Ortho,Effects in organic molecules on electron impact: X—unusual ortho effects of the methyl group in o-picolinotoluidide

The mass spectrum of o-picolinotoluidide gives rise to three major fragments at m/z 184, m/z 169 and m/z 168, corresponding to the loss of CO from the molecular ion followed by the loss of ?H₂ and ?H₃ by independent pathways. It has been shown that the ortho methyl group and the nitrogen of the pyridine ring in the 2-position are involved in the formation of these three major fragments observed in the mass spectrum of o-picolinotoluidide. The mass spectrum of 2-(o-toluidino) pyridine, the molecular ion of which can resemble the [M? CO]⁺ ion in o-picolinotoluidide, also shows loss of CH₃ and NH₂ radicals from the molecular ions. Based on these observations coupled with the high resolution data, the mass analysed ion kinetic energy spectrometry and high voltage scans of these fragments in both the compounds, two mechanistic pathways have been proposed for the formation of these ions in o-picolinotoluidide.     

Isotope abundance analysis for improved sample identification with tandem mass spectrometry

Tandem mass spectrometry (MS/MS) is widely used for trace level sample analysis in complex mixtures. However, sample identification in MS/MS is challenging and not as trustworthy as with electron ionization (EI) mass spectral libraries. This paper presents a novel method for the combination of isotope abundance analysis (IAA) and EI‐MS/MS for improved sample identification even at trace level in complex matrices. Accordingly, the first quadrupole is scanned in a narrow range around the molecular ion group of isotopomers such as M⁺, [M+1]⁺ and [M+2]⁺, Q2 serves for collision‐induced dissociation to produce product ions while Q3 transfers the major sample product ions with low resolution, thus encompassing and uniformly transmitting all the product ion isotopomers. IAA can then be used to derive elemental formula information from the cleansed experimental data. IAA‐MS/MS was experimentally tested with perfluorotributylamine and a very good matching factor of 995 (out of 1000) was obtained for IAA on m/z 502, 503 and 504 (fragment ion isotopomers) while Q3 transmitted the m/z 264 product ion with a mass window of 6 m/z units. The IAA‐MS/MS method was further tested with the pesticide diazinon on its molecular ions m/z 304, 305 and 306 while Q3 was locked on its m/z 179 product ion with a mass window of 6 m/z units. Again, very good matching factors were obtained, even for 40 pg diazinon on‐column during its GC/MS analysis (match = 981). IAA‐MS/MS combines the traditional benefits of MS/MS in the removal of matrix interferences with the IAA power of elemental analysis. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass spectra of 2-thioacylaminothiazoles, 2-thioacylaminobenzothiazoles and their ‘fixed structure’ amino and imino tautomeric forms

The fragmentation patterns of the thioacyl derivatives of 2-aminothiazole and 2-aminobenzothiazole and of their ‘fixed structure’ imino and amino tautomeric forms give evidence of the predominance of the imino tautomer in the molecular ions of the trifluorothioacetyl compounds. On the other hand, the M⁺˙ ions of the thioacetyl and thiobenzoyl derivatives are mainly the amino tautomers. A rearrangement with elimination of RCN and formation of the 2-thiazolothione (2-benzothiazolothione) ion is characteristic for ail the compounds investigated. Additional evidence for the interpretation of the main fragmentation paths is provided by the mass spectra of the N-trideuteromethyl and N-deuterated derivatives, high-resolution mass spectrometry being used to determine the elemental composition of some selected peaks.     

Structural characterization of pregnane glycosides from Cynanchum auriculatum by liquid chromatography on a hybrid ion trap time‐of‐flight mass spectrometer

A method coupling high‐performance liquid chromatography with hybrid ion trap time‐of‐flight mass spectrometry (TOFMS) using an electrospray ionization source was firstly used to characterize ten major pregnane glycosides including one novel compound auriculoside IV from the roots of Cynanchum auriculatum Royle ex Wight. In the MS/MS spectra, fragmentation reactions of the [M+Na]⁺ were recorded to provide abundant structural information on the aglycone and glycosyl moieties. Experiments using TOFMS allowed us to obtain precise elemental compositions of molecular ions and subsequent product ions with errors less than 6 ppm. The pregnane glycosides in C. auriculatum were classified into two major core groups: one is caudatin characterized by the neutral loss of one ikemamic acid molecule (128 Da) from the precursor ion, and the other is kidjoranin characterized by the neutral loss of cinnamic acid (148 Da) from the precursor ion. Meanwhile, a series of sugar‐chain fragment ions provided valuable information about the compositions of the sugar residues and the sequences of the sugar chain. Logical fragmentation pathways for pregnane glycosides have been proposed and are useful for the identification of these compounds in natural products especially when there are no reference compounds available. Copyright © 2009 John Wiley & Sons, Ltd.     

