition pathways. Guest–host chemistry of onium ions and various hosts (crowns, calixarenes) can be probed. The decomposition chemistry of the resulting ion–molecule clusters can be studied. Examples of onium salts and carbocation salts that have been studied are few, probably because research in this area has been mainly a domain for analytical chemists. This paper calls attention to the possibilities for dealing with topical physical organic problems.

Selective Injection and Isolation of Ions in Quadrupole Ion Trap Mass Spectrometry Using Notched Waveforms Created Using the Inverse Fourier Transform

Broad-band excitation of ions is accomplished in the quadrupole ion trap mass spectrometer using notched waveforms created by the SWIFT (stored waveform inverse Fourier transform) technique. A series of notched SWIFT pulses are applied during the period of ion injection from an external Cs[sup +] source to resonantly eject all ions whose resonance frequencies fall within the frequency range of the pulse while injecting only those analyte ions whose resonance frequencies fall within the limits of the notch. This allows selective injection and accumulation of the ions of interest and continuous ejection of the unwanted ions. This is shown to result in significant improvement in S/N ratio, resolution, and sensitivity for the analyte ions of interest. Selective ion injection is demonstrated by injecting the protonated molecules of peptides VSV and gramicidin S and the intact cation of l-carnitine hydrochloride, using singly notched SWIFT pulses. Multiply notched SWIFT pulses are used to simultaneously inject ions of different m/z values of l-carnitine hydrochloride into the ion trap. A new coarse/fine ion isolation procedure, which employs a doubly notched SWIFT pulse, is demonstrated for isolating ions of a single m/z value of 4-bromobiphenyl from a population of trapped ions. 36 refs., 10 figs.,more » 2 tabs.« less