Photodissociation of alkane ions in the source of a mass spectrometer: A potential analytical technique

Alkane ions generated by 70 eV electron ionization are irradiated in the source of a reversed geometry, double focusing mass spectrometer by 2.41 eV (514.5 nm) photons from an argon ion laser. The ions in the chamber are extracted, accelerated to 6 keV and spectra obtained by scanning the field of the magnet. The laser light is mechanically chopped and changes in the peak heights of the mass spectra resulting from photodissociation of ions in the ion chamber are recorded using a phase-sensitive detector to give what are referred to as photoinduced spectra. The peaks for the molecular ions of alkanes are large relative to other peaks in the photoinduced spectra, a situation which contrasts with that for the mass spectra. The relative photodissociation cross-sections of several alkanes, C₆? C₁₀, C₁₂ and C₁₆, have been measured and those of linear and branched [C₁₀H₂₂]⁺˙ ions are compared. Some of the characteristic features of the photoinduced spectra make the identification of the sample simpler than from mass spectra. The method is applied to the analysis of some alkane mixtures and its potential in analytical work discussed.     

Fast,very fast,and ultra-fast gas chromatography-mass spectrometry of thermally labile steroids,carbamates, and drugs in supersonic molecular beams

Gas chromatography-mass spectrometry (GC-MS) analyses of thermally labile compounds have been studied by using a short column fast gas chromatograph, coupled with fly-through electron ionization in supersonic molecular beams. Thirty-two compounds, which include steroids, carbamate pesticides, antibiotic drugs, and other pharmaceutical compounds, have been analyzed and the details of their GC-MS analysis are provided. The ability to analyze thermally labile compounds is discussed in relation to the speed of analysis. A new term, “speed enhancement factor” (SEF), is defined as the product of column length reduction and the carrier gas linear velocity increase, as compared with normal GC-MS conditions. Fast, very fast, and ultra-fast GC-MS are defined with a SEF in the ranges of 5–30, 30–400, and 400–4000, respectively. Trade-offs in the degree of dissociation, speed, gas chromatograph resolution, and sensitivity were studied and examined with thermally labile molecules. The experimental factors that affect the dissociation are described with emphasis on its reduction. We claim that the use of supersonic molecular beams for sampling and ionization provides the ultimate capability in the GC-MS of thermally labile compounds. The obtained 70-eV electron ionization mass spectra are shown, and an enhanced relative abundance of the molecular ion is demonstrated together with library search capability of these mass spectra, which is better than that reported with particle beam liquid chromatography-mass spectrometry. The performance of fast GC-MS in supersonic molecular beams is compared with other methods of fast GC-MS and with particle beam liquid chromatography-mass spectrometry.     

Mass spectral behavior of n-alkynes

The 70 e V-electron impact mass spectra of the C₇–C₁₀ n-alkynes have been determined as well as the metastable ion spectra of the molecular ions and the [CS₂]⁺ and [N₂O]⁺ charge exchange mass spectra of the C₇-C₉ n-alkynes. The metastable ion mass spectra provide only a limited opportunity to distinguish between isomers; however, the 70-eV EI mass spectra of isomeric compounds permit a ready distinction between isomers. The [CS₂]⁺ charge exchange mass spectra of isomeric compounds also show substantial differences. The [N₂O]⁺ charge exchange mass spectra do not show the enhancement of β-fission fragments observed in field ionization experiments, despite representing ions of similar internal energy, and it is concluded that field dissociation is responsible for the β-fission fragments in the field ionization experiments.     

Mass spectral fragmentation pattern of 4,4′-bipyridines. Part II. 4,4′-oxybispyridine and 4,4′-thiobispyridine

The mass spectra of 4,4′-oxybispyridine and 4,4′-thiobispyridine are reported. In the former the base peak is due to the molecular ion and the fragmentation routes involve loss of H, CO, HCN, C₂H₂N and C_sHO from the molecular ion as well as rupture of the central bonds. In the latter the base peak is also due to the molecular ion and the fragmentation routes involve loss of H, CS, S, HCN and C₂HS as well as central bond rupture.     

