Field ionization mass spectrometry—I. Nucleosides

The field ionization mass spectra of a series of nucleosides are reported, and compared with spectra obtained by conventional electron-impact ionization. The latter are complex, with structurally significant molecular ion and sugar cleavage peaks often of low intensity or completely absent. In contrast, the field ion spectra are extremely simple, with all except guanosine (highest mass peak [M — 18]) showing intense molecular ion peaks, and the characteristic sugar (S) and base (B + H) cleavage products as the only other important fragments.     

Field ionization mass spectrometry—IV. Isotopically labeled heptanals

The field ionization (FI) mass spectra of n-heptanal and a series of deuterium labeled analogs have been studied, with the objectives of initiating systematic investigations of reaction mechanisms of FI produced ions and to permit comprison with those found for other ionization processes. It is now recognized that FI ions have: (a) lower average internal energies and (b) shorter residence times than similar ions generated by electron-impact (EI), and the possibility exists of H/D-randomization occuring in ions formed by desorption from the emitter, by unimolecular decomposition close to the emitter and by either ‘fast’ or ‘slow’ metastable decompositions. In this study only the peak shifts of normal ions could be utilized; accurate mass measurements of all major ions revealed elemental compositions similar to EI. A site-specific McLafferty rearrangement gave the base peak at m/e 44 ([C₂H₄O]^+.), although the apparently complementary ion at m/e 70 ([C₅H₁₀]^+.) arose in a less specific process. Ions at m/e 43 ([C₃H₇]⁺) and 71 ([C₅H₁₁]⁺ 80%; [C₄H₇O]⁺ 20%) were apparantly generated without significant H/D-scrambling. Of special interest was the observation of the rearrangement ion at m/e 86 ([C₅H₁₀O]^+.) caused by loss of C-2 and C-3 as C₂H₄, as found for EI. It is concluded that at least in this system, decomposing molecular ions formed: (a) in the gas phase extremely close to the emitter and/or (b) on the emitter surface, have lifetimes sufficiently short to preclude complete H/D randomization. The results also provide evidence for common fragmentation mechanisms for heptanal molecular ions at both the low end and the high end of the energy distribution.     

Field ionization mass spectrometry—II: Benzyloxycarbonyl and t-butyloxycarbonyl derivatives of some simple peptides

A brief discussion is given of some factors that merit consideration in the analysis of peptides by mass spectrometry, with emphasis on sequence determination. The first systematic study of simple peptides by field ionization (FI) is described, and comparison made with the corresponding electron-impact (EI) ionization spectra, both at low resolution (approx. 1500). The substances examined were either benzyloxycarbonyl or t-butyloxycarbonyl derivatives of di-through pentapeptide methyl esters, containing the amino acids glycine, alanine, leucine, serine, threonine, proline and tyrosine. The incidences of sequence-characteristic cleavage peaks and rearrangement ion peaks were both slightly lower in the FI than the EI spectra, although the peak relative intensities generally were higher under FI conditions.     

Field-ionization mass spectrometry—fragmentation and metastables

Field-ionization (FI) mass spectrometry is an established method for obtaining abundant molecular ions¹ of a variety of compounds, and recently has been used in conjunction with exact mass measurements for determination of elemental compositions of molecules.^2,3 Very little, however, has been reported on the value of normal fragment ions and metastable ions appearing in the FI spectrum as a source of structural information.     

Negative ion chemical ionization mass spectrometry—binding of molecules to bromide and iodide anions

The analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro-methane (CF₂Br₂) and iodomethane (CH₃I)/methane (CH₄) as reagent gases is examined. The NICI mass spectrum of CF₂Br₂ contains Br^?, [HBr₂]^? and [CF₂Br₃]^? anions. Weak acids (i.e. those acids with approximately ΔH°_(acid) values between 1674 and 1464 kJ mol^?1) react with Br^? to produce minor yields of the hydrogen?bonded bromide attachment [MH + Br]^? anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°_(acid) > 1464 kJ mol^?1) produce primarily [MH + Br]^? anions with a minor yield of proton transfer [M ? H]^? anion. The NICI spectrum of CH₃I/CH₄ is dominated by I^?. Weak acids react with I^? to yield minor amounts of [MH + 1]^? or are unreactive. Strong acids produce only [MH + l]^? anions. From a consideration of the gas-phase basicity of the halide anion and the binding energy of the hydrogen-bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.     

