Kinetic study of proton-bound dimer formation using ion mobility spectrometry

A method to measure the rate constant for the formation of symmetrical proton-bound dimers at ambient pressure was proposed. The sample is continuously delivered to the drift region of an ion mobility spectrometer where it reacts with a swarm of monomer ions injected by the shutter grid. Dimer ions are formed in the drift tube and a tail appears in the ion mobility spectrum. The rate constant is derived from the mobility spectra. The proposed approach was typically examined for methyl isobutyl ketone (MIBK), 2,4-dimethyl pyridine (DMP), and dimethyl methyl phosphonate (DMMP). The rate constants measured in this study were: 0.25 × 10⁻⁹, 0.86 × 10⁻¹⁰, and 0.47 × 10⁻¹⁰ cm³ s⁻¹ for MIBK, DMP and DMMP, respectively. The logarithm of the measured rate constants were found to be almost independent of reciprocal temperature within 303 to 343 K, indicating that no activation energy is involved in the formation of proton-bound dimers.     

Characterization of proton-bound acetate dimers in ion mobility spectrometry

Ionized acetates were used as model compounds to describe gas-phase behavior of oxygen containing compounds with respect to their formation of dimers in ion mobility spectrometry (IMS). The ions were created using corona discharge at atmospheric pressure and separated in a drift tube before analysis of the ions by mass spectrometry. At the ambient operational temperature and pressure used in our instrument, all acetates studied formed dimers. Using a homolog series of n-alkyl-acetates, we found that the collision cross section of a dimer was smaller than that of a monomer with the same reduced mass. Our experiments also showed that the reduced mobility of acetate dimers with different functional groups increased in the order n-alkyl 相似文献   

New structural insights into mechanically interlocked polymers revealed by ion mobility mass spectrometry

Mechanically interlocked polymers can possess significant additional physical properties, in comparison to those associated with their constituent parts. Their unique properties make them attractive for a range of potential applications, such as as biomaterials and molecular machines. Their efficient and reproducible synthesis is therefore of much interest. Both their synthesis and subsequent characterization are intriguing yet demanding. The properties of mechanically interlocked polymeric systems depend not only on the properties of their individual components but also on the topology of the subsequent product. Here traveling wave ion mobility mass spectrometry has been used to investigate the structural properties of a polyrotaxane system. Ion mobility studies reveal that this system remains linear in form with increase in size. Both ion mobility studies and tandem mass spectrometry studies indicate that the macrocycle preferentially remains associated with the ammonium moiety of the polymeric repeat unit and is impeded from moving freely along the axle. This is consistent with NMR observations of the average structure. Analysis of mechanically interlocked polymers by ion mobility mass spectrometry provides additional structural insights into these systems relating to dynamics, heterogeneity, and topology. This molecule-specific information is vital in order to understand the origin of a system's functional properties.     

Electrospray mass spectrometry: Ethylene glycol as a solvent and its effects on ion desorption

Ethylene glycol solutions of gramicidin S, myoglobin and tetrabutylammonium bromide were analysed by means of electrospray mass spectrometry and their spectra were compared with those of aqueous solutions. The evaporation of water and ethylene glycol droplets, initially at room or elevated temperature, in air at room temperature was modelled. It was found that under conditions where a water droplet's radius would shrink by ～30%, an ethylene glycol droplet shrinks negligibly. Further, droplets that are initially hot (such as those that are ejected from a heated electrospray needle) cool very rapidly owing to evaporation and heat loss to ambient air, and subsequently evaporate much like droplets that are initially at room temperature. For gramicidin S, the ion abundances in ethylene glycol as solvent were ～200 times lower than those in water under room temperature operating conditions. In experiments where the spray probe was heated to ～100°C to reduce the viscosity of ethylene glycol, the gramicidin response difference between the solvents decreased to about a factor of 40. Similar trends were observed for myoglobin and the tetrabutylammonium ion. The gramicidin abundances in ethylene glycol, relative to those in water, are orders of magnitude too large to be accounted for using the conventional solvent evaporation model. It is speculated that decreasing the viscosity increases the velocity of ions drifting in ethylene glycol towards the solution/air interface and increases the total number of analyte ions desorbed at the Taylor cone during electrospray.     

Collision induced dissociation of protonated N-nitrosodimethylamine by ion trap mass spectrometry: Ultimate carcinogens in gas phase