Positive and negative ion mass spectra of Aryl-ONN-azoxy compounds containing electron withdrawing groups

The positive and negative ion mass spectra, at 70 eV, of p-RC₆H₄N(O)?NCOOCH₃ (R?H, Cl, Br, NO₂), C₆H₅N(O)?NCOOC₂H₅, p-RC₆H₄N(O)?NCONH₂ (R?H, Cl, Br, NO₂) and p-RC₆H₄N(O)?NCOC₆H₅ (R?H, Cl, Br, NO₂) are reported. The azoxyester derivatives show abundant molecular ions and a number of weak fragment and rearrangement ions in the positive ion mass spectra, whereas weak molecular ions and abundant low mass fragment ions are present in the negative ion mass spectra. Similar behaviour is observed in the mass spectra of the azoxyamides. Conversely, for the azoxycarbonyl compounds the positive molecular ion is absent. A ready cleavage of the N? CO bond occurs and only few fragments of low diagnostic value are formed, whereas the negative molecular ion is the base peak for all these compounds with the exception of the p-NO₂ derivative, where [M? O]^?? is the base peak and [M]^?? is the second major ion. The behaviour under electron impact of these classes of compounds is compared with that of azoxycyanides reported previously.     

Mass spectra of new heterocycles: XII. Main fragmentation pathways of the molecular ions of 5-methylsulfanyl-1-vinyl-1H-pyrrol-2-amines under electron impact and chemical ionization

Fragmentation patterns of 5-methylsulfanyl-1-vinyl-1H-pyrrol-2-amines under electron impact (70 eV) and chemical ionization (methane as reactant gas) were studied for the first time. The electron impact mass spectra of all the examined compounds contained a strong peak of molecular ion which decomposed along four pathways. Two pathways involved cleavage of the C-S bonds with elimination of methyl (major) and MeS radicals (minor), and the two others, decomposition of the pyrrole ring. The chemical ionization mass spectra displayed strong molecular, [M + H]⁺, and odd-electron [M + H ? SMe]⁺ ion peaks. N,N-Dimethyl-5-methylsulfanyl-4-phenyl-1-vinyl-1H-pyrrol-2-amine under chemical ionization with methane as reactant gas characteristically decomposed with formation of [M ? C₄H₉N]⁺ as the only fragment ion.     

Charge transfer reactions in alkane and cycloalkane systems. Estimated ionization potentials

Rate constants of change transfer reactions k_CT, involving C₃? C₉ alkanes and cycloalkanes, have been determined in an ion cyclotron resonance mass spectrometer. The rate constants are significantly lower than the corresponding rate constants for collision when the reaction is less than about 0.5 eV exothermic for linear alkane ions, or less than about 0.2 eV exothermic for cycloalkane ions. In this region of low reaction efficiency, the efficiency of reaction with linear or branched alkanes seems to depend primarily on reaction exothermicity. (The efficiencies of reaction of a given ion with cyclic alkanes also depend on ΔH_rn, but are higher than for reactions with other compounds). Although the lowered reaction efficiencies probably result, at least in part, from unfavorable Franck–Condon factors in the energy range near the ionization onset, quantitative correlations between reaction efficiency and estimated relative Franck–Condon factors were not observed. When the enthalpy of reaction is small (less than about ?0.15 eV), it is seen that the reverse charge transfer can also occur, and equilibrium is established under the conditions of these experiments. From the observed equilibrium constants, values for the standard free energy change are derived, and assuming that ΔS is small for electron transfer equilibria, values of ΔH_rn are estimated. In the case of the equilibria involving cyclohexane ion, these values of ΔH_rn lead to estimates of the ionization potentials of methylcyclopentane, 3-methylpentane, n-octane, 2,2-dimethylbutane, and 2,3-dimethylbutane, which are lower than the ionization potentials of cyclohexane, that is, <9.88 eV, although all these compounds had previously been reported to have ionization potentials above 10.03 eV. That the ionization potentials are indeed lower than 10.03 eV is confirmed by determining the quantum yields of ionization with 10.03-eV photons. It is pointed out that the conclusions reached here apparently also apply to the charge transfer reactions of alkane ions in the liquid phase.     

