Collisional activation mass spectra of ions containing polyisotopic elements

If a mass selected ion containing multiple isotopes of an element fragments by loss of a neutral containing the element, the product ions will be observed as multiple peaks. This ‘isotopic cluster’ of peaks, although of a different appearance than those found in normal mass spectra, also indicates the isotopic composition of the precursor ion.     

Collisional activation mass spectra of pentene and hexene molecular ions

The collisional activation spectra of the molecular ions from fourteen pentene and hexene isomers show substantial differences, some of which can be rationalized using known ion fragmentation mechanisms. These differences, and their negligible dependence on ion internal energy, are advantageous for the structural characterization of such isomeric ions, including those produced by fragmentation of larger molecules. The decompositions of metastable ions of the pentenes were nearly identical, but those of the hexenes showed some differences useful for structural charactrization.     

Stereospecific adduct ion formation in the chemical ionization mass spectra of E- and Z-1,2,3-triaryl-2-propen-1-ones

Stereospecific adduct ion formation has been observed in the chemical ionization mass spectra (positive and negative) of certain E- and Z-1,2,3-triaryl-2-propen-1-ones. The Z isomers are found to give higher relative abundances of adduct ions than the E isomers. This has been interpreted in terms of the differences in the proton affinities of the isomers originating from their different degrees of enone resonance. Halide ion (CI^? and Br^?) attachment spectra of these compounds also show stereochemical differences in the relative abundances of [M]^?˙ and [M+halide]^? ions, though the effect is not as pronounced as in the case of the positive ion spectra.     

Collisional ionization of monocharged positive ions

Competitive fragementations of doubly charged ions formed by charge stripping have been observed for some hydrocarbon ions. The reproducibility of the abundance ratios indicates that they may be used to characterize the structural identity or non-identity of non-decomposing monocharged positive ions.     

Gas phase substitution under ammonia chemical ionization

Evidence for the isomerization of the [M + NH₄]⁺ adducts of nitrobenzene and triphenylnitrocyclohexenes to the isonitroso derivatives in the first field free region has been obtained. The isomerized adducts undergo successive loss of ?O and ?H or NH₃ to give ions corresponding to the substitution products. Mass analysed ion kinetic energy and collisionally activated dissociation/mass analysed ion kinetic energy data reveal that the adduct decomposes by similar pathways in the second field free region. In the ion source an addition elimination reaction and nucleophilic substitution contribute to the formation of the substituted product ion.     

Collisional activation mass spectra of [C6H8]+ ions

Collisional activation spectra and isotopic labeling indicate that for [C₆H₈]⁺ ions at least six stable isomers can be formed from a variety of precursors.     

Reduction of olefinic bond under ammonia chemical ionization

The unsaturated $\text{trans}$dicarboxylates (III and IV) form stable (M+N₂H₇)⁺ adducts in addition to the (M+NH₄)⁺ adduct. At high ammonia pressure in the ion source, bimolecular reactions between (M+NH₄)⁺ (M+N₂H₇) adducts and NH₃ result in the reduction of olefinic bonds in unsaturated $\text{cis}$ and $\text{trans}$ dicarboxylates. Evidence was obtained from MIKE/CAD data on the possible structure of the product ion.     

Electrophilic aromatic substitution under chemical ionization conditions. Methylamine and ammonia as reagent gases

For compounds C₆H₅X (X?Cl, Br, I) under chemical ionization conditions, methylamine causes ipso substitution of X by [NH₂CH₃]⁺ and by [NH₂]⁺˙. C₆H₅F is less reactive; it gives some [C₆H₅NH₂]⁺˙. Nitrobenzene gives an adduct ion [M+CH₃NH₃]⁺, a reduction product ion [C₆H₅NO₂]⁺˙, and an ion at m/z93, probably a substitution product [C₆H₅NH₂]⁺˙, but no [C₆H₅NH₂CH₃]⁺. It is also shown that the ion m/z94, formed from nitrobenzene with ammonia as reagent gas, is a substitution product rather than a reduction product ion. Carbonyl compounds C₆H₅. CO. X give adduct ions and some substitution, mainly [C₆H₅NH₂]⁺˙.     

Behavior of acetals under chemical ionization conditions

Russian Chemical Bulletin -     

Collisional activation spectra of field ionised molecules

The collision of field ionised molecular ions of high kinetic energy (8 keV) with neutral target atoms (helium) gives rise to a large variety of fragments. In general the types of fragments, as well as their abundance ratios, closely resemble those formed under 70 eV electron-impact. The differences observed between the collisional activation spectra of several field ionised[C₇H₈]+. ions, as well as the differences observed between several [C₈H₈]+. ions can be rationalised assuming different degrees of isomerisation. The usefulness of collisional fragmentation spectra for structure determination of organic compounds has been demonstrated for a variety of compounds and some mixtures.     