Collision induced dissociation of protonated N-nitrosodimethylamine (NDMA) and isotopically labeled N-nitrosodimethyl-d6-amine (NDMA-d6) was investigated by sequential ion trap mass spectrometry to establish mechanisms of gas phase reactions leading to intriguing products of this potent carcinogen. The fragmentation of (NDMA + H⁺) occurs via two dissociation pathways. In the alkylation pathway, homolytic cleavage of the N–O bond of N-dimethyl, N′-hydroxydiazenium ion generates N-dimethyldiazenium distonic ion which reacts further by a CH₃ radical loss to form methanediazonium ion. Both methanediazonium ion and its precursor are involved in ion/molecule reactions. Methanediazonium ion showed to be capable of methylating water and methanol molecules in the gas phase of the ion trap and N-dimethyldiazenium distonic ion showed to abstract a hydrogen atom from a solvent molecule. In the denitrosation pathway, a tautomerization of N-dimethyl, N′-hydroxydiazenium ion to N-nitrosodimethylammonium intermediate ion results in radical cleavage of the N–N bond of the intermediate ion to form N-dimethylaminium radical cation which reacts further through α-cleavage to generate N-methylmethylenimmonium ion. Although the reactions of NDMA in the gas phase are different to those for enzymatic conversion of NDMA in biological systems, each activation method generates the same products. We will show that collision induced dissociation of N-nitrosodiethylamine (NDEA) and N-nitrosodipropylamine (NDPA) is also a feasible approach to gain information on formation, stability, and reactivity of alkylating agents originating from NDEA and NDPA. Investigating such biologically relevant, but highly reactive intermediates in the condensed phase is hampered by the short life-times of these transient species.     

Fragmentation pathways of heroin-related alkaloids revealed by ion trap and quadrupole time-of-flight tandem mass spectrometry

The electrospray ionization (ESI) ion trap and quadrupole time-of-flight (QqToF) mass spectra of heroin and seven related alkaloids, i.e., morphine, codeine, O-6-monoacetylmorphine (6-MAM), thebaine, acetylcodeine, papaverine and narcotine, have been extensively investigated in this work. The ESI mass spectrometric fragmentation pathways of protonated 6-MAM, heroin, acetylcodeine, and thebaine were comprehensively elucidated for the first time with the aid of high-resolution mass spectrometry. It was found that cleavage of the piperidine ring was the featured fragmentation route of six of the compounds, although not of papaverine and narcotine. In addition, a simple high-performance liquid chromatography (HPLC)-based separation method gave baseline resolution of all eight components. This study could play an important role in the screening for these alkaloids in different matrices by HPLC coupled to tandem mass spectrometry (MS/MS).     

A non-covalent dimer formed in electrospray ionisation mass spectrometry behaving as a precursor for fragmentations

A non-covalent-bonded dimer was detected in the positive ion electrospray ionisation (ESI) mass spectra of a synthetic impurity. In tandem mass spectrometry (MS/MS) experiments using collision-induced dissociation (CID), the ion was found to behave as a [M+H]+-type precursor ion for fragmentation until MS5. The dimer was probably formed through multi-hydrogen bonds over a proton bridge. When the fragmentation occurred at the center of the bridge, the dimer was broken apart to give monomer fragments at MS6. However, no corresponding deprotonated dimer [2M-H]- was found in the negative ion ESI spectra. The dimer was extremely stable, and it could still be observed when a fragmentation voltage of up to 50 V was applied in the ionisation source. The formation of the non-covalent dimer was also found to be instrument-dependent, but independent of sample concentration. Accurate mass measurements of the [2M+H]+ and [M+H]+ ions, and their MSn product ions, provided the basis for assessing the fragmentation mechanism proposed for [2M+H]+. The fragmentation pathway was also illustrated for the deprotonated molecule [M-H]-.     

Use of a kinetic energy orifice as a probe of metastable dissociation in Fourier transform ion cyclotron resonance mass spectrometry

Although Fourier transform ion cyclotron resonance mass spectrometry is a powerful tool in the qualitative observation of gas phase reactions, ion detection is on the millisecond time scale, orders of magnitude longer than typically found when using a sector instrument. Observations of short-lived species such as chemically activated adduct ions can be accomplished using selective ion excitation as a probe of intermediate lifetime. Whereas ion elimination has been shown to be effective in monitoring ion lifetimes on the microsecond time scale, problems associated with detecting ions produced with high kinetic energies limits the technique. Use of a kinetic energy orifice as an ion skimmer effectively eliminates ions near the center of the ion cell at relatively low kinetic energies. By modifying a single section cell to include a kinetic energy orifice, the lifetimes of chemically activated adduct ions have been investigated.     

Drug binding revealed by tandem mass spectrometry of a protein-micelle complex

The protein-micelle complex formed between the protein EmrE and the lipid dodecylmaltoside has been examined by mass spectrometry. The results show that despite the unfavorable hydrophobic environment in the mass spectrometer it is possible to preserve protein submicelle complexes in the gas phase. The peaks assigned to the submicelle complexes are broad in nature and consistent with a heterogeneous distribution of lipid molecules attached to the protein complex. As such, the spectrum cannot be interpreted. To simplify this complexity we used a tandem mass spectrometry procedure in which discrete m/z values are isolated from the peak and subjected to collision-induced dissociation. These spectra reveal clusters of DDM molecules as well as sequential release of TPP+ and EmrE from the complex as the collision cell voltage is raised. Taken together, the results provide direct evidence for drug binding within a relevant gas-phase protein-micelle complex.     

