Negative ion mass spectra of highly fluorinated compounds

The negative ion mass spectra of highly fluorinated molecules with varying functionality are discussed. Attempts to obtain negative molecular ions where no positive molecular ion was observed met with little success. An interesting structural correlation for fluorinated ethers was developed based upon the observation of the abundant perfluoro alkoxide anions, CF₃O^? and CF₃(CF₂)₂O^?, in the negative ion mass spectrum.     

Negative ion tandem mass spectrometry of prenylated fungal metabolites and their derivatives

Liquid chromatography negative ion electrospray ionisation tandem mass spectrometry has been used for characterisation of naturally occurring prenylated fungal metabolites and synthetic derivatives. The fragmentation studies allow an elucidation of the decomposition pathways for these compounds. It could be shown, that the prenyl side chain is degraded by successive radical losses of C₅ units. Both the benzoquinones and the phenolic derivatives display significant key ions comprising the aromatic ring. In some cases, the formation of significant oxygen-free key ions could be evidenced by high-resolution MS/MS measurements. Furthermore, the different types of basic skeletons, benzoquinones and phenol type as well as cyclic prenylated compounds, can be differentiated by their MS/MS behaviour.

Negative ion liquid secondary ion mass spectrometry of carbamate-linked oligodeoxynucleosides. II

Carbamate-linked Oligodeoxynucleosides, in which the backbone consists of carbamate and N-methylcarbamate linkages, have been analyzed by negative ion liquid secondary ion mass spectrometry. Bidirectional sequence-determining fragmentations are postulated to occur from a common radical anion intermediate that is produced by capture of an ionizing electron by the neutral sample molecule. Fragmentation reactions appear to be related to whether a proton or methyl group is present on the amide nitrogen.     

Negative ion formation in electrospray mass spectrometry

Analytical and Chemical Sciences, Research Triangle Institute, Research Triangle Park, North Carolina, USA Negative ion electrospray (ES) operating on a single quadrupole mass spectrometer for the detection of low-molecular-weight molecules is discussed. The ES interface was operated at a positive cylindrical electrode potential to produce negative ions, and the results obtained were compared to the positive ion mode. As in the case of operation in the more common positive mode, negative ions with varying degrees of solvation and structurally relevant fragments can be obtained from a variety of solute species, including β-lactam antibiotics, aminoglycosides, aminocyclitols, tetracyclines, sulfonamides, nucleotides, peptides, and explosives. No fragmentation of parent species, except those from some labile explosives, was provided because low potential differences are applied between the capillary and the first skimmer, and electrical discharge is avoided in the gas phase. An increase in the capillary voltage resulted in collision-induced decomposition to produce structurally relevant fragment ions in both operation modes. An evaluation of representative chromatographic solvents indicated that 2-propanol added with oxygen in the ES bath gas is best suited to suppress electrical (corona) discharge phenomena in negative ion operation, whereas it aids in solution nebulization, desolvation, and transfer of ions in solution to the gas phase. For positive ion mode, no such precaution was necessary. Conditions that promote the formation of ions in solution usually improve ES response. Therefore, an increase in the solvent pH can increase the sensitivity in negative ion ES ionization. Negative ion ES offers the advantage of providing complementary structural information to help in the characterization of an unknown compound or to confirm a certain tentatively proposed structure. Nucleotides and explosives were best characterized in negative ion mode owing to the ease with which they form anions in solution, and they could be detected down to the l-pg /gML level.     

Secondary ion mass spectrometry of organic compounds

Sputtering is a very effective means to ionize large organic molecules, even those which are involatile and thermally labile. It can be applied to organic trace detection, identification and structural elucidation, to the analysis of body fluids, and as a detector in HPLC and TLC.     

Negative ion chemical ionization mass spectrometry of some isocyanates

Summary Negative ion mass spectra for 3 aliphatic and 4 aromatic isocyanates have been obtained by low pressure chemical ionization, using CH₄, CO₂ and N₂O as reagent gases. All compounds furnished intense anions at m/z 42. With CH₄, quasi-molecular anions were observed at m/z M+1 for aliphatic and m/z M+1 and M–1 for aromatic isocyanates. With N₂O, anionic substitution products at m/z M+15 and M+30 were observed, and with CO₂ and N₂O, peaks at m/z M–12 could be detected for all aromatic isocyanates. Studies with ¹³CO₂ and C¹⁸O₂ as reagent gases showed that the anions at m/z M–12 and M+15 correspond to [M–CO+O]^– and [M–H+O]^–, respectively.

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Negativionen-Massenspektrometrie mit chemischer Ionisierung von einigen Isocyanaten

Zusammenfassung Die Negativionen-Massenspektren von 3 aliphatischen und 4 aromatischen Isocyanaten wurden mittels chemischer Ionisation bei tiefem Quellendruck aufgenommen, und zwar mit den Reagensgasen CH₄, CO₄ und N₂O. Alle Verbindungen lieferten intensive Anionen mit m/z 42. Mit CH₄ erhielten wir die quasi-molekularen Anionen M+1 für aliphatische sowie M+1 und M–1 für aromatische Isocyanate. Das Reagens N₂O ergab die anionischen Substitutionsprodukte M+15 und M+30. Sowohl CO₂ als auch N₂O führten mit aromatischen Isocyanaten zur Bildung von M–12 Anionen. Versuche mit ¹³CO₂ und mit C¹⁸O₂ als Reagensgase zeigten, daß die Anionen M–12 und M+15 den Ionen [M–CO+O]^– und [M–H+O]^– entsprechen.

