Stereospecific fragmentations in the mass spectra of stereoisomeric isoindoloquinazolines

The mass spectrometric behavior of stereo- and regioisomeric, partially saturated isoindoloquinazolines was studied by positive-ion electron ionization (EI) and fast-atom bombardment (FAB/LSIMS) mass spectrometry combined with collision-induced dissociation (CID). A highly stereospecific retro-Diels-Alder process was observed in the cyclohexene-fused isomers under the EI conditions, and a corresponding (although less specific) fragmentation was observed in their FAB spectra. In the absence of RDA fragmentations, regio- and stereoisomers of the cyclohexane-fused heterocycles could be distinguished based on their FAB/CID spectra.     

Chromatographic and mass spectral methods of identification for the side-chain and ring regioisomers of methylenedioxymethamphetamine

The popular drug of abuse 3,4-methylenedioxymethamphetamine (MDMA) is one of a total of 10 regioisomeric 2,3- and 3,4-methylenedioxyphenethylamines of MW 193 that yields regioisomeric fragment ions with equivalent mass (m/z 58 and 135/136) in the electron-impact (EI) mass spectrum. Thus, these 10 methylenedioxyphenethylamines are uniquely isomeric; they have the same molecular weight and equivalent major fragments in their mass spectra. The specific identification of one of these compounds (i.e., Ecstasy or 3,4-MDMA) in a forensic drug sample depends upon the analyst's ability to eliminate the other regioisomers as possible interfering or coeluting substances. This study reports the synthesis, chemical properties, spectral characterization, and chromatographic analysis of these 10 unique regioisomers. The ten 2,3- and 3,4-regioisomers of MDMA are synthesized from commercially available precursor chemicals. In the EI mass spectra, the side-chain regioisomers show some variation in the relative intensity of the major ions, with the exception of only one or two minor ions that might be considered side-chain specific fragments. The position of substitution for the methylenedioxy ring is not easily determined by mass spectral techniques, and the ultimate identification of any one of these amines with the elimination of the other nine must depend heavily upon chromatographic methods. The chromatographic separation of these 10 uniquely regioisomeric amines are studied using reversed-phase liquid chromatographic methods with gradient elution and gas chromatographic techniques with temperature program optimization.     

Mass spectral study of meso-alkyl and meso-cycloalkyl calix(4)pyrroles under electron impact conditions

A series of meso-cycloalkyl calix(4)pyrroles (I) and meso-dialkyl calix(4)pyrroles (II) has been studied under electron ionization (EI) mass spectral conditions. All the calix(4)pyrroles showed prominent molecular ions. The cleavage of the C--C bond linked at position 2 of the pyrrole ring (beta-cleavage) is the foremost and dominant fragmentation process. The beta-cleavage process, either through ring opening or directly, results in the loss of an alkyl radical from the molecular ion. The collision-induced dissociation (CID) spectra of I showed specific sequential expulsion of pyrrole and/or cycloalkyl rings from the molecular ion with or without hydrogen migrations, revealing more information on the structure of individual compounds than was available from the EI spectra. The isomeric cycloalkyl calix(4)pyrroles showed distinguishable CID spectra, indicating structure specificity in initial ring opening whereas, in the case of II, the EI mass spectrum contains all the structure-indicative fragment ions. The CID spectra of II resulted in a dominant [M-R]+ ion, with other characteristic ions being less abundant.     

Ionization and fragmentation of monochloro-isomers of 3-hydroxy-2-phenyl-4(1H)-quinolinone

Electron ionization (EI), methane chemical ionization (CI), and collision-induced dissociation (CID) mass spectra of complete series of positional monochloro-isomers of 3-hydroxy-2-phenyl-4(1H)-quinolinone are evaluated and discussed. It is shown that in the CI experiments, in addition to the protonated precursor molecules, odd-electron molecular ions are formed and this affects the appearance of the CID spectra. The influence of different direct probes and other experimental parameters such as the pressure of the reagent gas, isolation width, or collision energy was studied. EI, CI and CID spectra of the positional isomers show essentially the same fragmentation pathways but comparisons of the relative signal intensities of various product ions reveal some positional effects. Different isomers are also distinguished. The compounds can be divided into two groups using diagnostic ions (chloro substitution of the quinolinone moiety or the phenyl ring) or identified using a created spectral database. It was demonstrated that the reproducibility of the CID spectra is fully satisfactory for isomer identification, and that the created database can be applied for comparison of spectra measured over an extended time period (1 month) or spectra obtained during the direct analysis of a reaction mixture extract. Explanation of the fragmentation of the isomers is supported by exploratory density functional theory (DFT) calculations, e.g. rationalization of the relatively higher importance of the M(+.)-H(.)-Cl(.)-CO fragmentation pathway during EI than during CID, and vice versa for the pathway M(+.)-Cl(.)-CO. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Electron and chemical ionization mass spectrometry in stereochemical differentiation of some 1,3-amino alcohols