Identification of butyryl derivatives of cyclic nucleotides by positive ion fast atom bombardment mass spectrometry and mass-analysed ion kinetic energy spectrometry

The syntheses of the dibutyryl derivatives of the 3′,5′-cyclic monophosphates of adenosine, guanosine and cytidine are described. The fast atom bombardment mass spectra of these compounds are discussed, together with the mass-analysed ion kinetic energy spectra of their protonated molecular ions and of diagnostic fragments. A protocol for the identification of the derivatives is reported which includes criteria for confirming retention of the cyclic phosphodiester moiety, substitution of both heterocyclic base and ribose ring, and butyrylation of the 2′-O-position. The origins of significant fragments in the spectra are discussed.     

Detection of C-nitroso dimer formation by field desorption mass spectrometry

Field desorption mass spectrometry is shown to be a useful technique for observing dimer formation in C-nitroso compounds. Mixed dimer formation (RN₂O₂R′) is shown to be readily observed by field desorption mass spectrometry.     

Polyelectrolytes as new matrices for secondary ion mass spectrometry

Significant enhancements in ion yields in time-of-flight secondary ion mass spectrometry (TOF-SIMS) are observed when water-soluble analytes are mixed with a polyelectrolyte, e.g., poly(diallyldimethylammonium chloride) or poly(sodium 4-styrenesulfonate), and then deposited in the layer-by-layer method on a surface. This previously unobserved effect is demonstrated for 5-chloro-8-methoxyquinoline appended diaza-18-crown-6, 5-(2-aminoethoxy)methyl-5-chloro-8-methoxyquinoline appended diaza-18-crown-6, acridine, 9-anthracenecarboxylic acid, and ferrocenecarboxylic acid. By optical ellipsometry film thicknesses range from ca. 5-20 angstroms. X-ray photoelectron spectroscopy shows significantly less analyte in the polyelectrolyte-analyte films than in the neat analytes. However, TOF-SIMS generally shows significant enhancements in ion yields from the polyelectrolyte films compared with either the neat compounds or the compounds solubilized with acid or base and then dried on a surface. These significant enhancements in ion yields also appear to extend to analyte fragments and cationized molecular species. Some enhancement is also observed for dried droplets of analytes mixed with a polyelectrolyte on surfaces.     

Stereochemical effects in mass spectrometry XI. Negative ion fast atom bombardment mass spectrometry of saccharides and glycosides using phenylboronic acid as reagent

The negative ion fast atom bombardment (NIFAB) mass spectra of mono-, di-saccharides and glycosides using phenylboronic acid (PBA) as reagent have been studied. In the ion source, PBA reacts stereospecifically with the molecules containing cis-vicinal glycols to form characteristic ions, from which the stereo-isomers of saccharides can be definitely distinguished. Disaccharides and glycosides with β-glycosidic linkage seem to be unfavorabale to react with PBA, therefore, by comparison of the abundances of the characteristic ions, the configuration of the glycosidic linkage in these compounds may be inferred.     

Diffusion of beryllium in copper as determined by ion and spark probe mass spectrometry

Diffusion profiles of Be in Cu, which extend over some millimeters, are studied laterally by both SSMS and SIMS. The diffusion of Be in Cu is investigated in the temperature range from 650 to 900 (± 5)°C for annealing times from 313 to 502 h. The Arrhenius equation is obeyed in the chosen temperature range. The diffusion coefficients obtained from both experimental approaches are in reasonable agreement.     

Depth profile measurement by secondary ion mass spectrometry

The possibilities of measuring depth profiles by secondary ion mass spectrometry are evaluated. The influence of different instrumental and experimental parameters on depth resolution in the profiles are studied: the effects of primary ion beam characteristics, reactive gas adsorption and mechanical aperturing in secondary ion extraction are discussed. Beam effects are studied from the point of view of surface damage. The effects of secondary processes, such as crater edge effects, element mixing, preferential sputtering, background signals, (residual) gas contamination and ion-induced topographical and compositional changes are studied for thin metal and binary materials.     

Tandem mass spectrometry of peptides. Relationship between translational energy loss and fragment ion mass

Incident ion energy E₁ and collision gas pressure have been adjusted so as to obtain a satisfactory tandem mass spectrum of the m/z 1882 ion formed from valme-gramicidin A (relative molecular mass 1881). Translational energy losses ΔE have been determined (at E_i = 14.8 keV) from the precise positions of a large number of fragment ion peaks in the spectrum. The variation in ΔE for different fragment ions is great and the magnitudes of ΔE are large (being commonly of the order of 10² eV). It is shown how very large energy losses ΔE can arise, if the parent ion decomposes to a small fragment ion which itself collides and decomposes further. Implications of the dependence of ΔE upon fragment ion mass for the scan laws of four-sector instruments are discussed briefly.     

High-pressure ion trap mass spectrometry

The effects of buffer gas pressure on ion trap stability, mass resolution/calibration, and choice of mass scanning are described. Pressure effects were treated phenomenologically by adding a drag term to the ion equations of motion. The resulting collisional damping enlarges the mass-dependent stability region but reduces the region in which mass-selective resonance ejection can be performed. The pressure effects can be reduced by increasing the frequency of the alternating quadrupole field.