Formation and reactivity of metal carbonyl anions in the gas phase

The reactions of metal carbonyl anions (M(CO)_n^?; M = Cr, Mn and Fe; n = 1–3) with n-heptane, water and methanol were studied with use of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an external ion source. The M(CO)_n^? ions were formed in the FT-ICR cell by collision-induced dissociation of the most abundant primary ion generated by electron impact of the appropriate metal carbonyl compound present in the external ion source. The M(CO)_n^? ions were allowed subsequently to undergo non-reactive collisions with argon in order to remove possible excess internal/translational energy prior to the ion/molecule reaction. Only the Cr(CO)₃^?, Mn(CO)₃^? and Fe(CO)₂^? ions react with n-heptane. This reaction proceeds by loss of H₂ from the collision complex and the Cr(CO)₃^? and Fe(CO)₂^? ions react about three times more efficiently than the Mn(CO)₃^? ion. With water, Mn(CO)^? and Fe(CO)₃^? are unreactive, whereas the other ions react by loss of one or two CO molecules from the collision complex. The rate of the reaction with water decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^?, Fe(CO)^?, Mn(CO)₃^? and Mn(CO)₂^?. With methanol, the Cr(CO)₂^? ion reacts by loss of two CO molecules from the collision complex, whereas loss of one CO molecule and elimination of CO + H₂ occur in the reaction with Cr(CO)₃^?. Competing loss of CO and one or two H₂ molecules occurs in the reactions of Mn(CO)₃^? and Fe(CO)₂^? with methanol. The rate of the reaction with methanol decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^? and Mn(CO)₃^?.     

Elemental compositions from daughter ion spectra of m1+ and [m1 + 1]+: Some applications of the method

The elemental compositions of ions can be determined in tandem mass spectrometry by comparing the daughter ion spectra of the m₁⁺ and [m₁ + 1]⁺ ions. The method is demonstrated for mass-analyzed ion kinetic energy spectra but is applicable to all types of daughter ion spectra, including complex collisionally activated dissociation spectra. In this work, the method is applied to compounds that produce daughter ions of known elemental compositions, and the errors and limitations are evaluated. Following that test, the procedure is applied to a compound that may produce daughters of more than one possible elemental composition. The method is sometimes useful even if the formula of the parent is not known; that is, the formulae of unknown parent and daughter ions may be found. Locating a specific atom in an isotopically labeled molecule is another capability of the method. The basic equation of the method was generalized and incorporated into a computer program for performing the calculations.     

Electron impact ionization mass spectra of some substituted dipyrido[1,2-a:4,3-d]pyrimidinones

The electron impact ionization mass spectra of dipyrido [1,2-a:4,3-d]pyrimidinones are strongly influenced by the degree of aromaticity of the fused rings. The molecular ions of the compounds are fairly stable. The main routes of fragmentation involve formation of the [M – H]⁺ ion and loss of substituents, H₂CN˙, CO and alkyl radicals.     

CID of singly charged antioxidants applied in lubricants by means of a 3D ion trap and a linear ion trap-Orbitrap mass spectrometer

The aim of this study was to investigate the fragmentation behavior induced by low‐energy collision‐induced dissociation (LE‐CID) of four selected antioxidants applied in lubricants, by two different types of ion trap mass spectrometers: a three‐dimensional ion trap (3D‐IT) and a linear IT (LIT) Orbitrap MS. Two sterically hindered phenols and two aromatic amines were selected as model compounds representing different antioxidant classes and were characterized by positive‐ion electrospray ionization (ESI) and LE‐CID. Various types of molecular ions (e.g. [M]^+?, [M + H]⁺, [M + NH₄]⁺ or [M + Na]⁺) were used as precursor ions generating a significant number of structurally relevant product ions. Furthermore, the phenolic compounds were analyzed by negative‐ion ESI. For both IT types applied for fragmentation, the antioxidants exhibited the same unusual LE‐CID behavior: (1) they formed stable radical product ions and (2) C? C bond cleavages of aliphatic substituents were observed and their respective cleavage sites depended on the precursor ion selected. This fragmentation provided information on the type of structural isomer usually not obtainable for branched aliphatic substituents utilizing LE‐CID. Comparing the two instruments, the main benefit of applying the LIT‐Orbitrap was direct access to elemental composition of product ions enabling unambiguous interpretation of fragmentation trees not obtainable by the 3D‐IT device (e.g. loss of isobaric neutrals). It should be emphasized that the types of product ions formed do not depend on the type of IT analyzer applied. For characterizing degradation products of antioxidants, the LIT‐Orbitrap hybrid system, allowing the determination of accurate m/z values for product ions, is the method of choice. Copyright © 2011 John Wiley & Sons, Ltd.     