Mass spectral fragmentations of two cyclopentane, eight cyclohexane and four norbornane/one 1,3-amino alcohols were studied under electron ionization (EI) by low-resolution, high-resolution, metastable ion analysis and collision-induced dissociation (CID) techniques. All stereoisomeric compounds gave rise to identical 70 eV EI mass spectra. However, the spectra of positional isomers clearly differed. The main fragmentation pathway for the saturated compounds began as an α-cleavage reaction with respect to the nitrogen atom. For the norbornene compounds a retro-Diels—Alder reaction was favoured. Relative to the aminomethyl-substituted compounds the fragmentation patterns for the compounds having the amino group connected directly to the ring were more complicated. The chemical ionization (CI) mass spectra were recorded using ammonia, isobutane, methane, dichloromethane and acetone as reagent gas. From the norbornane/One compounds the di-exo isomers decomposed more easily than the di-endo isomers with most of the reagent gases used. Differences between stereoisomers were observed directly only under methane CI. The decomposition products of the [M + H]⁺ ions generated under ammonia and isobutane CI were studies by recording their CID mass spectra. These spectra allowed the differentiation of the stereoisomers, at least to some extent.     

Structure-specific fragmentations of 2,2,4-trisubstituted 1,2-dihydroquinazolines

Fragmentations of a series of 2,2,4-trisubstituted 1,2-dihydroquinazolines were studied by electron impact (EI) ionization and positive- and negative-ion fast atom bombardment (FAB) coupled with collision-induced decomposition (CID). The EI mass spectra give information that directly indicates the specific location of each substituent. The FAB–CID spectra provide data regarding the masses of each substituent, but no definitive information regarding location.     

Mass spectral fragmentation pathways in cyclic difluoramino and nitro compounds

The recently synthesized compounds 4, 4-bis(difluoramino)-1-nitropiperidine (I), 1,4,4-trinitropiperidine (II), 1,1,4,4-tetranitrocyclohexane (III), 1,1,4, 4-tetrakis(difluoramino)cyclohexane (IV) and 3,3,7, 7-tetrakis(difluora-mino)octahydro-1,5-dinitro-1,5-diazocine (HNFX, V) are being considered as potential energetic materials. The mass spectra of these compounds were studied using electron ionization (EI) mass spectrometry. A collision-induced dissociation (CID) study of the major EI peaks was carried out using a Finnigan TSQ 700 tandem mass spectrometer. The mass fragmentation pathways are constructed and discussed. The decomposition of HNFX (V), under EI, appeared to parallel the thermal decomposition of nitramines where N-NO(2) cleavage is often the first step. However, the two nitramines with a six-membered ring structure (I and II) underwent initial loss of a geminal substituent; loss of a nitramine nitro group was the secondary step. The two cyclohexane structures (III and IV) showed similar initial fragmentation pathways, featuring successive losses of nitro or difluoramino groups. Copyright 2000 John Wiley & Sons, Ltd.     

Mass spectra of tetrahydroisoquinoline-fused 1,3,2-O,N,P- and 1,2,3-O,S,N-heterocycles: influence of ring size and fusion, of present heteroatoms, substituent effects and of the stereochemistry on fragmentation

The electron ionization (EI) mass spectra of a variety of stereoisomeric tricyclic 1,3,2-oxazaphosphino[4,3-a]isoquinolines (1-4), 1,2,3-oxathiazino[4,3-a]isoquinoline-4-oxides (5-7) and the -4,4-dioxides (8-10) of oxazaphospholo- and oxathiazolo[4,3-a]- (11, 12, 15 and 16) and -[3,4-b]isoquinolines (13, 14 and 17) were recorded. Ring size and fusion, the different heteroatoms (P and S) and substituents on the ring systems strongly influence the mass spectra. In addition, mass spectra of the stereoisomers of compounds 1, 2 and 13, 14 revealed stereochemically relevant differences which are not observed for the other pairs of isomers.     

