Formation of M+. ions under matrix fast atom bombardment conditions: Does charge exchange reaction occur?

The formation of molecular ions, M^+., under fast atom bombardment (FAB) conditions using a liquid matrix was examined by using a new type of synthesized compounds in which preferential M^+. peaks appear in their FAB spectra. The FAB spectra were compared with the corresponding mass spectra obtained by the electron impact (EI) ionization, chemical ionization (CI) and charge-exchange ionization (CEI) methods. All of the spectra showed preferential peaks of M^+. ion and a characteristic intense fragment ion peak originating from a β-fission. The FAB spectra were similar in the fragment ions appearing in the EI spectra and were very similar in the fragmentation pattern to the CEI spectra using Ar^+. and Xe^+. as the reagent ions. Further, the FAB spectra did not show any doubly charged ion peaks, while the 70 eV EI spectra showed the peaks of doubly charged molecular and/or fragment ions. The isobutane CI spectra of the synthesized compounds suggested that the formation of M^+. ions occurred through the CE reaction with isobutane ion, C₄H₁₀^+., and the CI spectra showed a marked intense fragment ion peak originating from the β-fission which seemed to occur characteristically in CEI processes. The results obtained suggested that the formation of M^+. ions under matrix FAB conditions occurred mainly by CE reactions between the analytes M and matrix molecular ions B^+. and/or fragment ions b^+..     

Development of a new electron ionization/field ionization ion source for gas chromatography/time‐of‐flight mass spectrometry

We have developed a combined EI/FI source for gas chromatography/orthogonal acceleration time‐of‐flight mass spectrometry (GC/oaTOFMS). In general, EI (electron ionization) and FI (field ionization) mass spectra are complementary: the EI mass spectrum contains information about fragment ions, while the FI mass spectrum contains information about molecular ions. Thus, the comparative study of EI and FI mass spectra is useful for GC/MS analyses. Unlike the conventional ion sources for FI and EI measurements, the newly developed source can be used for both measurements without breaking the ion source vacuum or changing the ion source. Therefore, the combined EI/FI source is more preferable than the conventional EI or FI ion source from the viewpoint of the reliability of measurements and facility of operation. Using the combined EI/FI source, the complementarity between EI and FI mass spectra is demonstrated experimentally with n‐hexadecane (100 pg): characteristic fragment ions for the n‐alkane such as m/z 43, 57, 71, and 85 are obtained in the EI mass spectrum, while only the parent peak of m/z 226 (M⁺) without any fragment ions is observed in the FI mass spectrum. Moreover, the field desorption (FD) measurement is also demonstrated with poly(ethylene glycol)s M600 (10 ng) and M1000 (15 ng). Signals of [M+H]⁺, [M+Na]⁺ and [M+K]⁺ are clearly detected in the FD mass spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass spectra of organolanthanide complexes

The EI mass spectra of seven 1,1′-(3-oxa-pentamethylene)-dicyclopentadienyl lanthanide and yttrium chlorides ( 1–7 ), and eleven dicyclopentadienyl lanthanide and yttrium chlorides ( 8–18 ) were investigated. Fragmentation patterns of these complexes were studied by using metastable ion measurements. The EI spectra of complexes 1–7 exhibited strong molecular ion peaks, the fragmentation of molecular ions were more complicated. The EI mass spectra of complexes 8–18 supported the dimeric structure under gaseous state. In the spectra of dimers (C₅H₅)₃LN⁺ observed were resulted from the skeletal rearrangement involving the migration of cyclopentadienyl moiety.     

Relative molecular mass information from aliphatic nitro compounds: A chemical ionization study

The mass spectra of a number of aliphatic nitro compounds have been studied using electron Ionization (EI) and a variety of chemical Ionization (CI) techniques in attempts to obtain relative molecular mass information. The use of positive ion ammonia chemical Ionization techniques gave very satisfactory results, providing abundant [M + NH4]⁺ ions, not only from both primary and secondary nitro compounds, but also from the much more labile tertiary nitro compounds. However, the use of methane and isobutane positive ion CI or EI conditions resulted in facile fragmentation with little relative molecular mass information being made available. Negative ion CI using methane, isobutane or ammonia as moderating gases all gave abundant [M ? 1]^? ions with primary and secondary nitro compounds but at much reduced sensitivity.     