A study of the electron impact induced retro-diels-alder process with selected 5,6,6a, 7,12,12a-hexahydrobenzo[a]anthracenes

The mass spectra of 5,6,6a,7,12,12a-hexahydrobenzo[a]anthracene and 2-methoxy, 3-methoxy-, 4-methoxy and 1-methyl-4-methoxy derivatives are reported. Among the fragment ions observed under electron impact ionization, [C₈H₈]^+· and [M? C₈H₈]^+· can be generated by a retro-Diels-Alder process. Studies of metastable ion reactions show these ions to be formed by fragmentation directly from the molecular ion. The CA spectra of the [C₈H₈]^+· ions from the subject compounds were compared with spectra from ions of the same composition from various sources. From these data, kinetic energy release measurements and stereochemical considerations, it is concluded that these ions are formed by a stepwise, rather than a concerted mechanism.     

Mass spectral fragmentation pattern of 2,2′-bipyridyls. Part XII. 2,2′-Selenodipyridine

The mass spectrum of 2,2′-selenodipyridine obtained by electron impact is reported. The base peak in the spectrum is due to the C₅H₄N⁺ ion formed principally by rupture of the central bonds. The molecular ion gives rise to a peak of 50% of the intensity of the base peak. Other fragmentations include loss of H, Se and CSe from the molecular ion and HCN from the M-1 ion.     

Doubly charged ion mass spectra: IV—chlorinated and brominated alkanes

Doubly charged ion mass spectra have been obtained for 42 chlorinated and brominated n-alkane (methyl through octyl) hydrocarbons. A double focusing Hitachi RMU-7L mass spectrometer, operated at 1.6kV accelerating voltage, has been used to measure the spectra. Molecular doubly charged ions have not been observed. Intense fragment ions have been produced from extensive H and halogen loss as well as C? C bond rupture of the parent molecule. The most abundant ions in the doubly charged ion spectra observed in this investigation resulted from reactions of [Cl]²⁺˙, [Br]²⁺˙, [CCL₂]²⁺, [C₂H₂Cl]²⁺˙, [C₃H₂]²⁺, [C₃HCl]²⁺, [C₃HBr]²⁺, [C₄H₃]²⁺˙, [C₄H₄]²⁺, [C₄H₆Br]²⁺˙, [C₄H₈Br]²⁺˙, [C₅H₂]²⁺, [C₆H₆]²⁺, [C₆H₈]²⁺ and [C₇H₈]²⁺. The prominent doubly charged fragment ions formed by electron impact of the smaller halogenated alkanes generally contained halogen, whereas ions of the type [C_nH_x]²⁺ were dominant in the spectra of higher molecular weight mono- and dihalogenated alkanes. Appearance energies of several ions have been measured. A geometry optimized quantum mechanical SCF treatment has been used to compute energies, charge densities and structures of doubly charged halogenated alkane ions.     

Negative ion mass spectra of dicarboxylic acids

The negative ion mass spectra of dicarboxylic acids show [M]^?˙ and prominent [M – H]^?ions. These ions can therefore be used to determine the molecular weight of dicarboxylic acids which do not give positive molecular ions. The [C₂H₃]^? ion is a base peak in the spectra of maleic and fumaric acids. Isomeric phthalic acids are readily differentiated.     