Chromatographic and mass spectral studies on methoxymethcathinones related to 3,4-methylenedioxymethamphetamine

The methoxymethcathinones are uniquely regioisomeric with the controlled drug substance 3,4-methylenedioxymethamphetamine (3,4-MDMA) or Ecstacy. The various isomeric forms of the methoxymethcathinones have mass spectra essentially equivalent to 3,4-MDMA. They all have a molecular weight of 193 and major fragment ions in their electron ionization mass spectra at m/z 58 and 135/136. Differentiation by mass spectrometry was only possible after formation of the perfluoroacyl derivatives, pentafluoropropionylamides (PFPA), and heptafluorobutrylamides (HFBA). Gas chromatographic separation on nonpolar stationary phases successfully resolved the three methcathinones from 2,3- and 3,4-MDMA as the PFPA and HFBA derivatives.     

GC-MS analysis of acylated derivatives of the side-chain regioisomers of 4-methoxy-3-methyl-phenethylamines related to methylenedioxymethamphetamine

The five side-chain regioisomers of 4-methoxy-3-methylphenethylamine constitute a unique set of compounds having an isobaric relationship with the controlled drug substance 3,4-methylenedioxymethamphetamine (3,4-MDMA or Ecstasy). These isomeric forms of the 4-methoxy-3-methylphenethylamines have mass spectra essentially equivalent to 3,4-MDMA, and all have a molecular weight of 193 and major fragment ions in their electron ionization mass spectra at m/z 58 and 135/136. Mass spectral differentiation of 2,3- and 3,4-MDMA from primary and secondary amine regioisomeric side chains of 4-methoxy-3-methylphenethylamines was possible after formation of the perfluoroacyl derivatives, pentafluoropropionamides and heptafluorobutyrylamides. The mass spectra for these derivatives are significantly individualized, and the resulting unique fragment ions allow for specific side-chain identification. The individualization is the result of fragmentation of the alkyl carbon-nitrogen bond, which yielded unique hydrocarbon fragments. The heptafluorobutyrylamide derivatives offer more fragment ions for molecular individualization among these regioisomeric substances. Gas chromatographic separation on relatively non-polar stationary phases successfully resolves these derivatives.     

Mass spectra of new substituted 2-amino-4H-pyrans: a retro-Diels-Alder reaction pattern

New substituted 2-amino-3-cyano-4H-pyrans have been studied by electron ionization (EI), chemical ionization (CI) and electrospray ionization (ESI) mass spectrometry. The retro-Diels-Alder reaction (RDA) is the main fragmentation pattern observed in the EI spectra forming an unsaturated ketone as the diene fragment. In contrast, a different RDA reaction takes place yielding an unsaturated amide as diene fragment together with the unsaturated ketone in the CI spectra. The MS/MS spectra obtained using an ESI source reveal that the favoured fragmentation by collision induced dissociation (CID) is the elimination of the substituent at the C4 position with formation of a stable pyrilium cation.     

用高分辨质谱研究二茂铁类化合物

研究了八种N－二茂铁甲基苯胺衍生物的电子电离质谱（EI－MS），并与快原子轰击质谱（FAB－MS）作了比较，讨论了这些化合物的质谱特征。通过精确质量测量，确认了M^ -2 OH离子，并采用碰撞诱导裂解（CID）探讨了化合物质谱中主要离子间的关系。     

Analysis of polycyclic aromatic hydrocarbons by ion trap tandem mass spectrometry

An ion-trap mass spectrometer with a wave board and tandem mass spectrometry software was used to analyze gas chromatographically separated polycyclic aromatic hydrocarbons (PAHs) by using collision-induced dissociation (CID). The nonresonant (multiple collision) mode was used to determine the conditions for CID ionization of 18 PAHs. Unlike in electron impact (EI) analysis, the relative abundances of progeny ions of isomers were statistically different (using Student’s t-test) in CID analysis, thus making isomer identification by CID possible. For comparison, CID and EI were applied to the analysis of used motor oil. CID analysis was shown to be more sensitive than EI analysis of the used motor oil. Precision at the 10-ppb level for EI and CID showed relative standard deviations of 5. 2 and 7. 7%, respectively.     

