Comparative study of [Xe]+ and [Ar]+ bombardment in secondary ion mass spectrometry for bioorganic compounds

Secondary ion mass spectra obtained by [Xe]⁺ bombardment are compared with those obtained by [Ar]⁺ bombardment. Although [Ar]⁺ ions are commonly used as primary ions in secondary ion mass spectrometry for organic compounds, [Xe]⁺ ions seem better as primary ions because they give a larger sputtering yield for a metal substrate than [Ar]⁺ ions. Cationized molecular intensities of sucrose, raffinose and stachyose, and quasimolecular ion intensities of tuftsin and eledoisin related peptide are investigated using [Xe]⁺ and [Ar]⁺ bombardments. The observed molecular species are 2–4 times more intense for [Xe]⁺ bombardment than for [Ar]⁺ bombardment, although the secondary ion mass spectra are almost the same in both cases.     

Low-energy mass spectra of some aliphatic ketones

The charge exchange mass spectra of a selection of C₅-C₇ ketones have been measured using [CS₂]⁺˙, [COS]⁺˙ and [N₂O]_+. as reagent ions. The low energy charge exchange with [CS₂]⁺˙ or [COS]⁺˙ provides simple primary ion mass spectra, which readily permit structure elucidation in contrast to metastable ion spectra. In several cases, isomer distinction is easier from the charge exchange mass spectra than from the electron impact mass spectra. The energy transfer from [N₂O]⁺˙ is sufficiently high for complex spectra resembling electron impact mass spectra to be obtained.     

High resolution studies of the origins of polyatomic ions in inductively coupled plasma-mass spectrometry,Part I. Identification methods and effects of neutral gas density assumptions,extraction voltage,and cone material

Common polyatomic ions (ArO⁺, NO⁺, H₂O⁺, H₃O⁺, Ar₂⁺, ArN⁺, OH⁺, ArH⁺, O₂⁺) in inductively coupled plasma-mass spectrometry (ICP-MS) are identified using high mass resolution and studied using kinetic gas temperatures (T_gas) determined from a dissociation reaction approach. Methods for making accurate mass measurements, confirming ion identifications, and correcting for mass bias are discussed. The effects of sampler and skimmer cone composition and extraction voltage on polyatomic ion formation are also explored. Neutral species densities at several locations in the extraction interface are estimated and the corresponding effects of the T_gas value are calculated. The results provide information about the origins of background ions and indicate possible locations for their formation or removal.     

Fast atom bombardment mass spectra of telluronium salts

The fast atom bombardment (FAB) mass spectra of telluronium salts were studied. The spectra exhibit the intact cation (C⁺) and cluster ions ([M + C]⁺). The principal fragment ions in the FAB mass spectra of telluronium salts are [RTe]⁺, [R₂Te]⁺˙, [R₂Te − H]⁺, [RTeR′]⁺˙, and [RTeR′ + H]⁺. When the anion was [BPh₄]⁻, interesting cluster ions such as [M + C − BPh₃]⁺ appeared.     

Metastable ions of methane and their use in calibrating a low mass scale

The decomposition of metastable positive ions of methane formed by electron bombardment is discussed. Some new collision-induced decompositions involving the formation of H⁺, H₂⁺˙ and H₃⁺ have been found in the ion kinetic energy spectrum as well as in the mass spectrum of methane. In the latter case, the ‘metastable peaks’ occur below mass 1 on the mass scale and can be used for mass calibration in this region of the mass spectrum.     

Fast atom bombardment (FAB) mass spectrometry of piperidine N-oxide derivatives and free radical quenching under FAB conditions

Mechanisms are proposed for the formation of M⁺, [M + 2H]⁺ and [M + 3H]⁺ ions in the fast atom bombardment (FAB) mass spectra of 4-(2,2,6,6-tetramethyl-1-oxyl)-piperidol and its carboxylates. Free radical quenching induced by the fast atom beam has been observed. The effects of temperature on the radical quenching and of acid on the FAB mass spectra are discussed. The experiment showed that the volatile liquid samples with vapour pressures higher than that for glycerol produced M⁺ even-electron molecular ions, and the FAB mass spectra were similar to the corresponding electron ionization mass spectra. For the solid samples, it was found that the free radicals were quenched during the FAB process so that the mononitroxide and dinitroxide compounds produced [M + 2H]⁺ and [M + 3H]⁺ ions, respectively. Further experiments showed that the intensities and stabilities of [M + 2H]⁺ and [M + 3H]⁺ ions could be improved by addition of acids.     

Anomalies in the molecular ion region in the electron impact mass spectra of α- and β-hydroxyesters

In the electron impact mass spectra of some alkyl α- and β-hydroxyesters (introduced using the gas chromatography/mass spectrometry (GC/MS) technique), the absence of the molecular ion M^+· and the presence of the [M + 1]⁺ ion instead is observed. This phenomenon is especially characteristic of C₃? C₆ glycolates and diethyl malate, and is due to chemical auto-ionization—ion-molecule reactions in the high concentration gradient at the top of the GC peak. The existence of the [M ? 2]^+·, [M ?1]⁺ and M^+· ions in the mass spectra of other β- and α-hydroxyesters is discussed.     