Detection of negative ions in glow discharge mass spectrometry for analysis of solid specimens

A new method is presented for elemental and molecular analysis of halogen-containing samples by glow discharge time-of-flight mass spectrometry, consisting of detection of negative ions from a pulsed RF glow discharge in argon. Analyte signals are mainly extracted from the afterglow regime of the discharge, where the cross section for electron attachment increases. The formation of negative ions from sputtering of metals and metal oxides is compared with that for positive ions. It is shown that the negative ion signals of F^? and TaO₂F^? are enhanced relative to positive ion signals and can be used to study the distribution of a tantalum fluoride layer within the anodized tantala layer. Further, comparison is made with data obtained using glow-discharge optical emission spectroscopy, where elemental fluorine can only be detected using a neon plasma. The ionization mechanisms responsible for the formation of negative ions in glow discharge time-of-flight mass spectrometry are briefly discussed.     

Studies in negative ion mass spectrometry: XIV—Electron attachment reactions of a series of η6-arenetricarbonylchromium(O) complexes

Electron attachment reactions of a series of (η⁶-arene)tricarbonylchromium(O) complexes have been examined in the gas phase. The electron capture process has been shown to be influenced by the structure of the η⁶-arene ligand and its substituents. Whereas (η⁶-benzene)- and (η⁶-mesitylene)tricarbonylchromium(O) undergo dissocative electron capture, or reductive decarbonylation, yielding [M? CO]^?˙ ions of highest abundance in their negative ion mass spectra, [η⁶-(2,2-dimethylindan-1,3-dione)]tricarbonylchromium(O) forms a molecular negative ion which undergoes sequential CO eliminations and finally a demetallation to give the arene radical ion. A localization of charge on the coordinated arene ligand is proposed for the formation of [M]^?˙ in this case. (η⁶-Methylbenzoate)tricarbonylchromium(O) also forms a molecular negative ion by secondary electron attachment which decomposes by simultaneous and consecutive eliminations of up to four CO molecules. The elucidation of a mechanism and sequence for these CO eliminations has been achieved by synthesizing and examining the negative ion mass spectrum of [η⁶-(C₆H₅·¹³CO₂Me)]Cr(CO)₃. The first CO loss in the principal fragmentation pathway occurs solely from the –Cr(CO)₃ group of [M]^?˙. The effect of para substituent groups on the stabilities of molecular negative ions and their fragmentations has been ascertained using a series of para-substituted (η⁶-methylbenzoate)tricarbonylchromium(O) compounds containing the groups NH₂, OH, OCH₃, CL and COOMe. The stabilities of the [M]^?˙ ions have been rationalized in terms of the Hammett and Taft parameters σ_P, σ_I, σ_R^P, σ_P^O and σ_R^O. The overall electronic substituent effect transmitted to the carbonyl groups of the –Cr(CO)₃ unit involves both resonance and inductive components. In this series of compounds the stability of [M]^?˙ decreases as the electron withdrawing capacities of the para substituents increase.     

Structure determination of some C4H5N+˙ Ions from substituted pyridines by collisionally activated dissociation mass spectrometry

A standard procedure for recording and correcting collisionally activated dissociation mass spectra is proposed, and used to distinguish between the C₄H₅N^+˙ ions formed from hydroxy- and amino-pyridines after loss of CO and HCN, respectively. It is concluded that these ions are cyclic. From the 4-isomers the 3H-pyrrole ion is formed whereas from 2-hydroxypyridine the 1H-pyrrole ion is formed. In the other cases, mixtures of 2H- and either 1H- or 3H-pyrrole ions are generated, depending on the nature of the precursor.     