     

Negative ion field desorption mass spectrometry of anionic surfactants

Summary Negative ion field desorption mass spectrometry has been applied to detergents containing anionic surfactants. The mass spectra demonstrate a high selectivity for the detection of the sulphonates (concentration limit 10^–5 mol/l). The spectra do not contain fragment ions nor ionization products from non-ionic components of the mixture.

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Felddesorptions-Massenspektrometrie negativer Ionen zur Analyse von anionischen Tensiden

Zusammenfassung Die Felddesorptions-Massenspektrometrie negativer Ionen wurde zur direkten Mischungsanalyse von Detergentien, die anionaktive Tenside enthalten, angewendet. Die Massenspektren zeigen eine hohe Selektivität der Methode für den Nachweis der Sulfonate (Konzentrationsgrenze 10^–5 mol/l). Die Spektren enthalten weder Fragmentionen noch Ionisierungsprodukte nichtionischer Komponenten der Mischung.

     

Negative ion mode electrospray ionization mass spectrometry study of ammonium-counter ion clusters

Electrospray ionization mass spectrometry (ESI-MS) was used to examine clusters of protonated amine salt solutions with chloride counter ions in the negative ion mode. These ions have the general formula [(RNH₃)_xCl_x+1]⁻. Primary amines generate a wide cluster distribution with clusters up to 14 mer for methylamine hydrochloride clusters. Secondary and quaternary amines only generate the monomer ion under identical conditions. Collision induced dissociation (CID) of the cluster ions generates cluster ions of lower m/z with the next lower cluster being the most abundant. The product ions from MeNH₃Cl₂⁻, Me₂NH₂Cl₂⁻ and (MeNH₃)₂Cl₃⁻ have low threshold appearance energies of 1. 24 to 2. 22 eV center-of-mass frame. Secondary amine monomer ions have lower threshold CID energies than primary amine monomer ions. The amine threshold CID energy decreases as the carbon chain length increases. As an electrospray solvent, isopropyl alcohol (IPA) promotes the formation of counter ions and clustering.     

Negative ions in organic mass spectrometry. II—Negative ion mass spectra of simple thio and dithio esters.

The 70 eV negative ion mass spectra of some simple aromatic and aliphatic thio and dithio esters are discussed. A characteristic fragmentation process, a rearrangement analogous to the nitro-nitrite conversion of aromatic nitro compounds, is observed for aromatic thio esters.     

Electron impact studies. CIII—Negative ion mass spectra of naturally occurring compounds: Nitrobenzoyl derivatives of amino esters

The negative ion mass spectra of p-nitrobenzoyl derivatives of amino esters show pronounced molecular anions and characteristic fragmentation patterns. Fragment anions observed in the spectra of o-nitrobenzoyl analogues arise by various interactions between the nitro and amino ester groups. No fragmentation of the amino ester residue is observed for the m-nitrobenzoyl derivatives, but the +E spectra of these systems can be used to provide definitive structural information.     

Negative ion mass spectra of a series of fluorinated β-diketones

Electron capture by and the subsequent fragmentation of a series of eleven fluorinated β-diketones of general formula CF₃COCH₂COR has been studied in an MS-50 mass spectrometer. Consecutive loss of two HF molecules from molecular anions occurs with all compounds, as does elimination of CHF₃ from [M ? H]^? ions. Elimination of CO occurs from either \documentclass{article}\pagestyle{empty}\begin{document}$ \left[{{\rm M - HF}} \right]_{}^{_.^ - } $\end{document} or \documentclass{article}\pagestyle{empty}\begin{document}$ \left[{{\rm M - 2HF}} \right]_{}^{_.^ - } $\end{document} ions for five compounds where R ? CF₃ or a cyclic substituent. Kinetic energy release in metastable transitions associated with these HF and CO eliminations has been measured. Intensities of various fragment ions are in part influenced by the ion source temperature. Interaction of \documentclass{article}\pagestyle{empty}\begin{document}$ \left[{{\rm O}} \right]_{}^{_.^ - } $\end{document} reagent ions with five of the β-diketones under chemical ionization conditions gave [M ? H]^? ions as the only significant ion-molecule reaction product.     