Investigation of some pentasubstituted 1,4-dihydropyridines by means of fast ion bombardment,chemical ionization and electron impact mass spectrometry

Positive fast ion bombardment, positive chemical ionization (CI⁺) and positive electron impact (EI) ionization mass spectrometry were used to investigate a number of relatively large and structurally related organic molecules. Some of the major dissociation pathways observed in the CH₄-CI⁺ mass spectra are not present under NH₃-CI⁺ conditions, but are obtained in the collision-induced dissociation (CID) spectrum of the 50 eV MH⁺ molecular ion, formed in the latter reaction. The resemblance between the EI mass spectra and their fast ion bombardment counterparts, the effect of changing the energy of the bombarding Cs⁺ ion beam over the range 2–16 keV and the different degrees of internal excitation of ions formed in different CI reagent gases are discussed.     

Über quantitative Beziehungen Zwischen Fragmenten und Ihren Relativen Intensitäten im Massenspektrometer. III—Messung der Intensitätsverhältnisse aus der α-SWpaltung von Tertiären Alkjoholen und Äthern bei Niedriger Elektronenenergie

The ion intensity ratios from competing α-fissions of 30 tertiary aliphatic alcohols and 24 ethers of tertiary alcohols have been measured at 13 eV. The intensity ratios of ions [M ? alkyl1]+ and [M ? alkyl2]⁺ agree well with the reciprocal mass ratios of the respective ions in the case when the alkyl groups are not methyl (ion mass effect). The intensity ratios of [M ? alkyl]⁺ and [M ? methyl]⁺ are always too high, but intensity ratios of [M ? alkyl1]⁺ and [M ? alkyl2]⁺ may be derived indirectly from them, which also agree well with those values expected from the ion mass effect. By the indirect method it is shown, that for the 2,2-dialkyl-1,3-dioxolanes (ethylene ketals) the ion mass effect plays a dominant role too.     

Thermochemistry of HCOS+ and HCS2+

Heats of formation for HCOS⁺ and HCS₂⁺, obtained from high-quality ab initio calculations, have been used to determine the thermochemistry of several reactions which may form these ions in interstellar clouds or mass spectrometer experiments. Radiative association of HCO⁺ or HCS⁺ with either O or S may be important in interstellar clouds, whereas proton transfer from H₃⁺ or HCO⁺ to CS₂ or COS may be a means of synthesizing these ions by mass spectrometry. In the latter case, the relevant heats of formation are: HCS₂⁺(¹A₁) 1042 kJ mol^?1 and HCOS⁺ (¹A′) 996 kJ mol^?1.     

The behaviour of perchloro-3,4-dimethylenecyclobutene,-pentafulvene and- pentafulv-alene under electron-impact

The mass spectra of 1,2-dichloro-3,4-bis(dichloromethylene)cyclobutene (IV) and of hexachloropentafulvene (II) have been studied. Compound IV cannot be an intermediate in the formation of II from octachloro-1,2-dimethylenecyclobutane (III) under electron-impact, as previously suggested. In the mass spectra of II and IV the species [C₆]⁺ and [C₅]⁺ occur, obviously through cleavage of the semicyclic C-C bond. The mass spectrum of pentachlorofulvalene (VI) shows strikingly that successive elimination of an even number of CI atoms is preferred over that of an odd number of CI atoms; probably corresponding C-CI bonds in the two rings are broken simultaneously. Amongst the fragments, the species [C₁₀]⁺ and [C₇]⁺ and possibly also [C₈]⁺ and [C₉]⁺ have been observed.     

Evidence for the existence of phosphoazonium (iminophosphenium) and arsenoazonium ions in the gas phase by mass spectrometry

The first evidence for the existence of the methylphosphoazonium (methyliminophosphenium) ion CH₃NP⁺ and the methylarsenoazonium ion (named in this work) CH₃NAs⁺ (analogues of the methyldiazonium ion CH₃NN⁺) is provided using electron impact mass spectrometry The ions are formed as fragment ions of some highly air-sensitive phospholidinium and arsolidinium compounds that contain dicoordinate P and As centres. A combination of high-resolution mass measurements and high- and low-energy tandem mass spectra (fragment ion scans) suggest that the ions have N? C bonds rather than the isomeric azophosphonium (CH₃PN⁺) or azoarsenonium (CH₃AsN⁺) forms.     

Positive electron impact and chemical ionization mass spectra of some nitramine nitrates

The positive electron impact (EI) and isobutane chemical ionization (CI) mass spectra of six nitramine nitrates were studied with the aid of some accurate mass measurements. In the EI spectra, β fission relative to both the nitramine and nitrate ester is important. In the CI spectra a major ion occurs at [MH – 45]⁺ and was found to be mainly due to [M + 2H ? NO₂]⁺. All of the compounds except N-(2 hydroxyethyl)-N-(2′,4′,6′-trinitrophenyl)nitramine nitrate gave an [MH]⁺ ion. The [MH – 45]⁺ ion in the isobutane CI mass spectra of tetryl is also due to [M + 2H ? NO₂]⁺.     