Metastable ion study of organosilicon compounds. Part III—trimethoxyphenylsilane

The unimolecular decomposition of trimethoxyphenylsilane (1) was investigated by mass-analysed ion kinetic energy (MIKE) spectrometry, deuterium-labelling studies and from high resolution data. The characteristic fragmentations of metastable molecular ion of 1 were losses of C₆H₆ and C₇H₇^· with rearrangements. Almost complete H/D scrambling occurred in the molecular ion prior to these decompositions. The other important fragmentation routes corresponded to expulsions of CH₃O^· and C₆H₅^·. These fragmentations were followed by consecutive elimination of an H₂CO molecule, as commonly observed in the mass spectra of alkoxysilanes. In these fragmentation processes, H/D scrambling increased as the internal energy of the molecular ion was lowered. The fragmentations of 1 were compared with those of its carbon analogue, α,α,α-trimethoxytoluene.     

Ambient analysis of saturated hydrocarbons using discharge-induced oxidation in desorption electrospray ionization

Saturated nonfunctionalized hydrocarbons can be oxidized in situ by initiating an electrical discharge during desorption electrospray ionization (DESI) to generate the corresponding alchohols and ketones. This form of reactive DESI experiment can be utilized as an in situ derivatization method for rapid and direct analysis of alkanes at atmospheric pressure without sample preparation. Betaine aldehyde was incorporated into the DESI spray solution to improve the sensitivity of detecting the long-chain alcohol oxidation products. The limit of detection for alkanes (C₁₅H₃₂ to C₃₀H₆₂) from pure samples is ∼20 ng. Multiple oxidations and dehydrogenations occurred during the DESI discharge, but no hydrocarbon fragmentation was observed, even for highly branched squalane. Using exact mass measurements, the technique was successfully implemented for analysis of petroleum distillates containing saturated hydrocarbons.     

10.5-eV photoionization mass spectrometry of aliphatic compounds

Vacuum ultraviolet photoionization by coherent 10.5-eV radiation is evaluated for soft ionization of organic molecules. Coherent 10.5-eV radiation is produced by frequency tripling the third harmonic of an Nd:YAG laser (355 nm) in a mixture of xenon and argon. A number of intermediate size, C₆ to C₈, aliphatic compounds are studied to determine the extent and characteristics of fragmentation. Compared with 12-eV electron impact ionization, all show higher molecular ion abundances, less fragmentation, and significant enhancement of low energy rearrangement ions. n-Alkanes, alkenes, ketones, carboxylic acids, and ethers all form predominant molecular ions. Aldehydes and amines show significant molecular ion abundances but also extensive fragmentation. Branched alkanes, dienes, alcohols, and esters show little or no molecular ion, but do have a single dominant fragment ion. Metastable broadening in these spectra gives qualitative information on the energetics of low energy rearrangements.     

Fragmentation of selected C6H10O isomers: Evidence for a common [C5H7O]+ ion generated by methyl loss from cyclohexene oxide and 5,6-dihydro-4-methyl-2H-pyran

The loss of CH₃˙ from the molecular ions of cyclohexene oxide and 5,6-dihydro-4-methyl-2H-pyran has been investigated. On the basis of metastable peak shape analysis, collision-induced dissociation/mass-analysed ion kinetic energy spectra and thermochemical data it is concluded that the same [C₅H₇O]⁺ ion is formed in both cases.     

Characterization of cationic surfactant markers and their abiotic degradation products by CGC-EI/PICI MS

Summary The electronic impact (EI) and positive ion chemical ionization (PICI) mechanisms of a variety of environmental relevant secondary and tertiary trialkylamines (TAMs) (NR₁R₂R₃, where R_n=H, CH₃, C₁₄H₂₉, C₁₆H₃₃ or C₁₈H₃₇) have been determined. The base peak of EI mass spectra corresponds to the carbon-carbon bond -cleavage with respect to nitrogen, contrasting with a low abundant molecular ion [M]⁺. On the other hand, the [M-1]⁺ was the base peak of the PICI. In addition, the photoproduct composition of irradiated (>290 nm) TAMs ((CH₃)₂NR, R=C₁₆H₃₃ and C₁₈H₃₇) aqueous solutions in the presence of hydrogen peroxide has been characterized by CGC-MS. In this regard, oxidation products in the vicinal carbons with respect to nitrogen and a variety of photolysis products including fatty acids, aldehydes, alkenes, alkanes and 2-methylketones were identified as major components.     

