A laser desorption fourier transform mass spectrometric study of dimethyl 8-acetyl-3,7,12,17-tetramethylporphyrin-2,18-dipropanoate

Laser desorption Fourier transform ion cyclotron resonance positive- and negative-ion mass spectra are presented for dimethyl 8-acetyl-3,7,12,17-tetramethylporphyrin-2,18-dipropanoate. The 248-nm laser ionization thresholds for both positive and negative ions are observed to be about 2.5 MW cm^?2. The M⁺˙ molecular ion is assigned to the base peak in the low-power spectra whereas it is the M^?˙ ion for the corresponding anion spectra. Increased intensities of [M + H]⁺ and [M ? H]^? are observed with increased laser fluences of up to 38 MW cm^?2. At high laser powers the negative-ion results reveal that a series of carbon-nitrogen cumulene and polyacetylene cluster ions are formed. Laser evaporation/multiphoton ionization/ and thermal evaporation/electron impact ionization/collision-induced dissociation experiments carried out on the porphyrin M⁺˙ and [M + H]⁺ ions over a range of translational kinetic energies and delay times after acceleration are compared and used to obtain mechanistic and structural information. In contrast to the electron impact experiments, which show only side-chain cleavage, the laser-based collision-induced dissociation experiments reveal that, in addition to side-chain cleavage, it is possible to cleave the porphyrin ring to various extents depending on the ion translational energy selected.     

Characterization of synthetic N-acetylcysteine conjugates by positive- and negative-ion 252Cf plasma desorption mass spectrometry

N-Acetylcysteine and nine N-acetylcysteine conjugates of synthetic origin were characterized by positive- and negative-ion plasma desorption mass Spectrometry. For sample preparation the electrospray technique and the nitrocellulose spin deposition technique were applied. The fragmentation of these compounds, which are best seen as S-substituted desaminoglycylcysteine dipeptides, shows a similar behaviour to that of linear peptides. In the positive-ion mass spectra intense protonated molecular ion peaks are observed. In addition, several sequence-specific fragment ions (A⁺, B⁺, [Y + 2H]⁺, Z⁺), immonium ions (I⁺) and a diagnostic fragment ion for mercap-turic acids (R_M⁺) are detected. The negative-ion mass spectra exhibit deprotonated molecular ions and in contrast only one fragment ion corresponding to side-chain specific cleavage ([R_XS]^?) representing the xenobiotic moiety. In the case of a low alkali metal concentration on the target, cluster molecular ions of the [nM + H]⁺ or [nM - H]^? ion type (n = 1-3) are observed. The analysis of an equimolar mixture of eight N-acetylcysteine conjugates shows different quasi-molecular ion yields for the positive- and negative-ion spectra.     

Differentiation among the four diastereomers of benzyloxycarbonyl-protected γ-hydroxyornithine in negative-ion fast atom bombardment mass spectrometry

Discrimination among the four γ-hydroxyornithine diastereomers was studied by fast atom bombardment mass spectrometry (FABMS). It is impossible to distinguish among the four diastereomers of this amino acid by positive- and negative-ion FAB and collisionally activated dissociation MS, but benzyloxycarbonyl group protection of the α- and δ-amino groups in γ-hydroxyornithine allows differentiation among the diastereomers in negative-ion FABMS. The negative-ion mass spectra of benzyloxycarbonyl-protected γ-hydroxyornithine diastereomers showed differences among the abundances of the molecule ion [M – H]^-, the dehydrated ion [M — H — H₂O]^- due to the loss of the γ-hydroxyl group and the fragment ions formed from both [M — H]^- and [M — H — H₂O]^- ions. On the other hand, no difference was found between the fragmentations of the benzyloxycarbonyl-protected enantiomers of ornithine in negative-ion FABMS. These results indicate that the orientation of the γ-hydroxyl group and the existence of two benzene rings in the benzyloxycarbonyl group are important factors which are responsible for the fragmentations of the four benzyloxycarbonyl-protected γ-hydroxyornithine diastereomers in negative-ion FABMS. These studies also showed that the negative-ion FABMS for benzyloxycarbonyl-protected γ-hydroxyornithine diastereomers is a useful method for determining the configuration of each diastereomer of γ-hydroxyornithine.     