Investigation of some pentasubstituted 1,4-dihydropyridines by means of fast ion bombardment,chemical ionization and electron impact mass spectrometry

Positive fast ion bombardment, positive chemical ionization (CI⁺) and positive electron impact (EI) ionization mass spectrometry were used to investigate a number of relatively large and structurally related organic molecules. Some of the major dissociation pathways observed in the CH₄-CI⁺ mass spectra are not present under NH₃-CI⁺ conditions, but are obtained in the collision-induced dissociation (CID) spectrum of the 50 eV MH⁺ molecular ion, formed in the latter reaction. The resemblance between the EI mass spectra and their fast ion bombardment counterparts, the effect of changing the energy of the bombarding Cs⁺ ion beam over the range 2–16 keV and the different degrees of internal excitation of ions formed in different CI reagent gases are discussed.     

An isomer-specific high-energy collision-induced dissociation MS/MS database for forensic applications: a proof-of-concept on chemical warfare agent markers

Spectra database search has become the most popular technique for the identification of unknown chemicals, minimizing the need for authentic reference chemicals. In the present study, an isomer‐specific high‐energy collision‐induced dissociation (CID) MS/MS spectra database of 12 isomeric O‐hexyl methylphosphonic acids (degradation markers of nerve agents) was created. Phosphonate anions were produced by the electrospray ionization of phosphonic acids or negative‐ion chemical ionization of their fluorinated derivatives and were analysed in a hybrid magnetic‐sector–time‐of‐flight tandem mass spectrometer. A centre‐of‐mass energy (E_com) of 65 eV led to an optimal sequential carbon–carbon bond breakage, which was interpreted in terms of charge remote fragmentation. The proposed mechanism is discussed in comparison with the routinely used low‐energy CID MS/MS. Even‐mass (odd‐electron) charge remote fragmentation ion series were diagnostic of the O‐alkyl chain structure and can be used to interpret unknown spectra. Together with the odd‐mass ion series, they formed highly reproducible, isomer‐specific spectra that gave significantly higher database matches and probability factors (by 1.5 times) than did the EI MS spectra of the trimethylsilyl derivatives of the same isomers. In addition, ionization by negative‐ion chemical ionization and electrospray ionization resulted in similar spectra, which further highlights the general potential of the high‐energy CID MS/MS technique. Copyright © 2011 John Wiley & Sons, Ltd.     

Mass spectral studies of N,N-dialkylaminoethanols

Some dialkylaminoethanols, precursors of chemical warfare agents such as V-agents and nitrogen mustards, were analyzed by electron impact (EI) and electrospray ionization (ESI) mass spectrometry. The fragmentation pathways in EI and ESI-MS/MS methods are rationalized. The collision-induced dissociation (CID) spectra of [M+H](+) ions of aminoethanols in ESI mode are clearly distinguishable from one another, including those of isomeric normal and branched chain dialkylaminoethanols. Structures can be proposed based on the general fragmentation pathways of these molecules.     

Elucidation of urinary metabolites of fluoxymesterone by liquid chromatography-tandem mass spectrometry and gas chromatography-mass spectrometry

The suitability of liquid chromatography tandem mass spectrometry (LC-MS/MS) and gas chromatography mass spectrometry (GC-MS) for the elucidation of fluoxymesterone metabolism has been evaluated. Electrospray ionization (ESI) and collision induced dissociation (CID) fragmentation in LC-MS/MS and electron impact spectra (EI) in GC-MS have been studied for fluoxymesterone and two commercially available metabolites. MS(n) experiments and accurate mass measurements performed by an ion-trap analyser and a QTOF instrument respectively have been used for the elucidation of the fragmentation pathway. The neutral loss scan of 20 Da (loss of HF) in LC-MS/MS has been applied for the selective detection of fluoxymesterone metabolites. In a positive fluoxymesterone doping control sample, 9 different analytes have been detected including the parent compound. Seven of these metabolites were also confirmed by GC-MS including 5 previously unreported metabolites. On the basis of the ionization, the CID fragmentation, the accurate mass of the product ions and the EI spectra of these analytes, a tentative elucidation as well as a proposal for the metabolic pathway of fluoxymesterone has been suggested. The presence of these compounds has also been confirmed by the analysis of five other positive fluoxymesterone urine samples.     

