Substituent effects in mass spectrometry—II. The effect of substituents on the dissociation of para and meta disubstituted benzenes

The effect of substituents on the activation energy for primary dissociation processes in the molecular ions of mono- and para and meta di-substituted benzenes has been examined. Where the daughter ion retains the substituent group, variation of the energy of activation derives from a combination of the effects of substituents on the ionisation potential of the molecular ion and the appearance potential of the daughter ion. An equation relating the energy of activation for the fragmentation of the molecular ion of a mono-substituted benzene to that of related para and meta di-substituted benzenes is presented.     

Substituent effects in mass spectrometry: The correlation of the ionisation potentials of substituted benzenes with σp+

It is demonstrated that the ionisation potentials of mono- and di-substituted benzenes are related to the sum of Brown's σ_p⁺ of the substituents for both the meta and para isomers. From this it is concluded that the charge, in substituted arenes, lies predominantly on the aromatic ring rather than on either of the substituents. An empirical equation relating the IP's of mono- and disubstituted benzenes to the sum of σ_p⁺ is presented.     

Substituent effects in mass spectrometry—III: Substituent effects in the dissociation of the molecular ions of para and meta substituted benzoic acids

The effect of substituents on the electron-impact-induced fragmentation of the molecular ions of para and meta substituted benzoic acids has been examined. The substituent is observed to exert an effect on the ionisation potential of the molecular ion, on the appearance potentials of the primary daughter ions and on the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ analogues prior to the loss of hydroxyl therefrom. The energy of activation for the loss of hydroxyl from the molecular ion is in general dependent upon the nature but not the position of the substituent, while the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ derivative is dependent upon both the nature and the position of the substituent. No correlation of the relative ion abundances with σ⁺ constants was observed. The results are consistent with the molecular ions of each compound having a dissimilar energy distribution, which could arise either by different energy transfers from the electron beam to the molecule or by the participation of different isolated electronic excited states (or similar states but to varying extents) in the dissociation of the molecular ions.     

Theoretical calculation of ionization potentials for disubstituted benzenes: additivity vs non-additivity of substituent effects

The ionization potentials of 55 para- and 55 meta-disubstituted benzenes, consisting of all binary combinations of electron-withdrawing groups (-NO2, -CF3, -CHO, -COOH) and electron-donating groups (-Cl, -CH3, -OH, -OCH3, -NH2, and -N(CH3)2) have been calculated using density functional theory with the B3LYP functional and a 6-31G(d) basis set. Relative ionization potentials (delta IP), referred to benzene, are compared with experimental values and shown to be in good agreement. The disubstituted data were correlated with monosubstituted delta IP data and shown to require quadratic terms in order to achieve a good fit; the validity of this conclusion was possible due to the low scatter in the calculated data. A simple MO analysis gives a semiquantitative interpretation of the observed trends in substitutent effects, including a discussion of combinations of substituents for which nonadditivity should be expected.     

Spectral characteristics of ortho, meta and para dihydroxy benzenes in different solvents, pH and beta-cyclodextrin

Spectral characteristics of ortho, meta and para dihydroxy benzenes (DHB's) have been studied in different solvents, pH and beta-cyclodextrin. Solvent study shows that: (i) the interaction of OH group with the aromatic ring is less than that of amino group both in the ground and excited states, (ii) in absorption, the charge transfer interaction of OH group in para position is larger than ortho and meta positions. pH studies reveals that DHB's are more acidic than phenol. The higher pK(a) value of oDHB (monoanion-dianion) indicates that the formed monoanion is more stabilized by intramolecular hydrogen bonding. DHB's forms a 1:1 inclusion complex with beta-CD. In beta-CD medium, absorption spectra of DHB's mono and dianions shows unusual blue shifts, whereas in the excited state, the spectral characteristics of DHB's follow the same trend in both aqueous and beta-CD medium.     

Hydrogen migrations in mass spectrometry. IV—formation of [C6H7]+ in the chemical ionization mass spectra of alkylbenzenes

Using specific deuterium labelling the mechanisms of the olefin elimination reactions leading to formation of [C₆H₇]+ in the H₂ and CH₄ chemical ionizatin mass spectra of ethylbenzene and n-propylbenzene (and to [C₂H₅C₆H₆]+ in the CH₄ chemical ionization mass spectra) have been investigated. The results show that the reaction does not occur by specific migration of H from the β position of the alkyl group to the benzene ring. For ethylbenzene 23–29% of the migrating H originates from the α-position, while for n-propylbenzene H migration from all propyl positions is observed in the approximate ratio, position 1:position 2:position 3=0.30:0.22:0.48. It is proposed that the results can be explained on the basis of competing H migration from each alkyl position involving cyclic transition states of different ring sizes, rather than by H randomization within the alkyl chain.     