Competition in the formation of ion–neutral complexes. Expulsion of methane from the molecular ion of acetamide

An ion–neutral complex is a non-covalently bonded aggregate of an ion with one or more neutral molecules in which at least one of the partners rotates freely (or nearly so) in all directions. A density-of-states model is described, which calculates the proportion of ion–neutral complex formation that ought to accompany simple bond cleavages of molecular ions. Application of this model to the published mass spectrum of acetamide predicts the occurrence of ions that have not hitherto been reported. Relative intensities on the order of 0.1 (where the abundance of the most intense fragment ion = 1) ere predicted for [M – HO]⁺ and [M – CH₄]⁺˙ ions, which have the same nominal masses as the prominent [M – NH₃]⁺˙ and [M – NH₂]⁺ fragments. High-resolution mass spectrometric experiments confirm the presence of the predicted fragment ions. The [M – HO]⁺ and [M – CH₄]⁺˙ fragments were observed with relative abundances of 0.02 and 0.04, respectively. Differences between theory and experiment may be ascribed to effects of competing distonic ion pathways.     

Formation of an unusual MH− ion in the methane negative-ion chemical ionization mass spectra of chlorprothixene and aromatic sulfur compounds

The methane negative-ion chemical ionization (NCI) mass spectrum of chlorprothixene shows an unusual MH^? ion. This ion can be accounted for by electron capture followed by H˙ transfer from the reagent gas. The most probable site of electron attachment was concluded to be related to the sulfur atom of the thioxanthene ring based on the observation of analogous ions for structurally related compounds, all containing a heterocyclic sulfur. The MH^? ion observed with methane as the reagent gas was shifted to MD^? when tetradeuteromethane was used in place of methane. The ratio of [M ? H]^? to MH^? did not change with emission current suggesting that the process is independent of the radical concentration in the CI plasma. Consistent with this observation is the lack of CH₃˙ or C₂H₅˙ adduct ions in the NCI mass spectrum and the fact that gold-plating the ion source did not decrease the proportion of MH^?. Also, this mechanism is consistent with thermochemical considerations of reactions of a phenyl radical with various alkanes and observations of ions formed by methane NCI from model compounds. Therefore, unlike other MH^? ions observed in methane NCI mass spectra, the mechanism of formation does not appear to involve a hydrogen radical addition followed by electron capture.     

Analysis of amino acid mixtures by plasma desorption mass spectrometry

Plasma desorption mass spectrometry (PDMS) was investigated as a means of analysing mixtures of three, four and five amino acids in both positive- and negative-ion modes. Fifteen mixtures were tested; each mixture contained equimolar amounts of selected amino acids. The PD mass spectra exhibited MH⁺ and [M – H]^? molecular ions for all the aminoacids with different desorption–ionization yields. The spectra were more easily interpreted in the negative- than the positive-ion mode. The desorption order of the amino acids was progressively established by comparing the molecular ion desorption–ionization yields for each mixture. This desorption order was well correlated in both the positive- and negation-ion modes with the acid–base thermodynamic data for the amino acids in the gas phase. This observation gives some insight into the desorption–ionization mechanisms under PDMS conditions.     

Structural characterization of isomeric triazolocoumarins by high- and low-energy collision spectrometry of M+˙, [M + H]+ and [M − H]− species

Two monometayl- and four dimethyl-triazolocoumarin isomers were characterized by their electron impact mass spectra and by low-energy collision experiments performed on molecular ions M⁺˙ and other fragment ions with an ion-trap mass spectrometer. High-energy collision-activated dissociation measurements were performed on the protonated [M + H]⁺ and deprotonated [M ? H]^? molecular ion obtained by fast atom bombardment and M⁺˙ species produced by electron impact ionization on a double-focusing, reverse-geometry instrument. The data obtained allowed unequivocal structural identification of all the compounds investigated.     