Origin of the [C4H9O]+ ions in the mass spectrum of n-butyl ethyl ether

The mechanisms of formation of m/z 73 ions in the mass spectrum of the ionized title compound were investigated by deuterium substitution and by examining the decompositions of metastable ions. Two routes to the [C₄H₉O]⁺ ions were found in the normal spectrum. The ethyl lost by the major pathway contains the α- and β-hydrogens and a γ-hydrogen from the butyl group. The minor route involves the loss of ethylene from the [M? H]⁺ ion. There were metastable peaks for losses of ethyl, ethanol and methyl from the molecular ion. The ethyl contains the α- and β-methylenes and a γ-hydrogen, while the methyl is the δ-methyl of the butyl group. The labeling data rule out a previous mechanistic proposal for the loss of ethyl and support a mechanism involving stepwise isomerization to the sec-butyl ethyl ether molecular ion. However, the metastable ion chemistries of the molecular ions from the n- and sec-butyl ethyl ethers are highly dissimilar, perhaps due to decompositions from different electronic states. The n-pentyl methyl ether ions loses both ethyl and propyl, apparently following rearrangements to the 3-pentyl and 2-pentyl ether ions. Di n-butyl and n-butyl methyl ethers also give metastable peaks for loss of methyl, ethyl and the shorter chain alcohol.     

甾体磷酰胺类缀合物在电喷雾质谱中的裂解特征

Novel steroidal phosphoramidate conjugates of 3'-azido-2',3'-dideoxythymidine(AZT)and amino acid esterswere synthesized and determined by positive and negative ion electrospray ionization mass spectrometry.The MSfragmentation behaviors of the steroidal phosphoramidate conjugates have been investigated in conjunction withtandem mass spectrometry of ESI-MS/MS.There were three characteristic fragment ions in the positive ion ESImass spectra,which were the Na adduct ions with loss of steroidal moiety,amino acid ester moiety from pseudomolecular ion(M Na)~ ,and the phosphoamino acid methyl ester Na adduct ion by α-cleavage of the phosphora-midate respectively.The main fragment ions in negative ion ESI mass spectra were the ion(M-HN_3)~-,the ion(M-AZT-H)~-,and the ion(M-steroidal moiety-H)~- besides the pseudo molecular ion(M-H)~-.Thefragmentation patterns did not depend on the attached amino acid ester moiety.     

On the formation and decomposition of the [M ? HNO]+ ion from o-nitrobenzaldoxime

The molecular ion of o-nitrobenzaldoxime appears to eliminate a molecule of HNO, the latter as shown by deuterium labelling containing the hydroxyl hydrogen atom. This reaction may be followed either by loss of CO2 or C2O2 (2 × CO), which requires complicated skeletal rearrangements. To determine the structure of the [M ? HNO]+ ion, the kinetic energies, released in the loss of CO2 and of C2O2 from this ion, have been measured. Similar measurements have been made on the appropriate metastable ions from o-nitrosobenzaldehyde, 2,1-benzisoxazoline-3-one, 2-benzoxazolinone and 3-hydroxy-1,2-benzisoxazole, whose molecular ions have the same elemental composition as the [M ? HNO]+ ion from o-nitrobenzaldoxime. In the loss of HNO from the molecular ions of o-nitrobenzaldoxime o-nitrosobenzaldehyde molecular ions are generated first, which then, at least upon decomposition, rearrange further to 2,1-benzisoxazoline-3-one molecular ions. Part of the latter finally rearrange to 2-benzoxazolinone molecular ions, which eliminate CO via two routes. One of these corresponds to a low energy process and the other to a higher energy path whose excess energy induces the subsequent rapid loss of a second CO molecule. The observed effect of the rate-determining isomerizations of the molecular ions of o-nitrobenzaldoxime, o-nitrosobenzaldehyde, 2,1-benzisoxazoline-3-one and 3-hydroxy-1,2-benzisoxazole to 2-benzoxazolinone on the intensity and shape of the metastable peak for the low energy process for CO expulsion from the molecular ions of 2-benzoxazolinone supports the proposed rearrangements.     

