Modification of Reflectron Time-of-Flight Mass Spectrometer for Photodissociation of Mass-Selected Cluster lons

We introduce a modification of reflectron time-of-flight mass spectrometer for laser photodissociation of mass-selected ions. In our apparatus, the ions of interests were selected by a mass gate near the first space focus point and decelerated right after the mass gate, were then crossed by a laser beam for dissociation. The daughter ions and surviving parent ions were re-accelerated and analyzed by the reflectron time-of-flight mass spectrometer. Compared to the designs reported by other research groups, our selection-deceleration-dissociation-reacceleration approach has better daughter-parent-ions-separation, easier laser timing, and better overlapping between the ion beam and laser beam. We also conducted detailed cal- culations on the parent ion and daughter ion flight times, and provided a simplified formula for the calibration of daughter ion mass.     

A collinear tandem time-of-flight mass spectrometer for infrared photodissociation spectroscopy of mass-selected ions

An apparatus based on collinear tandem time-of-flight mass spectrometer has been designed for the measurement of infrared photodissociation spectroscopy of mass-selected ions in the gas phase.The ions from a pulsed laser vaporization supersonic ion source are skimmed and mass separated by a Wiley-McLaren time-of-flight mass spectrometer.The ion of interest is mass selected,decelerated and dissociated by a tunable IR laser.The fragment and parent ions are reaccelerated and mass analyzed by the second time-of-flight mass spectrometer.A simple new assembly integrated with mass gate,deceleration and reacceleration ion optics was designed,which allows us to measure the infrared spectra of mass selected ions with high sensitivity and easy timing synchronization.     

Multiphoton Ionization of Monosilane

The (3 + 1) photon ionization spectra of SiH_4 have been measured. A basically continuous spectrum was recorded in laser wavelengths from 428 to 458 nm.Two vibrational progressions were found in the UV laser region of 347-392 nm, anda fine structure of the band at 363.5 nm was assigned.The fragment ions were detected by a time-offlight mass spectrometer. No parent ion SiH_4~+ was observed whereas SiH_2~+ and SiH_3~+ were the major products. The ratio of SiH_2~+/SiH_3~+ was approximately equal to that of single photon ionization at the same energy, but the fraction of Si~+ was increased. Some additional bands appeared in Si~+ spectrum in the range of 384-390 nm implicating that the increased Si~+ might be generated from SiH_2~+ and SiH~+ ions.     

Characterization of heavy petroleum fraction by positive-ion electrospray ionization FT-ICR mass spectrometry and collision induced dissociation: Bond dissociation behavior and aromatic ring architecture of basic nitrogen compounds

This paper examined the bond dissociation behavior and aromatic ring architecture of basic nitrogen compounds in Sudan heavy petroleum fraction. Both broadband and quadrupole isolation modes positive-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with collision induced dissociation (CID) techniques were used to characterize a low sulfur crude oil derived vacuum residuum (VR). The appropriate CID operating condition was selected by comparing the molecular weight distributions of the basic nitrogen compounds under various CID operating conditions. Both oddand even-electron fragment ions were observed from the mass spectrum, indicating that the heterolytic and homolytic bond cleavages occurred simultaneously during the CID process. The odd-electron fragment ions were predominant in each class species, indicating preferential heterolytic bond cleavages. At the optimal CID condition, the alkyl groups decomposed deeply and just left the aromatic cores of the nitrogen compounds. No significant variation in double bond equivalent (DBE) value was observed between the fragment and parent ions, revealing that the domination of single core structure.     

激光溅射金属等离子体与醇类分子束反应的研究：产物离子对分子束状态的依赖性

The gas phase reactions of metal plasma with alcohol clusters were studied by time of flight mass spectrometry （TOFMS） using laser ablation-molecular beam （LAMB） method. The significant dependence of the product cluster ions on the molecular beam conditions was observed. When the plasma acted on the low density parts of the pulsed molecular beam, the metal-alcohol complexes M^＋An （M=Cu, Al, Mg, Ni and A=C2H5OH, CH3OH） were the dominant products, and the sizes of product ion clusters were smaller. While the plasma acted on the high density part of the beam, however, the main products turned to be protonated alcohol clusters H^＋An and, as the reactions of plasma with methanol were concerned, the protonated water-methanol complexes H3O^＋（CH3OH）n with a larger size （n≤12 for ethanol and n≤24 for methanol）. Similarly, as the pressure of the carrier helium gas was varied from 1 × 10^5 to 5 × 10^5 Pa, the main products were changed from M^＋An to H^＋An and the sizes of the clusters also increased. The changes in the product clusters were attributed to the different formation mechanism of the output ions, that is, the M^＋An ions came from the reaction of metal ion with alcohol clusters, while H^＋An mainly from collisional reaction of electron with alcohol clusters.     