Isomeric [C,H3,N,O]+˙ ions and their neutral counterparts

The isomeric ions [H₂NC(H)O]⁺˙, [H₂NCOH]⁺˙, [H₃CNO]⁺˙ and [H₂CNOH]⁺˙ were examined in the gas phase by mass spectrometry. Ab initio molecular orbital theory was used to calculate the relative stabilities of [H₂NC(H)O]⁺˙, [H₂NCOH]⁺˙, [H₃NCO]⁺˙ and their neutral counterparts. Theory predicted [H₂NC(H)O]⁺˙ to be the most stable ion. [H₂NCOH]⁺˙ ions were generated via a 1,4-hydrogen transfer in [H₂NC(O)OCH₃]⁺˙, [H₂NC(O)C(O)OH]⁺˙ and [H₂NC(O)CH₂CH₃]⁺˙. Its metastable dissociation takes place via [H₃NCO]⁺˙ with the isomerization as the rate-determining step. [H₂CNOH]⁺˙ undergoes a rate-determining isomerization into [H₃CNO]⁺˙ prior to metastable fragmentation. Neutralization-reionization mass spectrometry was used to identify the neutral counterparts of these [H₃,C,N,O]⁺˙ ions as stable species in the gas phase. The ion [H₃NCO]⁺˙ was not independently generated in these experiments; its neutral counterpart was predicted by theory to be only weakly bound.     

Ion-molecule reactions observed for nitroaromatic compounds in negative ion fast atom bombardment mass spectrometry

Analyses of a series of nitroaromatic compounds using fast atom bombardment (FAB) mass spectrometry are discussed. An interesting ion-molecule reaction leading to [M + O ? H]^? ions is observed in the negative ion FAB spectra. Evidence from linked-scan and collision-induced dissociation spectra proved that [M + O ? H]^? ions are produced by the following reaction: M + NO₂^? → [M + NO₂]^? → [M + O ? H]^?. These experiments also showed that M^?˙ ions are produced in part by the exchange of an electron between M and NO₂^? species. All samples showed M^?˙, [M ? H]^? or both ions in their negative ion FAB spectra. Not all analytes studied showed either [M + H]⁺ and/or M⁺˙ in the positive ion FAB spectra. No M⁺˙ ions were observed for ions having ionization energies above ～9 eV.     

A fast atom bombardment and field ionization/field desorption study of some isomeric unsaturated dicarboxylic acids

Geometrically isomeric dicarboxylic acids, such as maleic and fumaric acid and their methyl homologues, and the isomeric phthalic acids, have been investigated using fast atom bombardment, field ionization and field desorption mass spectrometry. The most intense peak in the positive ion fast atom bombardment spectra corresponds with the [M + H]⁺ ion. This ion, when derived from the E -acids, tragments either by successive loss of water and carbon monoxide or by elimination of carbon dioxide. In the case of the Z -acids only elimination of water from the [M + H]⁺ ions is observed to occur to a significant extent. The same is true for the [M + H]⁺ ions of the isomeric phthalic acids, that is the [M + H] ions derived from iso- and terephthalic acid exhibit more fragmentation than those of phthalic acid. All these acids undergo much less fragmentation upon field ionization, where not only abundant [M + H]⁺ ions, but also abundant [M]^+· ions, are observed. Upon field desorption only the [M + H]⁺ and [M + Na]⁺ ions are observed under the measuring conditions. Negative ion fast atom bombardment spectra of the acids mentioned have also been recorded. In addition to the most abundant [M? H]^? ions relatively intense peaks are observed, which correspond with the [M]^?˙ ions. The fragmentations observed for these ions appear to be quite different from those reported in an earlier electron impact study and in a recent atmospheric pressure ionization investigation.     