Stereochemical effects in mass spectrometry XIII—Determination of absolute configuration by fast atom bombardment mass spectrometry

It was found that in the fast atom bombardment mass spectra of some asymmetric secondary alcohols and amines, when a pair of enantiomers, such as (2R,3R)- and (2S,2S)-2,3-diacetoxysuccinic anhydride and (2R,3R)- and (2S,3S)-2,3-dibenzoyloxysuccinic anhydride, were used as reagents, the relative abundances of characteristic ions formed by the stereoselective reaction between a sample and a reagent of different configurations were much higher than those of ions formed by a sample and a reagent of the same configuration. The absolute configurations of the sample molecule may be predicted by examination of the mass spectra of the sample measured with reagents of R and S configurations. This approach proved to be a convenient way to determine the absolute configuration of organic molecules at the micromole level by fast atom bombardment mass Spectrometry, and it has advantages over the chemical ionization method reported previously for the analysis of polar and involatile compounds.     

Stereochemical effects in mass spectrometry. 3—Detection of chirality by chemical ionization mass spectrometry

It was found that the chemical ionization mass spectra of the enantiomers of some α-amino acids and α-hydroxy acids could be definitely distinguished when a chiral compound, l-amyl alcohol, was used as a component of the reagent gases. The relative abundances of some characteristic ions produced through ion-molecule reactions in the gas phase were much higher in the D isomers than in the L isomers. The results were highly reproducible. This approach proved to be a convenient way for the detection of the chirality of these compounds by chemical ionization mass spectrometry.     

Ambient ionisation mass spectrometry for in situ analysis of intact proteins

Ambient surface mass spectrometry is an emerging field which shows great promise for the analysis of biomolecules directly from their biological substrate. In this article, we describe ambient ionisation mass spectrometry techniques for the in situ analysis of intact proteins. As a broad approach, the analysis of intact proteins offers unique advantages for the determination of primary sequence variations and posttranslational modifications, as well as interrogation of tertiary and quaternary structure and protein‐protein/ligand interactions. In situ analysis of intact proteins offers the potential to couple these advantages with information relating to their biological environment, for example, their spatial distributions within healthy and diseased tissues. Here, we describe the techniques most commonly applied to in situ protein analysis (liquid extraction surface analysis, continuous flow liquid microjunction surface sampling, nano desorption electrospray ionisation, and desorption electrospray ionisation), their advantages, and limitations and describe their applications to date. We also discuss the incorporation of ion mobility spectrometry techniques (high field asymmetric waveform ion mobility spectrometry and travelling wave ion mobility spectrometry) into ambient workflows. Finally, future directions for the field are discussed.     

Studies in the chemistry of erythrina alkaloid derivatives IV—mass spectral studies of erythrinane derivatives

The electron impact induced fragmentations of erythrinane, erythrinone and their derivatives, have been investigated and rationalized.     

Characterisation of oxazepam degradation products by high‐performance liquid chromatography/electrospray ionisation mass spectrometry and electrospray ionisation quadrupole time‐of‐flight tandem mass spectrometry

Oxazepam has been subjected to controlled degradation at 100°C for 3 h in 0.5 M HCl and 0.5 M NaOH. Following neutralisation of the degradation mixture and removal of salts by solid‐phase extraction (SPE), isocratic high‐performance liquid chromatography/mass spectrometry (HPLC/MS) using water/methanol (25:75 v/v) as the mobile phase was carried out using a flow diverter to collect fractions prior to their characterisation by electrospray ionisation multi‐stage mass spectrometry (ESI‐MSⁿ) and proposal of the corresponding fragmentation patterns. The elemental compositions of the degradation products and their MS fragments were evaluated using electrospray ionisation quadrupole time‐of‐flight tandem mass spectrometry (ESI‐QTOF‐MS/MS) which was then used to support the proposed fragmentation patterns. Copyright © 2010 John Wiley & Sons, Ltd.     

Activation energies for the singlet excited state processes of substituted benzenes: para, meta, and ortho isomers of methylbenzonitrile and methylanisole in acetonitrile

The rate constants of decay of the excited singlet states of the methylbenzonitriles (1-3) and the methylanisoles (4-6) have been determined by the measurement of fluorescence lifetimes over a broad range of temperatures (-45 to +65 degrees C) in acetonitrile. By fitting this data to a nonlinear expression that includes the Arrhenius equation, rate constants for the activated process (reaction) and the unactivated ones (fluorescence and intersystem crossing) can be reliably obtained. Available literature data for benzene, toluene, and ortho-xylene were also analyzed. The results indicate that the excited singlet state of substituted benzenes is quite reactive and forms a prefulvene biradical intermediate efficiently (quantum yield = 0.69 for benzene itself) by an activated route. In contrast, the efficiency of isolable product formation is quite low because the dominant process for this intermediate is returned to starting material. These observations explain why Ermolaev's rule does not apply to benzene derivatives.     

Peptides containing aminophosphonic acids. IV—mass spectra of the peptides containing aminomethylphosphonic acid

The fragmentation behavious under electron of di- and tri-peptide analogues containing amino acid (aa) and aminomethylphosphonic acid (ap) groups are described. Successive fragmentations of the molecular ions provided a convenient method for the sequence determination of peptides containing aa and ap units.     