Chemical ionization mass spectrometry studies—VII: Deuterium labeled decanes

CI mass spectra of the five isomeric vicinal d₂-decanes have been recorded using methane and d₄-methane as reagent gases. In contrast to earlier suggestions, we find that a large fraction of the alkyl fragment ions from n-decane are formed by elimination of olefins from the abundant [M – 1] ion. Only the C₉ and C₈ fragment ions are produced completely by a one-step reaction between the decanes and the methane reagent ions. Isotope exchange does not occur between the hydrocarbon and the reagent ions derived from d₄-methane but extensive scrambling of the deuterium label in the d₂-decanes does take place in the [M – 1] ion.     

Chemical ionization mass spectrometry with amines as reactant gases. V—Amine chemical ionization mass spectrometry of flavonoid glycosides

The chemical ionization mass spectra of flavonoid glycosides (O-glycosides, C-glycosides and acetylated glycosides) have been investigated. Triethylamine, ethylenediamine, diethylamine, methylamine and ammonia were used as reactant gases. The fragmentation mechanism is discussed, and the perspectives for establishing the molecular weights of glycosides, aid the nature of both the sugar residue and the aglycone, are outlined.     

Chemical ionization mass spectrometry with amines as reactant gases. I—amine chemical ionization mass spectrometry of flavonoid aglycones

Chemical ionization mass spectrometry of 34 flavones, isoflavones, flavanones, chalcones and aurones with aliphatic amines and ammonia as reactant gases have been investigated. Some unusual ions have been obtained and are discussed. This method can be used to determine the type of flavonoid and the location of some functional groups in the molecule.     

High resolution field desorption mass spectrometry—I: Nucleosides and nucleotides

The first complete high resolution (HR) field desorption (FD) mass spectra (MS) are presented and constitute a helpful tool in the structure elucidation of biologically significant compounds. This is especially relevant for the determination of the molecular weight of substances with very low volatilities which suffer from thermal decomposition when evaporated into the ion source. An additional advantage is the very small sample consumption (in the submicrogram range). Further improvements in the ion production method are the introduction of high temperature activated emitters and a specially designed micromanipulator for optimal adjustment of the field anodes. These enabled values of mass resolution between 15000 and 25000 (10% valley definition) to be achieved when vacuum evaporated Ag Br plates were used for photographic recording.     

Chemical ionization mass spectrometry with amines as reactant gases. VII—Amine chemical ionization mass spectrometry of some iridoid and secoiridoid glucosides

The chemical ionization mass spectra of iridoid glucosides of the loganin type and secoiridoid glucosides of the gentiopicroside type were investigated. Triethylamine, diethylamine, ethylenediamine and ammonia were used as reactant gases. The possibilities of establishing the relative molecular masses and the masses of both the sugar residue and the aglycone and some structural and stereochemical features are discussed.     

Native mass spectrometry—A valuable tool in structural biology

Proteins and the complexes they form with their ligands are the players of cellular action. Their function is directly linked with their structure making the structural analysis of protein‐ligand complexes essential. Classical techniques of structural biology include X‐ray crystallography, nuclear magnetic resonance spectroscopy and recently distinguished cryo‐electron microscopy. However, protein‐ligand complexes are often dynamic and heterogeneous and consequently challenging for these techniques. Alternative approaches are therefore needed and gained importance during the last decades. One alternative is native mass spectrometry, which is the analysis of intact protein complexes in the gas phase. To achieve this, sample preparation and instrument conditions have to be optimised. Native mass spectrometry then reveals stoichiometry, protein interactions and topology of protein assemblies. Advanced techniques such as ion mobility and high‐resolution mass spectrometry further add to the range of applications and deliver information on shape and microheterogeneity of the complexes. In this tutorial, we explain the basics of native mass spectrometry including sample requirements, instrument modifications and interpretation of native mass spectra. We further discuss the developments of native mass spectrometry and provide example spectra and applications.     

Negative ion mass spectrometry—I: The spectra of some stabilised alkylidenetriphenyl-phosphoranes

The 70 eV negative ion mass spectra of a series of phosphoranes exhibit significant peaks for ions resulting from skeletal rearrangements. The primary fragmentation reactions of the molecular anions, which involve reduction of the phosphorus from the penta- to the ter-valent state, have been rationalised in terms of the relative stabilities of the products formed. The processes occurring are in some cases analogous to and in other cases complementary to, those previously observed in the positive ion spectra of similar compounds.     