Mass spectra of arylhydroxyoximes and their complexes with transition metals

The EI mass spectra of several arylhydroxyoximes and of their coordinated complexes with transition metals, Cu, Ni, Co and Fe have been studied using high resolution mass spectrometry, metastable ion measurements and the stable isotopes ¹⁵N, ⁶³Cu and ⁶⁵Cu labelling to elucidate the fragmentation pathways. It has been found that the spectra of all compounds 1–24 except Fe-complex 24 show molecular ions. The relative intensity of molecular ions of the complexes, which contained the same kind of metal and the arylhydroxyoximes with different substituents R , is closely related to the electronegativity of R. In all metal-complexes 9–22 , the main decomposition pathways involve the successive loss of O, NO and then H₂O or LH. In these spectra the fragment ions of non-metal- containing species resulting from the cleavage of the dioxime ligands can be discriminated.     

Field desorption mass spectrometry of amino acids

The mass spectra of some amino acids have been studied using the field desorption method. All amino acids yield molecular or quasi-molecular ions, even in the case of arginine and cystine, where these ions cannot be detected with the electron-impact (EI) or chemical ionization (CI) methods. The fragmentation is reduced as compared to EI, CI and FI.     

Comparative studies on photo-ionization and electron-impact ionization of peptide derivatives

Photo-ionization (PI) mass spectrometry performed with a monochromatic photon beam was applied to a series of peptide derivatives. PI mass spectra of ten N-acylpeptide methyl esters containing two to four residues of glycine, alanine, valine, leucine, proline, tryptophan, tyrosine, phenylalanine, methionine, carboxymethylcysteine, lysine and ornithine were studied. Comparative analysis of PI (10.2 eV) and electron-impact ionization (EI) (70 eV) mass spectra shows the total number of peaks on PI to be much less than that obtained with EI, especially in the low m/e region (< 250 to 300). At the same time the relative abundance of ‘heavy’ ions, including molecular ions, is much higher in PI. The amino acid fragmentation pattern followed by N-acylpeptide esters in PI was found to be the same as for EI.     

Electron impact,electron capture negative ionization and positive chemical ionization mass spectra of organophosphorus flame retardants and plasticizers

Phosphate esters are important commercial products that have been used both as flame retardants and as plasticizers. To analyze these compounds by gas chromatographic mass spectrometry, it is important to understand the mass spectra of these compounds using various ionization modes. This paper is a systematic overview of the electron impact (EI), electron capture negative ionization (ECNI) and positive chemical ionization (PCI) mass spectra of 13 organophosphate esters. These data are useful for developing and optimizing analytical measurements. The EI spectra of these 13 compounds are dominated by ions such as H₄PO₄⁺, (M ? Cl)⁺, (M ? CH₂Cl)⁺ or (M)⁺ depending on specific chemical structures. The ECNI spectra are generally dominated by (M ? R)^?. The PCI spectra are mainly dominated by the protonated molecular ion (M + H)⁺. The branching of the alkyl substituents, the halogenation of the substituents and, for aromatic phosphate esters, ortho alkylation of the ring are all significant factors controlling the details of the fragmentation processes. EI provides the best sensitivity for the quantitative measurement of these compounds, but PCI and ECNI both have considerable qualitative selectivity. Copyright © 2013 John Wiley & Sons, Ltd.     

Cluster chemical ionization and deuterium exchange mass spectrometry in supersonic molecular Beams

A cluster-based chemical ionization method has been developed that produces protonated molecular ions from molecules introduced through a supersonic molecular beam interface. Mixed clusters of the analyte and a clustering agent (water or methanol) are produced in the expansion region of the beam, and are subsequently ionized by “fly through” electron impact (EI) ionization, which results in a mass spectrum that is a combination of protonated molecular ion peaks together with the conventional EI fragmentation pattern. The technique is presented and discussed as a tool complementary to electron impact ionization in supersonic molecular beams. Surface-induced dissociation on a rhenium oxide surface is also applied to simplify the mass spectra of clusters and reveal the analyte spectrum. The high gas flow rates involved with the supersonic molecular beam interface that enable the easy introduction of the clustering agents also have been used to introduce deuterating agents. An easy-to-use, fast, and routine on-line deuterium exchange method was developed to exchange active hydrogens (NH, OH). This method, combined with electron impact ionization, is demonstrated and discussed in terms of the unique information available through the EI fragmentation patterns, its ability to help in isomer identification, and possible applications with fast gas chromatography-mass spectrometry in supersonic molecular beams.     

Electron ionisation mass spectral studies of bridgehead‐fused Δ2‐norbornanethiazolines

The electron ionisation (EI) mass spectra of a series of bridgehead‐fused Δ²‐norbornanethiazolines, a new class of bridgehead‐norbornane derivatives, have been studied and their cleavage mechanisms rationalised on the basis of the substituent shifts as well as on the identification of relevant peaks through accurate mass measurements and collision‐induced dissociation tandem mass spectrometric experiments. The fragmentation patterns of isomeric pairs of 6,6‐ and 10,10‐dimethylnorbornanethiazolines are almost identical, probably due to an initial isomerisation of molecular ion previous to the fragmentation. In general, the dominant peaks in the spectra of all the studied compounds originate from initial α‐cleavages of C(5)–C(6) or C(1)–C(10) bonds, followed by concomitant homolytic cleavage of C(1)–C(9) and C(7)–C(10) bonds. The driving force for this fragmentation pathway, directed by the gem‐dimethyl group, is the formation of a highly stabilised thiazolilmethyl cation which constitutes the base peak in all the spectra and allows the identification of these interesting ligands. Copyright © 2009 John Wiley & Sons, Ltd.     