Evaluation of nitric oxide chemical ionization mass spectra of substituted benzenes

Nitric oxide chemical ionization mass spectra of substituted benzenes obtained with the Townsend discharge technique were studied. There were four kinds of base peaks in the mass spectra, i.e. [M + NO]⁺˙, M⁺˙, [M ? H]⁺ and [M ? OR]⁺ (R = H, CH₃). The formation of the specific ion [M + NO]⁺˙ was highly dependent on the kind of substituent, and it was produced more abundantly in the case of substitutions involving electron-accepting groups. The measure of [M + NO]⁺˙ production was evaluated from the value of the ratio [M + NO]⁺˙/M⁺˙. In mono-substitutions, it was clarified that the ratios of [M + NO]⁺˙/M ⁺˙ were correlated with the Hammett substituent constant s?_p or the electrophilic substituent constant s?_p⁺. Monosubstitutions (C₆H₅R) and toluene substitutions (CH₃C₆H₄R) could be classified into six groups in terms of base peak species, [M + NO]⁺˙/M⁺˙ ratios and substituents. In disubstitutions, the mass spectral patterns were governed by the combination of substituents. Mass spectral distinctions among ortho, meta and para isomers could be made in many cases.     

Energy dependence of the fragmentation of some isomeric [C6H12]+˙ ions

The charge exchange mass spectra of 14 C₆H₁₂ isomers have been determined using [CS₂]^+˙, [COS]^+˙, [Xe]^+˙, [CO]^+˙, [N₂]^+˙ and [Ar]^+˙ as the major reactant ions covering the recombination energy range from ∼10.2 eV to ∼15.8 eV. From the charge exchange data breakdown graphs have been constructed expressing the energy dependence of the fragmentation of the isomeric [C₆H₁₂]^+˙ molecular ions. The electron impact mass spectra are discussed in relation to these breakdown graphs and approximate internal energy distribution functions derived from photoelectron spectra.     

Mass spectrometric determination of the configuration of 2-methyl-and 1,2-dimethyl-4-vinyl-ethinyl(n-butyl)-4-hydroxyperhydroquinolines

The configuration at C-2 and C-4 in the molecules of 2-methyl- and 1,2-dimethyl-4-vinylethinyl(n-butyl)-4-hydroxyperhydroquinolines was determined by mass spectrometry. The principal conclusions concerning the stereochemistry were made on the basis of differences in the values of the I_[M?15]⁺/I_[M]⁺·, I_[M?17]⁺/I_[M]⁺·, I[_M?43]⁺/I_[M]⁺· and I_[M?57]⁺/I_[M]⁺· ratios in the mass spectra of the epimeric vinylethinylic alcohols, and of the I_[M?15]⁺/I_[M]⁺· and I_[M?15]⁺/I_[M]⁺· ratios in the case of the n-butylic alcohols.     

The observation of broad metastable ion peaks in the surface-induced dissociation mass spectra of protonated and cationated peptides

The product ion mass spectra of protonated and cationated peptides of relative molecular mass (RMM) 555–574 Da have been obtained by surface-induced dissociation of MH⁺ and [M + Cat]⁻ ions in a four-sector tandem mass spectrometer equipped with a specially designed collision cell. A linked scan of the electric and magnetic sector field strengths of the second mass spectrometer was used to transmit the fragment ions arising from collisions with a stainless steel surface. The resulting mass spectra contained broad metastable ion peaks produced by the dissociation of MH⁺ and [M + Cat]⁺ ions before the second magnetic sector, in the fourth field-free region of the instrument.     

Isomeric molecular identification by UV laser photofragmentation

An apparatus is described which provides an alternative to collisional activation for distinguishing some selected isomeric molecular ions. A mass filter selects the molecular ion beam, which is crossed by the output of an excimer laser. Photofragmentation products are monitored by a magnetic sector mass spectrometer downstream of the interaction zone. Five criteria can be used to distinguish between isomeric molecular ions: (1) the photofragment mass spectra; (2) the normalized photogfragment intensities; (3) the kinetic energy spectra of the observed photofragments; (4) the dependence of criteria (1)–(3) on laser polarization; and (5) the dependence of criteria (1)–(4) on the laser wavelength. The scheme is applied to the isomeric pair HCO⁺−COH⁺. At 193 nm, decomposition into the products CO⁺ +H is barely endothermic for HCO⁺ but ca. 1.7 eV exothermic for COH⁺. The normalized intensity of CO⁺ from photofragmentation of a presumed COH⁺ target is found to be nine times that from an HCO⁺ target.     

A high resolution mass spectrometric measurement system for a vacuum fusion apparatus

A high resolution mass spectrometric read-out system for a vacuum fusion apparatus is described. Rapid analyses are possible and the rate at which various gases are evolved can be monitored. Gas mixture assays are accomplished by scanning the nominal mass 28 peak. Serial measurements of CO⁺, N₂⁺ and C₂H₄⁺ are made. Thus detection and measurement of evolved hydrocarbons are possible. The results for silver and tin from a conventional vacuum fusion apparatus and the mass spectrometer apparatus are compared.