Low energy,low temperature mass spectra: I—selected derivatives of n-octane

The low voltage, low temperature mass spectra of a series of octane derivatives n-C₈H₁₇X with X=CH₃, OH, OCH₃, NH₂, NHCH₃, N(CH₃)₂, CO₂H, CO₂CH₃, CO₂C₂H₅, CHO and COCH₃ are reported and discussed, using arguments involving thermochemistry where appropriate. The structures of these compounds can be uniquely assigned on the basis of such mass spectra.     

Stereochemical applications of mass spectrometry. II—Chemical ionization mass spectra of isomeric dicarboxylic acids and derivatives

The H₂ and CH₄, chemical ionization mass spectra of the cis dicarboxylic acids, maleic and citraconic acid, show much more extensive loss of H₂O from [MH]⁺ than the trans isomers, fumaric acid and mesaconic acid. Similarly, esters of maleic acid show a much more facile loss of ROH (R=alkyl or phenyl) from [MH]⁺ than do esters of fumaric acid. Similar differences are observed in the chemical ionization mass spectra of the isomeric phthalic and isophthalic acids and derivatives, where the ortho isomers show more extensive fragmentation of [MH]⁺ than the meta isomers. The facile fragmentation of [MH]⁺ for the cis and ortho isomers is attributed to ROH elimination involving interaction between the two carboxylate functions and forming the stable cyclic anhydride structure for the fragment ion. By contrast ROH elimination from [MH]⁺ for the trans and metu isomers requires a symmetry-forbidden [1,3]-H migration in the carboxyl protonated species and cannot lead to the cyclic anhydride structure. The chemical ionization mass spectra of cis and trans cyclohexane-1,2-dicarboxylic acids are essentially identical and show extensive fragmentation of the [IMH]⁺ ion. Experiments using deuterium labelling show extensive carboxyl group interactions for both isomers. The chemical ionization mass spectra of maleanilic and phthalanilic acids and of the related anhydrides and imides also are reported, as are the electron impact mass spectra of diphenyl maleate, diphenyl fumarate, diphenyl phthalate, maleanilic acid and phthalanilic acid.     

Stereochemical effects in mass spectrometry: 2—Chemical ionization mass spectra of some cyclic glycols and mono- and di-saccharides using trimethyl borate as reagent gas

The chemical ionization mass spectra of cyclic glycols and mono- and di-saccharides using trimethyl borate as reagent gas have been studied. In the gas phase, the trimethyl borate ions react stereospecifically with molecules of cis-cyclic glycols to form characteristic ions, from which the stereochemical isomers of 1,2-cyclopentanediols, 1,2-cyclohexanediols and mono- and di-saccharides can be definitely distinguished.     

High resolution mass spectrometry—I. Some substituted δ-lactones

The mass spectra of some δ-lactones substituted in various positions by methyl groups have been studied both by accurate mass measurements at high resulution and by deuterium labelling. The loss of carbon dioxide has been shown to be important only for a mono-substituted lactone. A fragmentation mode common to all the lactones studied is the elimination of the ring oxygen atom, plus the adjacent carbon atom with its substituents, as a neutral carbonyl molecule such as formaldehyde, acetaldehyde or acetone. Deuterium labelling has uncovered the existence of a rearrangement involving hydrogen transfer in an even-electron ion, and a mechanism involving a six-membered transition state is proposed for this. The bond fissions, common to all the lactones, by which the principal ions in the mass spectra arise, are summarised.     

High resolution mass spectrometry—III: Loss of water from a nitro compound under electron-impact

Deuterium labelling has been utilised to elucidate the mechanism by which a molecule of water is eliminated from a heterocyclic nitro compound under electron-impact.     

High resolution mass spectrometry—V: Cyclic esters of aliphatic α-hydroxy acids

The main fragmentation sequences of glycollide and its homologues are initiated by fission of a CO? O bond, leading to the formation of fragment ions of low, m/e, such as [R¹CO]⁺ and [CR¹R²CCO]⁺. When a hydrogen atom is present on a ring carbon atom, 1,3 hydrogen migration occurs to produce [CHR²OH]⁺. In case where a ring carbon atom carries an alkylchain ? C₂H₅, a McLafferty rearrangement occurs with the adjacent carbonyl group. When both ring carbon atoms are dimethyl substituted, a 1,4 hydrogen migration must be invoked to account for the observed fragmentation sequence.