甾体磷酰胺类缀合物在电喷雾质谱中的裂解特征

Novel steroidal phosphoramidate conjugates of 3'-azido-2',3'-dideoxythymidine(AZT)and amino acid esterswere synthesized and determined by positive and negative ion electrospray ionization mass spectrometry.The MSfragmentation behaviors of the steroidal phosphoramidate conjugates have been investigated in conjunction withtandem mass spectrometry of ESI-MS/MS.There were three characteristic fragment ions in the positive ion ESImass spectra,which were the Na adduct ions with loss of steroidal moiety,amino acid ester moiety from pseudomolecular ion(M Na)~ ,and the phosphoamino acid methyl ester Na adduct ion by α-cleavage of the phosphora-midate respectively.The main fragment ions in negative ion ESI mass spectra were the ion(M-HN_3)~-,the ion(M-AZT-H)~-,and the ion(M-steroidal moiety-H)~- besides the pseudo molecular ion(M-H)~-.Thefragmentation patterns did not depend on the attached amino acid ester moiety.     

MS／MS Study of Deoxydinucleotides Bound with Alkali—metal Ions Using ESI—MS

Positive ion ESI-MS has been used to examine the fragmentation pathways of the complex ions of deoxydinucleotides with H^ ,Na^ ,K^ by LCQ instrument.It had been found that the dissociation Varied markedly due to the differences of the base sequence.The alkali-metal ion binding site and the charcaterization of dissociation were directed by the size of metal ion,the sequesce of base and the steric hindrance.     

Study on the atmospheric photochemical reaction of CF_3 radicals using ultraviolet photoelectron and photoionization mass spectrometer

A study of the atmospheric photochemical reaction of CF3 radical with CO and O2 was performed by using a homemade ultraviolet photoelectron spectrometer-photoionization mass spectrometer (PES- PIMS). The electronic structures and mechanism of ionization and dissociation of CF3OC(O)OOC(O)- OCF3 were investigated. It was indicated that the two bands on the photoelectron spectrum of CF3OC(O)OOC(O)OCF3 are the result of ionization of an electron from a lone pair of oxygen and a fluo- rine lone pair of CF3 group. The outermost electrons reside in the oxygen lone pair. The experimental and theoretical first vertical ionization energy is 13.21 and 13.178 eV, respectively, with the PES and OVGF method. They are in good agreement. The photo ionization and dissociation processes were discussed with the help of theoretical calculations and PES-PIMS experiment. After ionization, the parent ions prefer the dissociation of the C-O bond and giving the fragments CF3OCO and CF3 . It demonstrated that the ultraviolet photoelectron and photoionization mass spectrometer could be ap- plied widely in the study of atmospheric photochemical reaction.     

Characterizing some gossypol and gossypolone Schiff's bases by studying their fragmentation patterns with electrospray ionization tandem mass spectra

To investigate the structural form of gossypol and gossypolone Schiff's bases,seven relevant Schiff's bases were synthesized and the electrospray ionization-tandem mass spectrometry(ESI-MS/MS) with low-energy collision-induced dissociation was used to analyze their fragmentations.A common fragmentation pathway with the loss of RNH_2 from those schiff's bases quasi-molecular ions was observed and proposed on the basis of their MS/MS spectra data.This common pathway indicated that those Schiff's bases exis...     

Thermodynamics of mixture of boric acid with lithium borate and chloride

The first thermodynamic dissociation constants of boric acid in aqueous solution of lithium chloride with borate at five different temperatures from 278.15 to 318.15 K were evaluated from emf values of the cell without liquid junction potential with the improved extrapolation and polynomial approximation on the basis of Pitzer's theory. Values obtained from both methods are in good agreement with each other. Pitzer's parameters β(0) and β(1) of LiB(OH)4 and other thermodynamic quantities of dissociation process of boric acid are obtained. Results have been discussed.     

Fragmentation dynamics of methane by few-cycle femtosecond laser pulses

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.     