GC-MS analysis of acylated derivatives of a series of side chain regioisomers of 2-methoxy-4-methyl-phenethylamines

Five side chain regioisomers of 2-methoxy-4-methylphenethylamine constitute a unique set of compounds having an isobaric relationship with the controlled drug substance 3,4-methylenedioxymethamphetamine (3,4-MDMA or Ecstasy). These isomeric forms of the 2-methoxy-4-methyl-phenethylamines have mass spectra essentially equivalent to 3,4-MDMA; all have molecular weight of 193 and major fragment ions in their electron ionization mass spectra at m/z 58 and 135/136. Mass spectral differentiation of 2,3 and 3,4-MDMA from primary and secondary amine regioisomeric side chains of 2-methoxy-4-methyl-phenethylamines was possible after formation of the perfluoroacyl derivatives, pentafluoropropionamides (PFPA) and heptafluorobutyrylamides (HFBA). The mass spectra for these derivatives are individualized and the resulting unique fragment ions allow for specific side-chain identification. The individualization is the result of fragmentation of the alkyl carbon-nitrogen bond yielding unique hydrocarbon fragments of varying mass. Gas chromatographic separation on relatively non-polar stationary phases gave essentially base line resolution for these compounds.     

Study of two diastereomeric pairs of regiosomeric 2-isoxazolines by tandem mass spectrometry with electron impact ionization

On electron impact (EI) ionization, two cis/trans pairs of 4-methyl-5-phenyl and 4-phenyl-5-methyl regioisomeric 3-carbethoxy-2-isoxazolines showed normal mass spectra and mass-analysed ion kinetic energy (MIKE) spectra of metastable (MI) and collision-activated (CA) molecule ions, allowing unequivocal differentiation of the regioisomers. The cis/trans stereoisomers of each regioisomer showed very similar normal mass spectra. Very interestingly, the cis- and trans-4-phenyl-5-methyl stereoisomers appeared reasonably differentiated by the molecule ion MIKE spectra, whereas the 4-methyl-5-phenyl regioisomeric pair of stereoisomers did not. The influence of the phenyl substituent to the fragmentation processes was notable. Some fragments of interest were studied by comparison of their MIKE spectra with those of model ions, generated by EI from suitable substrates, including (i) the isomeric α,β-unsaturated oxime, namely ethyl (Z)-2-(hydroxymino)-3-methyl-4-phenylbut-3-enoate, a by-product of importance for the mechanism(s) of the addition/cycloaddition reactions of nitrile oxides to alkenes and (ii) trans-β-methylstyrene, a dipolarophilic reactant in the same reactions. The favoured heterocyclic C(5)–O(1) bond cleavage occurred only for the ionized 4-methyl-5-phenyl 2-isoxazoline pair, leading to a distonic ion of relevance, as it can represent either a reasonable precursor for both the isomerization to the ionized α,β-unsaturated oxime and the EI-induced cycloreversion yielding ionized β-methylstyrene, or the ionized form of a zwitterionic intermediate, which had been proposed previously for the addition/cycloaddition mechanism(s) in the solution phase, currently under study.     

Cyclization of ortho-cyclopropylphenyl benzamides in gas and liquid phases

The electron ionization (EI) and collision-induced dissociation (CID) spectra of substituted N-(ortho-cyclopropylphenyl)benzamides 1-7 and N-[ortho-(1-methylcyclopropyl)phenyl]benzamides 8-12 were recorded. In addition to routine bond cleavages, the molecular ions (M+) of 1-12 undergo cyclization into the corresponding 3-aryl-1-alkyl-1-ethyl-1H-benzoxazines and isomeric 5-ethyl-2-oxodibenzoazepines. The presence of a methyl group in the cyclopropyl ring (compounds 8-12) makes the formation of 5-ethyl-2-oxodibenzoazepine less favorable. In accord with mass spectrometric predictions, compound 13 (3-p-tolyl-1-ethyl-1H-benzoxazine) was obtained as a major product of the reaction of N-(ortho-cyclopropylphenyl)-4-methylbenzamide 1 with sulfuric acid. Traces of 5-ethyl-2-oxodibenzoazepine were also detected in gas chromatography-mass spectrometry (GC-MS) analysis of the reaction mixture although the yield was too low to allow its isolation.