Selective detection of the tolyl cation among other [C7H7]+ isomers by ion/molecule reaction with dimethyl ether

The ion/molecule reaction of the tolyl cation with dimethyl ether has been investigated using triple quadrupole mass spectrometry. Three isomers with [C₇H₇]⁺ composition, the 3-tolyl, benzyl, and tropylium cations, were individually selected and reacted with dimethyl ether at a pressure of 1 mtorr in the second quadrupole (Q₂) collision cell. Only the tolyl ion reacted to yield a methoxylated product ion peak at m/z 122. This reaction product having m/z 122 is postulated to be identical in structure with the molecular ion of 3-methyl anisole, as supported by thermochemical data and the similarity of the collision induced dissociation (CID) daughter ion mass spectra of the product ion and the molecular ion of authentic 3-methyl anisole. The daughter ion mass spectra of the three [C₇H₇]⁺ isomers during CID, by using a triple quadrupole mass spectrometer, are nearly identical; on the other hand, the analytical approach based on the ion/molecule reaction with dimethyl ether clearly exhibits distinct gas-phase chemistry reflecting structural differences among the isomers. Sot     

193 nm Laser photoionization and photodissociation for isomer differentiation in Fourier-transform mass spectrometry

Energetic (6.4 eV) multiphoton ionization (MPI) or photodissociation, effected interchangeably in a Fourier-transform mass spectrometer, can differentiate isomers that yield similar electron ionization spectra. Selectivity is shown for isomers of C₇H₈, C₇H₉N, C₇H₇F, C₈H₁₀, but not of C₆H₃Cl₃ and C₁₄H₁₀. The contrasting MPI fragmentations and ionization efficiencies, as well as high sensitivities, are of substantial analytical utility. The high ionization efficiency makes possible high resolution MPI spectra, such as 470,000 (FWHH) for the molecular ion of anthracene, from a single laser pulse.     

Molecular ion rearrangements of benzenesulfonylhydrazides

The mass spectra of a series of substituted benzenesulfonylhydrazides are discussed. The main fragmentation mode of these compounds corresponds to rearrangement of the molecular ion with subsequent loss of N₂H˙ and/or N₂H₂. These fragmentations are confirmed by mass analysed ion kinetic energy spectra. Simple cleavage of the S? N bond with retention of the charge on nitrogen is observed for all the hydrazides studied but is favoured by electron withdrawing substituents. This latter fragmentation could not be confirmed by mass analysed ion kinetic energy spectra as it is probably very rapid and occurs mainly in the source.     

Fingerprinting stereoisomers by survivor-ion mass spectrometry

Survivor-ion mass spectrometry is used to distinguish stereoisomeric cis- and trans-4-methylcyclohexanol. The method involves producing ions by electron impact ionization and submitting them without mass selection to collisional neutralization and reionization, followed by selective monitoring of non-dissociating ions. The differences in the electron impact mass spectra of the stereoisomers, due to the different fragment ion elemental compositions and structures, are highlighted by collisional neutralization with Xe, NO and CH₃SSCH₃, followed by reionization with oxygen. The differences in the survivor-ion spectra are due to different neutralization efficiencies of the isobaric and isomeric ions produced by electron impact ionization, different stabilities of the intermediate neutral species, different reionization efficiencies and reionized ion stabilities. Neutralization-reionization spectra of the C₇H₁₂^+., C₆H₉⁺, C₃H₆O^+. and C₃H₅O⁺ ions from stereoisomeric 4-methylcyclohexanols are also reported.