Electron ionization and electrospray mass spectra of diaryl-substituted enaminoketones and their thio analogs

Electron ionization (EI) and positive electrospray ionization (ESI) mass spectra of selected diaryl enaminoketones and enaminothiones have been studied. In the EI mass spectra of both classes of compound, molecular ion peaks are accompanied by the peaks corresponding to the [M-H](+) ions. The formation of these ions can be rationalized by a cyclization reaction resulting in the formation of the respective isoxazolium and isothiazolium cations. Under positive ESI conditions, in the spectra recorded for the enaminoketones peaks corresponding to the [M+H](+), [M+Na](+) and [2M+Na](+) ions appeared, while in the spectra recorded for the enaminothiones, peaks corresponding to the [M-H](+) ions were dominant. These ions are most likely formed by oxidation of the neutral enaminothione molecules on the surface of the positively charged stainless steel capillary in the ESI ion source (anodic oxidation).     

Field desorption mass spectra of flavonoid acylglycosides. I. Natural acetyl derivatives

The field desorption mass spectra (FD spectra) of 17 natural flavonoid acetylglycosides have been studied. In the spectra of each of the 0-monoglycosides the molecular ion (M or M + H) appears as the main peak and it is accompanied by the ions of the aglycone (A or A + H) and of the acylated anhydrosugar (S). The intensity of the latter peak is largely connected with the structure of the substance. In the FD spectra of flavone 0-biosides, fragment S is absent but its mass can be calculated from the difference (M – A). Useful information for establishing the position of the acetyl group is given by the fragments S₁ and (M – S₁) corresponding to the detachment of the terminal sugar residue. The FD spectra of flavone C-glycosides differ greatly from the spectra of the 0-glycosides: In them the main peak is that of the ion (M), but peaks (A) and (S) are absent and the ions present resemble the fragmentation of the C-glycosides under the action of electron impact.All-Union Scientific-Institute of Medicinal Plants, Moscow. L. Ya. Karpov Scientific-Research Institute of Physical Chemistry, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 573–582, September–October, 1984.     

A comparison of electron ionization mass spectra obtained at 70 eV,low electron energies,and with cold EI and their NIST library identification probabilities

Electron ionization (EI) mass spectra of 46 compounds from several different compound classes were measured. Their molecular ion abundances were compared as obtained with 70‐eV EI, with low eV EI (such as 14 eV), and with EI mass spectra of vibrationally cold molecules in supersonic molecular beams (Cold EI). We further compared these mass spectra in their National Institute of Standards and Technology (NIST) library identification probabilities. We found that

1. Low eV EI is not a soft ionization method, and it has little or no influence on the molecular ion relative abundances for large molecules and those with weak or no molecular ions.
2. Low eV EI for compounds with abundant or dominant molecular ions in their 70 eV mass spectra results in the reduction of low mass fragment ions abundances thereby reducing their NIST library identification probabilities thus rarely justifies its use in real‐world applications.
3. Cold EI significantly enhances the relative abundance of the molecular ions particularly for large compounds; yet, it retains the low mass fragment ions; hence, Cold EI mass spectra can be effectively identified by the NIST library.
4. Different standard EI ion sources provide different 70 eV EI mass spectra. Among the Agilent technologies ion sources, the “Extractor” exhibits relatively abundant molecular ions compared with the “Inert” ion source, while the “High efficiency source” (HES) provides mass spectra with depleted molecular ions compared with the “Inert” ion source or NIST library mass spectra.

These conclusions are demonstrated and supported by experimental data in nine figures and two tables.     

Mass-spectrometric studies of new 6-nitroquipazines—serotonin transporter inhibitors

Six synthesized 6-nitroquipazine derivatives were examined by electron ionization (EI) and electrospray ionization (ESI) mass spectrometry in positive and negative ion mode. The compounds exhibit high affinity for the serotonin transporter (SERT) and belong to a new class of SERT inhibitors. The EI mass spectra registered in negative ion mode showed prominent molecular ions for all the compounds studied. All EI mass spectra and all ESI mass spectra showed similar fragmentation pathways of molecular ions, but the pathways differed between EI and ESI. The differences were explained with the aid of theoretical evaluation of the stability of the respective radical ions (EI MS) and protonated ions (ESI MS).     