Study of selected ions in the ammonia desorption chemical ionization mass spectra of peracetylated gentiobiose and two isotopically labelled peracetylated gentiobioses

The ammonia desorption chemical ionization (NH₃-DCI) mass spectra of peracetylated gentiobiose (1) and two isotopically labelled gentiobioses (2 and 3) were examined. Compound 2 is labelled with trideuteroacetyl groups in the non-reducing moiety and 3 with trideuteroacetyl groups in the reducing moiety. It is shown that the [M + NH₄ – 42]⁺ ion is not formed direct from [M + NH₄]⁺ by loss of ketene but appears to be formed by way of a nucleophilic acyl substitution reaction resulting in a neutral species which complexes with NH₄⁺. The disaccharides undergo cleavage at either side of the glycosidic oxygen joining the two sugar residues, a process which is accompanied by addition of H or CH₃CO to afford neutral species which complex with NH₄⁺. The structures of the ions resulting from H transfer have been inferred by comparison of their mass-analysed ion kinetic energy (MIKE) spectra with MIKE spectra of the [M + NH₄]⁺ ions of compounds of established structure. A ring fragmentation reaction of 1, 2 and 3 is reported.     

Sites of protonation in phenothiazines: Methane and ammonia chemical ionization mass spectra

Positive ion methane and ammonia chemical ionization mass spectra for ten phenothiazine derivatives are reported. The fragmentations observed in the chemical ionization mass spectra are rationalized in terms of the location of the added proton. High-resolution measurements are used to confirm empirical formulae of the ions in the mass spectra. Changes in the mass spectra with a change in the chemical ionization reagent gas from methane to ammonia are described. A comparison with positive ion secondary ion mass spectra of the same compounds show that the amount of fragmentation is higher in the secondary ion mass spectra, but the same types of ions are observed in spectra produced by both ionization methods.     

Oxidation reactions in triterpenes under chemical ionization conditions

From a collisional activation spectral study it has been found that certain triterpene alcohols with an ursane or oleanane skeleton undergo oxidation to the corresponding ketones under chemical ionization (NH₃) conditions giving rise to abundant [M + NH₄ ? 2]⁺ ions. Mass-analysed ion kinetic energy and B²/E scan results indicate that both [M + NH₄]⁺ and [M + N₂H₇ ? 2]⁺ ions contribute to the formation of the [M + NH₄ ? 2]⁺ ion.     

Anomalous ions in the chemical ionization mass spectra of aromatic nitro and nitroso compounds

The appearance of [MH-30]⁺ ions in the chemical ionization mass spectra of aromatic nitro compounds may be due to their initial reduction to the corresponding amines within the ion source. Aromatic nitroso compounds may be similarly reduced to yield [MH-14]⁺ ions. The hydroxy derivatives of the nitroso compounds yield further anomalous ions at [MH-16]⁺ probably due to the reduction of the hydroxy groups.     

Gas-phase alkylation of 2-methylfuran under chemical ionization conditions

In the gas phase, under chemical ionization conditions, the sites of attachment of [CH₃]⁺ and [C₂H₅]⁺ to 2-methylfuran have been studied by tandem mass spectrometry. Spontaneous metastable and collision-induced fragmentations have been compared to those observed for the protonated alkylfuran isomers. The results obtained show that alkylation occurs preferentially at the β-position.     

Behavior of aromatic ortho esters under chemical ionization conditions

Conclusions Ortho esters decompose under protolysis conditions in the gas phase to form carboxonium ions [MH-RHO]⁺, which can give stable associates with neutral molecules.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 579–583, March, 1988.     

Ammonium adduct ion in ammonia chemical ionization mass spectrometry: 2—Mechanism of elimination of neutrals from the ammonium adduct ion

The mechanism of elimination of ROH (R = H or CH₃) from the ammonium adduct ion, [M+NH₄]⁺, of 1-adamantanol and its methyl ether is examined by using linked-scan metastable ion spectra and by measuring the dependence of the peak intensity ratio [M+NH₄]⁺/[M+NH₄? ROH]⁺ on ammonia pressure. For 1-adamantanol both S_Ni and S_N1 reactions are suggested in metastable ion decomposition, while only the S_N1 mechanism is operative in the ion source. For 1-adamantanol methyl ether the S_N1 reaction predominates both in metastable ion decomposition and in the ion source reaction.