Hydrogen transfer in the retro diels–alder fragmentation of oxygen-containing heterocyclic compounds. II—Chromones

Mass Spectra of unsubstituted, 2-methyl-, 3-methyl and 2,3-dimethylchromones were examined. These compounds showed [RDA]⁺˙ and [RDA + H]⁺ ions as characteristc ions, together with [M? H]⁺,[M? CO]⁺˙,[M? CHO]⁺ and [RDA? CO]⁺˙ ions. Based on deuterium labelling experiments and measurement of metastable peaks by the ion kinetic energy defocusing technique, the origin of transferred hydrogen in the [RDA + H]⁺ ion was clarified. The mechanism of the [RDA + H]⁺ ion formation is discussed.     

Use of fast atom bombardment mass spectrometry for the characterization of nitroaromatic isomers

Positive and negative ion fast atom bombardment (FAB) mass spectra of some monosubstituted nitroaromatic isomers are reported. Generally ions carresponding to [M + H]⁺ and M⁺ are observed in the positive ion FAB spectra; ions such as [M ? H] ^? and M^?˙ are observed in the negative ion FAB spectra. The use of FAB mass spectra to distinguish the isomers is discussed. Comparisons of FAB, chemical ionization and electron impact mass spectra of the same isomers (wherever possible) are reported. The structural information obtained in the negative ion FAB spectra complement those obtained in the positive ion FAB spectra.     

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.     

Usefulness of field desorption mass spectrometry in determining molecular masses of carotenoids,natural carotenoid derivatives and their chemical derivatives

Field desorption (FD) mass spectrometry was applied to the determination of the molecular masses of carotenoids, natural carotenoid derivatives and their chemical derivatives. All the carotenoids examined gave the molecular ion as the base peak with negligible fragment ions. Carotenoid glucoside and its fatty acid monoester were successfully determined without acetylation, whereas carotenoic acids (carboxylate and sulphate) needed to be converted into methyl esters prior to analysis. The applicable ranges of molecular masses and polarity were very wide. In addition, carotenoid glycoside gave only [M]⁺˙ without [M + H]⁺˙ and [M + cation]⁺˙. The numbers of carbonyl groups, primary and/or secondary hydroxyl groups and total hydroxyl groups could be directly determined according to the increase in mass units of the carotenoids after chemical reduction, acetylation and trimethylsilylation, respectively. Owing to the negligible fragment ions, FD analysis was also suitable for carotenoids containing small amounts of impurities or other carotenoids. Hence this technique is useful for determining the molecular masses of carotenoids and the number of modifiable groups in carotenoids.     

Losses of isobaric neutral species from molecular ions of isomeric nitroanisoles

Alternative losses of the isobaric neutral species CH₂O and NO˙ have been assessed for molecular ions of isomeric nitroanisoles fragmenting in the ion source and the first field free region of a double focusing mass spectrometer. Mass analyses of the primary fragment ions indicate that specific loss of CH₂O occurs from molecular ions of 2-nitroanisole, while specific loss of NO˙ occurs from molecular ions of 3-nitroanisole. Although the peak due to [M? NO]⁺ ions is negligible in the mass spectrum of 2-nitroanisole, evidence is presented to show that they are transient intermediates in the consecutive fragmentation for loss of the elements of CNO₂ from the molecular ions.     

Electron-impact studies—LV: Skeletal rearrangement and hydrogen scrambling processes in the positive and negative-ion mass spectra of phenyl derivatives of elements of group IV and V

The mass spectra of many triphenyl/tetraphenyl derivatives of the Group IV and V elements exhibit the processes [M⁺˙ ? C₁₂H₁₀] and/or [M⁺˙ ? C₆H₅· ? C₁₂H₁₀]. These fragmentations are not preceded by hydrogen scrambling between all the phenyl rings. Hydrogen scrambling does occur in certain fragment ions prior to fragmentation in both the positive and negative-ion spectra. The process [M⁺˙ ? C₁₂H₁₀] occurs in the negative-ion mass spectrum of tetraphenylsilane.     