A detailed analysis of the complete hydrogen and carbon scrambling and fragmantation reactions of boron-containing cationic species in the mass spectrometer

A detailed analysis of the mass spectral behavior of trialkylboranes and deuterium and ¹³C labeled tri-n-butylborane, and comparison with the ion cyclotron resonance behavior, have revealed several unique features and herefore unrecognized rearrangement and fragmantation reactions:(1)the occurrence of intial fragmentation of the parent ion with apparently nearly exclusive loss of one complete alkyl radical; (2) the extensive formation of boron-containing spiecies (>% of the total ion current)in the mass spectrometer relative to the virtual absence (<5%) of such spieces in the ion cyclotron resonance spectrum; (3) the dominant formation of boron-containing species of the composition [CⁿH²ⁿ⁺²B]⁺ with [C²H⁶B]+ generally being the base peak; (4) the formation of appreciable quantities of alkane molecular ions (5 to 10% of the total ion current) up to [R²]+. (from R³B); (5) the characterization of a fragmentation reaction involving the loss of CH²; (6) the observation that very extensive hydrogen and carbon scrmbling occurs in the ions formed after the initial fragmentation of the parent ion. A mechanism which satisfactorily accounts for the fragmentation and rearrangement reactions is proposed which involves the reversible formation and rearrangement of protonated boracyclopropanes and cyclopropanes formed by cationic insertions in ß-C? H bonds.     

Electron impact mass spectra of chlorinated methyl propanoates

The mass spectral fragmentations of all eleven chlorinated methyl propanoates have been studied. Deuterium labelling and metastable ion analysis were used to elucidate the fragmentation mechanism. The molecular ion peaks of all compounds are small, except methyl 3,3-dichloropanoate (38%). In most cases α-cleavage gives the base peak [COOCH₃]⁺, and the loss of a chlorine atom from the molecular ion is characteristic of the 3-chloro, 3,3-dichloro and 3,3,3-trichloro compounds. Metastable ions showed the losses of small neutral molecules such as CH₃OH, CH₂CO, CO₂ and CO from the [M? Cl]⁺ ion. α-Cleavage and the loss of Cl^˙ gives an intense [M? COOCH₃? Cl]^+˙ peak, which is the base peak in the spectra of the 2,3-dichloro and 2,3,3-trichloro compounds.     

Mass spectral fragmentation of benzofurazan-1-oxide

Fragmentations in the mass spectrum of benzofurazan-1-oxide have been studied using linked scan, accelerating voltage scan and mass-analysed ion kinetic energy spectrometric techniques. Major pathways involve NO·+ NO· and NO·+CO loss, these double losses occurring in such rapid succession as to appear ‘concerted’ in some experiments. Minor pathways are loss of CO₂, C₂N₂O₂, or C₂HN₂O₂ from the molecular ion. The major fragment ion, m/z 76, in the conventional mass spectrum is not detected in a mass-analysed ion kinetic energy spectrometric experiment with the molecular ion until collision activation is provided. The conventional electron impact spectrum invariably includes ions from benzofurazan which is produced by thermal deoxygenation in the source.     

Unimolecular rearrangements of ketene-O,O-acetals and fragmentations occurring in the gas phase

Gas phase skeletal rearrangements of regioisomeric 3‐cyano‐2‐methoxy‐3a‐alkylfuro[2,3‐b]‐ and [3,2‐b]indoles were evidenced by product ions [M ? 32]^+?, consistent with loss of methanol, on electron ionization in their mass spectra. The rearranged products occurring in gas phase were demonstrated to have elemental composition and fragmentation properties identical to those of authentic samples of 2‐indolyl cyanomalonates. Isotopic labeling experiments support the formation mechanism of the [M ? 32]^+? ion. Additional thermal gas‐phase reaction products were characterized by comparison with an authentic sample. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of fragmentation pathways of lithiated α,β‐unsaturated thioesters by electrospray ionization mass spectrometry

The fragmentation pathways of lithiated α,β‐unsaturated thioesters with different substituents were investigated by electrospray ionization tandem mass spectrometry (ESI‐MS/MS) in positive ion mode. In mass spectrometry of the α,β‐unsaturated thioesters, Ar‐CH?CH‐CO‐S‐Ph, loss of PhSLi and elimination of a thiophenol were the two major fragmentation reactions of the lithiated molecules. The elemental compositions of all the ions were confirmed by high‐resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR‐MS/MS). The thioesters studied here were para‐monosubstituted on the phenyl ring of cinnamoyl and the electron‐withdrawing groups favored loss of a thiophenol, whereas the electron‐releasing groups strongly favored the competing reaction leading to the loss of PhSLi to form a cinnamoyl cation, Ar‐CH?CHCO⁺. The intensity ratios of the two competitive product ions were well correlated with the σ substituent constants. The mechanisms of these two competing routes were further investigated by density functional theory (DFT) calculations. Copyright © 2010 John Wiley & Sons, Ltd.