Improving IRMPD in a quadrupole ion trap

A focused laser is used to make infrared multiphoton photodissociation (IRMPD) more efficient in a quadrupole ion trap mass spectrometer. Efficient (up to 100%) dissociation at the standard operating pressure of 1 × 10⁻³ Torr can be achieved without any supplemental ion activation and with shorter irradiation times. The axial amplitudes of trapped ion clouds are measured using laser tomography. Laser flux on the ion cloud is increased six times by focusing the laser so that the beam waist approximates the ion cloud size. Unmodified peptide ions from 200 Da to 3 kDa are completely dissociated in 2.5–10 ms at a bath gas pressure of 3.3 × 10⁻⁴ Torr and in 3–25 ms at 1.0 × 10⁻³ Torr. Sequential dissociation of product ions is increased by focusing the laser and by operating at an increased bath gas pressure to minimize the size of the ion cloud.     

Selective parent ion axialization for improved efficiency of collision-induced dissociation in laser desorption-ionization fourier transform ion cyclotron resonance mass spectrometry

We have systematically established the excitation frequency, amplitude, duration, and buffer gas pressure for optimal axialization efficiency and mass selectivity of quadrupolar excitation-collisional cooling for isolation of parent ions for collision-induced dissociation in Fourier transform ion cyclotron resonance mass spectrometry. For example, at high quadrupolar excitation amplitude, ion axialization efficiency and selectivity are optimal when the applied quadrupolar excitation frequency is lower than the unperturbed ion cyclotron frequency by up to several hundred hertz. Moreover, at high buffer gas pressure (10^?6 Torr), quadrupolar excitation duration can be quite short because of efficient collisional cooling of the cyclotron motion produced by magnetron-to-cyclotron conversion. Efficiency, detected signal magnitude, and mass resolving power for collision-induced dissociation (CID) product ions are significantly enhanced by prior parent ion axialization. With this method, we use argon CID to show that C ₉₄ ⁺ (m/z 1128) formed by Nd:YAG laser desorption-ionization behaves as a closed-cage structure.     

Nucleophilic Substitution in Dipolar Aprotic Solvents: Hexamethylphosphoric triamide

The rate of the reaction between ethyl tosylate and chloride as well as bromide ion has been measured in hexamethylphosphoric triamide in the presence of several counterions. Rate constants for the free ions at 25.0° are 14.4 mole^?1 for chloride and 2.0 mole^?1min^?1 for bromide, both independent of the cation used. The equilibrium constants for ion pair dissociation of lithium-, ammonium- and tetra-n-butylammonium chloride are 3.97 × 10^?2, 1.52 × 10^?3 and 6.36 × 10^?3 mole, and for the corresponding bromides 3.30 × 10^?2, 9.65 × 10^?3, and 9.62 × 10^?3 mole.     

Channel-Resolved Ultrafast Dissociation Dynamics of NO2 Molecules Studied via Femtosecond Time-Resolved Ion Imaging

The ultrafast dissociation dynamics of NO₂ molecules was investigated by femtosecond laser pump-probe mass spectra and ion images. The results show that the kinetic energy release of NO⁺ ions has two components, 0.05 eV and 0.25 eV, and the possible dissociation channels have been assigned. The channel resolved transient measurement of NO⁺ provides a method to disentangle the contribution of ultrafast dissociation pathways, and the transient curvesof NO⁺ ions at different kinetic energy release are fitted by a biexponential function. The fast component with a decay time of 0.25 ps is generated from the evolution of Rydberg states. The slow component is generated from two competitive channels, one of the channel is absorbing one 400 nm photon to the excited state A²B2, which has a decay time of 30.0 ps, and the other slow channel is absorbing three 400 nm photons to valence type Rydberg states which have a decay time less than 7.2 ps. The channel and time resolved experiment present the potential of sorting out the complex ultrafast dissociation dynamics of molecules.     

Medium energy ion irradiation of Ge surface – search for a better understanding of the surface nano‐patterning