Influence of Amino Acid Composition and Phosphorylation on the Ion Yields of Peptides in MALDI-MS

The influence of arginine (Arg), lysine (Lys), and phenylalanine (Phe) residues and phosphorylation on the molecular ion yields of model peptides have been quantitatively studied using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry in both positive- and negative-ion mode. The results obtained from these experiments have been interpreted from the standpoint of two different components, namely, desorption and ionization, on the basis of the physicochemical properties of constituent amino acids of the model peptides. The presence of basic residues such as Arg and Lys enhanced the ion yields of protonated molecules [M + H]⁺. An N-terminal rather than a C-terminal Arg residue was advantageous for the formation of both [M + H]⁺ and [M – H]^–. The presence of the Phe residue resulted in the increase of the ion yields of both [M + H]⁺ and [M – H]^–. In contrast, the presence of phosphate group(s) contributed to the suppression of the yields of both [M + H]⁺ and [M – H]^– due to the loss of phosphate group. The detection limits for both [M + H]⁺ and [M – H]^– of model peptides have been evaluated.     

Charge localization and aromatic stabilization in large ring ions: Mass spectra of ms-porphyrins

The major mass spectrometric fragments of ms-tetraphenylporphin and ms-tetra(p-chloro)phenylporphin are [M ? H]⁺˙ and [M ? Cl]⁺˙, respectively. Metal derivatives of these compounds give a modified characteristic fragmentation pattern with peak groups ending in the ions [M ? 4H]⁺˙, [M ? ? ? 5H]⁺˙ and [M ? 2? ? 2H]⁺˙ for the metallo ms-tetraphenylporphins, and [M ? ?Cl ? 2Cl ? 3H]⁺˙ and [M ? 2?Cl ? Cl ? H]⁺˙ for Mgms-tetra(p-chloro)phenylporphin. Deuterated metal derivatives indicate random hydrogen loss from both phenyl and pyrrole carbons. However, metal substituents do not significantly modify the fragmentation pattern in the case of ms-tetra(p-methoxy)phenylporphin. These patterns can be explained in terms of aromatic stabilization of the fragmentation products, coupled with charge localization on the π system in the free base, on the metal atom in the metallo derivatives and on the methoxy function in the p-methoxyphenyl derivative.     

Chemical ionization and collisional activation spectra of α,β-unsaturated nitriles

A study of the chemical ionization (CI) and collisional activation (CA) spectra of a number of α, β-unsaturated nitriles has revealed that the even-electron ions such as [MH]⁺ and [MNH₄]⁺ produced under chemical ionization undergo decomposition by radical losses also. This results in the formation of M ⁺˙ ions from both [MH]⁺ and [MNH₄]⁺ ions. In the halogenated molecules losses of X˙ and HX compete with losses of H˙ and HCN. Elimination of X˙ from [MH]⁺ is highly favoured in the bromoderivative. The dinitriles undergo a substitution reaction in which one of the CN groups is replaced with a hydrogen radical and the resulting mononitrile is ionized leading to [M ? CN + 2H]⁺ under CI(CH₄) or [M ? CN + H + NH₄] and [M ? CN + H + N₂H₇]⁺ under CI(NH₃) conditions.     

Characterization of C8H10 alkylbenzenes by negative ion mass spectrometry

The O^?˙ chemical ionization mass spectrri of the C₈H₁₀ alkylbenzenes, o-, m-. andp -xylene and ethylbenzene, show formation of [M ? H + O]^?, [M ? H]^?, [M ? H₂]^?˙ and, for the xylenes, [M ? CH₃ + O]^? as primary reaction products; the relative importance of these products depends on the isomer. However, [OH]^? is a primary product from reaction of O^?˙ with both the C₈H₁₀ isomers and hydrogen-containing impurities; [OH]^? reacts further with the alkylbenzenes to produce [M ? H]^? with the result that the chemical ionization mass spectra depend on experimental conditions such as sample size and the presence of impurities. The collision-induced charge inversion mass spectra of the [M ? H + O]^? and [M ? H]^? products allow only distinction of ethylbenzene from the xylenes. However, the collision-induced charge inversion mass spectra of the [M ? H₂]^?˙ ions show differences which allow identification of each isomer.