Atmospheric pressure chemical ionization of fluorinated phenols in atmospheric pressure chemical ionization mass spectrometry, tandem mass spectrometry, and ion mobility spectrometry

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) for fluorinated phenols (C6H5-xFxOH Where x = 0-5) in nitrogen with Cl- as the reagent ion yielded product ions of M Cl- through ion associations or (M-H)- through proton abstractions. Proton abstraction was controllable by potentials on the orifice and first lens, suggesting that some proton abstraction occurs through collision induced dissociation (CID) in the interface region. This was proven using CID of adduct ions (M Cl-) with Q2 studies where adduct ions were dissociated to Cl- or proton abstracted to (M-H)-. The extent of proton abstraction depended upon ion energy and structure in order of calculated acidities: pentafluorophenol > tetrafluorophenol > trifluorophenol > difluorophenol. Little or no proton abstraction occurred for fluorophenol, phenol, or benzyl alcohol analogs. Ion mobility spectrometry was used to determine if proton abstraction reactions passed through an adduct intermediate with thermalized ions and mobility spectra for all chemicals were obtained from 25 to 200 degrees C. Proton abstraction from M Cl- was not observed at any temperature for phenol, monofluorophenol, or difluorophenol. Mobility spectra for trifluorophenol revealed the kinetic transformations to (M-H)- either from M Cl- or from M2 Cl- directly. Proton abstraction was the predominant reaction for tetra- and penta-fluorophenols. Consequently, the evidence suggests that proton abstraction occurs from an adduct ion where the reaction barrier is reduced with increasing acidity of the O-H bond in C6H5-xFxOH.     

Negative ion chemical ionization mass spectrometry of some nitroso compounds with CO2 as reagent gas

Summary Mass spectra of seven N-nitrosamines and five alkyl nitrites, the O-nitroso compounds, have been obtained by low pressure negative chemical ionization with CO₂ as reagent gas. Intense anions were observed at m/z M–32 for N-nitrosamines and at m/z M–30 for alkyl nitrites. Addition products were found at m/z M+12 and M+43 for N-nitrosamines and at m/z M+14 for alkyl nitrites. By using isotopically labeled CO₂, it could be shown that the anions at m/z M+12, M+14 and M+43 correspond to [M - H₂NO + CO₂ ^–, [M – NO + CO₂]^–, and [M – H + CO₂]^–, respectively.

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Negativionen-CI-Massenspektrometrie einiger Nitrosoverbindungen mit CO₂ als Reagensgas

Zusammenfassung Die Massenspektren von sieben N-Nitrosaminen und fünf Alkylnitriten (O-Nitrosoverbindungen) wurden durch negative chemische Ionisation bei niederem Druck mit CO₂ als Reagensgas erhalten. Intensive Anionen wurden bei m/z M–32 für N-Nitrosamine und bei m/z M–30 für Alkylnitrite beobachtet. Additionsprodukte fanden sich bei m/z M+12 und M+43 für N-Nitrosamine sowie bei m/z M+14 für Alkylnitrite. Mit Hilfe von Isotopen-markiertem CO₂ konnte gezeigt werden, daß die Anionen bei m/z M+12, M+14 und M+43, [M – H₂NO + CO₂]^–, [M – NO + CO₂]^– bzw. [M – H + CO₂]^– entsprechen.

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A full account of this work including the negative chemical ionization mass spectra obtained with other reagent gases will be submitted to the Journal of Mass Spectrometry.     

Negative ion atmospheric pressure ionization mass spectrometry: Ionization of aromatic hydrocarbons

A corona discharge atmospheric pressure ionization source generates the reagent ions, OH^? and O^? ions in addition to better known O₂^? ions, when ambient air is used as the carrier. All three ions are gas-phase bases that could form negative ions from organics via proton abstraction. Ionization of simple aromatic hydrocarbons by O₂^? is thermodynamically not feasible. Simple aromatic hydrocarbons are ionized only by O^? and/or OH^? to form [M ? H]^? ions. However, [M ? H]^? ions do not appear in the mass spectrum as they undergo stabilization via clustering with predominantly oxygen atoms.     

Negative ion electrospray high‐resolution tandem mass spectrometry of polyphenols

Representative compounds with a 1,3‐dihydroxybenzene substructure belonging to different important polyphenol classes (stilbenes, flavones, isoflavones, flavonols, flavanones, flavanols, phloroglucinols, anthraquinones and bisanthraquinones) were investigated based on detailed high‐resolution tandem mass spectrometry measurements with an Orbitrap system under negative ion electrospray conditions. The mass spectral behaviour of these compound classes was compared among each other not only with respect to previously described losses of CO, CH₂CO and C₃O₂ but also concerning the loss of CO₂ and successive specific fragmentations. Furthermore, some unusual fragmentations such as the loss of a methyl radical during mass spectral decomposition are discussed. The obtained results demonstrate both similarities and differences in their mass spectral fragmentation under MSⁿ conditions, allowing a characterization of the corresponding compound type. Copyright © 2015 John Wiley & Sons, Ltd.     

Negative ion field desorption mass spectra of some inorganic and organic compounds

Field desorption of negative ions can be achieved below the threshold of field electron emission. To this end a mixture of the sample with polyethylene oxide and water was applied to smooth wire cathodes. The mass spectra of some inorganic and organic compounds are reported. Anionization by [CI]^? ion attachment is demonstrated with the examples of 20-hydroxycholesterol and sucrose.