We report the morphological changes on Ge surfaces upon 50 keV Ar⁺ and 100 keV Kr⁺ beam irradiation at 60° angle of incidence. The Ge surfaces having three different amorphous–crystalline (a/c) interfaces achieved by the pre‐irradiation of 50 keV Ar⁺ beam at 0°, 30° and 60° with a constant fluence of 5 × 10¹⁶ ions/cm² were further processed by the same beam at higher fluences viz. 3 × 10¹⁷, 5 × 10¹⁷, 7 × 10¹⁷ and 9 × 10¹⁷ ions/cm² to understand the mechanism of nano‐scale surface patterning. The Kr⁺ beam irradiation was carried out only on three fresh Ge surfaces with ion fluences of 3 × 10¹⁷, 5 × 10¹⁷ and 9 × 10¹⁷ ions/cm² to compare the influence of projectile mass on surface patterning. Irrespective of the depth of a/c interface, the nanoscale surface patterning was completely missing on Ge surface with Ar⁺ beam irradiation. However, the surface patterning was evidenced upon Kr⁺ beam irradiation with similar ion fluences. The wavelength and the amplitude of the ripples were found to increase with increasing ion fluence. In the paper, the mass redistribution at a/c interface, the incompressible solid flow through amorphous layer, the angular distribution of sputtering/backscattering yields and the generation of non‐uniform stress across the amorphous layer are discussed, particularly in analogy with low energy experiments, to get better understanding of the mechanism of nanoscale surface patterning by the ion beams. Copyright © 2016 John Wiley & Sons, Ltd.     

Determination of chloride ion in bismuth oxide by capillary electrophoresis

A procedure is developed for determining chloride ion in bismuth(III) oxide by capillary electrophoresis with indirect photometric detection after precipitating the major component of the sample as bismuthyl acetate. The mixture containing chromate ion and diethanolamine is used as a separating electrolyte. The detection limit for chloride ions in the developed procedure is 5 × 10^?4%, the relative standard deviation is 15%. The absence of a systematic error is confirmed by atomic absorption and laser mass spectrometry.     

Fragmentation pathways of 2,3‐dimethyl‐2,3‐dinitrobutane cations in the gas phase

2,3‐Dimethyl‐2,3‐dinitrobutane (DMNB) is an explosive taggant added to plastic explosives during manufacture making them more susceptible to vapour‐phase detection systems. In this study, the formation and detection of gas‐phase [M+H]⁺, [M+Li]⁺, [M+NH₄]⁺ and [M+Na]⁺ adducts of DMNB was achieved using electrospray ionisation on a triple quadrupole mass spectrometer. The [M+H]⁺ ion abundance was found to have a strong dependence on ion source temperature, decreasing markedly at source temperatures above 50°C. In contrast, the [M+Na]⁺ ion demonstrated increasing ion abundance at source temperatures up to 105°C. The relative susceptibility of DMNB adduct ions toward dissociation was investigated by collision‐induced dissociation. Probable structures of product ions and mechanisms for unimolecular dissociation have been inferred based on fragmentation patterns from tandem mass (MS/MS) spectra of source‐formed ions of normal and isotopically labelled DMNB, and quantum chemical calculations. Both thermal and collisional activation studies suggest that the [M+Na]⁺ adduct ions are significantly more stable toward dissociation than their protonated analogues and, as a consequence, the former provide attractive targets for detection by contemporary rapid screening methods such as desorption electrospray ionisation mass spectrometry. Copyright © 2009 Commonwealth of Australia. Published by John Wiley & Sons, Ltd.     

Dissociation dynamics of halotoluene ions,production of tolyl,benzyl and tropylium ([C7H7]+)ions

The dissociation rates and energetics of the loss of halogen atoms from energy-selected halotoluene ions were investigated by photoelectron photoion coincidence (PEPICO) and collisional activation (CA) mass spectrometric experiments. Dissociation onsets, determined from the dissociation rates measured as a function of the internal energy of the parent ion, revealed the formation of three [C₇H₇]⁺ isomers, which were identified, on the basis of the CA data, as the tolyl, benzyl and tropylium ions. All of the ions investigated produced a mixture of isomeric ions. Only iodotoluene ions produced any tolyl product ions by a direct bond cleavage. The bromo- and chlorotoiuene ions produced mixtures of benzyl and tropyl ions. The observed two-component decay rates of the iodotoluene ions revealed the participation of a lower energy [C₇H₇I]^{+ ˙} isomer in the dissociation process. The identity of this isomer is not known but it probably does not have the cycloheptatriene ion structure because considerable kinetic energy was released in this dissociation.     

Detection of femtomole and sub-femtomole levels of peptides by tandem magnetic sector/reflectron time-of-flight mass spectrometry and matrix-assisted laser desorption ionization

This article describes results of low-level (sub-femtomole) detection of peptides by matrix-assisted laser desorption ionization. The matrix-assisted laser desorption ionization method can be used for low-level detection of the parent ion, either [M + H]⁺ or [M + Na]⁺, and collision-induced dissociation of the parent ion can be performed at the picomole level. The instrument used for these studies is a novel high-performance magnetic sector (electric(E)/magnetic(B) sector)/reflectron time-of-flight (TOP) tandem mass spectrometer (EB/TOF).