Direct assignment of positional isomers by mass spectrometry: ortho,meta and para acyl and amidyl anilines and phenols and derivatives

A direct MS/MS method for the ortho, meta or para configuration assignment of any single molecule that forms reference ions upon ionization and dissociation is demonstrated. Gas-phase structure diagnostic ion–molecule reactions with acetonitrile are shown to distinguish the isomeric 2-, 3- and 4-hydroxybenzoyl cations and the 2- from the 3- and 4-aminobenzoyl cations. These reference ions, which display indistinguishable 15 eV collision-induced dissociation product ion mass spectra, react with acetonitrile to yield characteristic ratios of product ions, most particularly for the 2-isomers. The reactivity of the 2-benzoyl cations is the most characteristic since the ortho configuration allows for [4⁺ + 2] polar cycloaddition that yields relatively stable heterocycles in N-protonated forms. Distinction of the reference isomeric 2-, 3- and 4-hydroxy- and aminobenzoyl cations permits, therefore, partially or completely, direct ‘MS-only’ positional assignment of either ortho, meta or para configuration for any single molecule that forms such reference ions upon ionization and dissociation. This “class-universal” method for direct MS assignment of a single positional isomer should therefore be applicable to many members of the homologous series of isomeric ortho, meta and para acyl and amidyl anilines and phenols and derivatives. Such molecules dissociate, or are likely to dissociate, after or during ionization processes to form the reference and structurally diagnostic ortho, meta or para hydroxy- or aminobenzoyl cations. Copyright © 2004 John Wiley & Sons, Ltd.     

Analysis of estrogens in river sediments by liquid chromatography-electrospray ionisation mass spectrometry. Comparison of tandem mass spectrometry and time-of-flight mass spectrometry

A method has been developed and optimised for the determination of two natural estrogens, estrone (E1) and 17beta-estradiol (E2), and one synthetic estrogen, 17alpha-ethynylestradiol (EE2), in river sediments at the sub-ng/g level. This procedure includes microwave-assisted solvent extraction (MASE), solid-phase extraction and high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) with electrospray ionisation. Using sediments spiked with the three estrogens at 10 ng/g wet weight, efficient extraction (>92%) of all the three analytes was achieved by MASE, and whole-procedure recoveries ranged from 82 to 98%. Optimisation of the LC separation allowed for substantial reduction of ionisation suppression in the electrospray source to a final level of <18% suppression. Time-of-flight mass spectrometry (TOF-MS) and MS/MS were compared for the analysis of sediment extracts, with the latter technique proving to be the most selective. The method detection limits achieved by LC-MS/MS were 15, 30 and 40 pg/g for E1, E2 and EE2, respectively, which were 13-fold lower than those obtained by LC-TOF-MS. Analysis of river sediments collected from the River Ouse, UK, showed the presence of the natural estrogens at sub-ng/g level. E1 levels ranged from 0.40 ng/g (dry weight) to 3.30 ng/g while E2 levels ranged from <0.03 to 1.20 ng/g and EE2 was never detected (<0.04 ng/g).     

Electrospray ionisation tandem mass spectrometry in the characterisation of isomeric benzofurocoumarins

A set of aminoalkoxy-substituted, differently annullated furocoumarins, differing in the position of the aminoalkoxy chain and in the unsaturation level of the fused ring, has been subjected to electron impact and electrospray ionisation (ESI) experiments. In order to achieve a distinct characterisation of isomeric compounds, which partially failed under electron impact conditions, collision-induced dissociation experiments were performed on protonated molecules. The breakdown curves obtained by varying the tickle voltage on an ion trap ESI instrument led to the desired characterisation.     

Mass spectrometry of onium compounds. XXIII—ionisation potentials in the structural assignment of [M – N2] ions from diazo-oxides

The ionisation potential shows that fulven-6one is formed pyrolytically from o-benzo-diazo-oxide. Appearance potential measurements show increasing pyrolytic contribution to the [M – N₂] species with increase in ion source temperature.     

Studies in mass spectrometry. IV—Fragmentation of aryltriflates. Substituent effects on [ArO]+ ion abundances: Looking for a best suited electron energy in the steady state approach. Experimental evidence for the ‘through bond’ stabilization of aryl cations by meta electron donor Substituents

The fragmentation patterns of 24 substituted phenyltrifluoromethanesulfonates has been determined by exact mass and metastable transition measurements. The influence of the ring substituent(s) on the abundance of the [ArO]+ ions has been investigated at low energies and a new standard parameter for plotting against Hammett's ó constants is proposed. The direct loss of CF₃SO₃ from the molecular ion of 3,5-(diisopropyl)-phenyl-, 3,5-(dicyclopropyl)phenyl- and 3,5-di(α-methylcyclopropyl)-phenyl-methanesulfonate, and further observations on the fragmentation processes of these compounds are consistent with M.O. calculations on the ‘through bond’ stabilization of the [Ar]+ σ state by meta electron donor substituents.