Fragmentation and rearrangement reactions in certain 1,2,3-triaryl-2-propen-1-ones following electron impact and chemical ionization

The electron impact (EI) and chemical ionization (CI) mass spectra of certain 1,2,3-triaryl-2-propen-1-ones (TAPs) have been studied in detail with the help of exact mass measurements, deuterium labelling and metastable data. The E- and Z-isomeric pairs do not show any difference in their behaviour under EI or CH₄ CI conditions. EI-induced rearangement reactions in the TAPs include aryl migration to carbonium ion centres. A study of the metastable transitions reveals aryl group interchange in the molecular ions prior to fragmentation. Under EI conditions loss of arene involves either C(2) or C(3) aryl groups while under CI conditions the C(1) aryl is lost as a neutral arene molecule. Mechanisms for the different fragmentation modes are given.     

Study of the fragmentation pattern of ketamine‐heptafluorobutyramide by gas chromatography/electron ionization mass spectrometry

Ketamine is an anaesthetic compound used in human and veterinary medicine with hallucinogen properties that have resulted in its increased illicit use by teenagers at rave parties. Although several gas chromatography/mass spectrometry (GC/MS) methods have been reported for the quantification of the drug both in urine and in hair, its electron ionization (EI) fragmentation after derivatization with different reagents has been not yet fully investigated. The present work reports the study of the fragmentation of ketamine, derivatized with heptafluorobutyric anhydride (HFBA‐Ket), using gas chromatography/electron ionization mass spectrometry (GC/EI‐MS). The complete characterization of the fragmentation pattern represented an intriguing exercise and required tandem mass spectrometry (MSⁿ) experiments, high‐resolution accurate mass measurements and the use of deuterated d⁴‐ketamine to corroborate the proposed structures and to characterize the fragment ions carrying the unchanged aromatic moiety. Extensive fragmentation was observed, mainly located at the cyclohexanone ring followed by rearrangement of the fragment ions, as confirmed by the mass spectra obtained from the deuterated molecule. The GC/EI‐MS analysis of HFBA‐Ket will represent a useful tool in forensic science since high‐throughput analyses are enabled, preserving both the GC stationary phase and the cleanliness of the mass spectrometer ion optics. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of gem-difluoropropargyl synthons through HF loss from protonated molecules in methane chemical ionization mass spectra

Electron impact and methane chemical ionization mass spectra were obtained following gas chromatography/mass spectrometry for several gem-difluoropropargyl compounds, which had been synthesized as potential intermediates for synthesis of gem-difluoromethylene-containing C-3 acetylenes. EI spectra were variable with respect to the presentation of molecular ions, depending on substituent functional groups present. Methane-CI spectra were characterized by loss of 19 mass units from molecular weight with all compounds examined. These [M − 19]⁺ ions often presented as base peaks of the CI spectra, and were more reliably present and abundant than [M + 1]⁺ ions for these compounds. These ions could have been formed by elimination of HF from the protonated molecules under conditions of methane chemical ionization.     

Positive chemical ionization mass spectra of polycyclic chlorinated pesticides

Methane chemical ionization (CI) mass spectra for a series of ten polycyclic chlorinated insecticides and metabolites have been examined. In all cases except heptachlor epoxide the base peak corresponded to elimination of Cl, or OH from the molecule ion. In the spectrum of heptachlor epoxide the [M + H]⁺ and [M ? Cl]⁺ clusters were of approximately equal intensity. The CI spectra were remarkably simple, invariably less complex than the corresponding electron-impact (EI) mass spectra and the intensity of the ions with high information content, e.g. [M ? CI]⁺ was uniformly high. All of these features are important to the analytical potential of these studies. Retro Diels-Alder (RDA) fragments were observed for the chlordanes, aldrin, isodrin, nonachlor and heptachlor epoxide. The reported preliminary data suggest that the relative intensity of RDA ions in CI mass spectra may be useful in establishing molecular configurations.     

Mass spectral studies on aryl-substituted N-carbamoyl/N-thiocarbamoyl narcotine and related compounds

Positive ion mass spectral fragmentation of new N-carbamoyl/N-thiocarbamoyl derivatives of narcotine and compounds closely related to it are reported and discussed. The techniques used include electron impact (EI), fast-atom bombardment (FAB), matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and electrospray ionization tandem mass spectrometry (ESI-MS/MS). Prominent peaks in the mass spectra of these compounds appear to involve C-C bond cleavage beta to the amine nitrogen with loss of the 4,5-dimethoxy(1H)isobenzofuranone moiety from their molecular ions, along with another prominent peak at m/z 382. No molecular ion peaks of these compounds were recorded in EI, whereas intense [M + H]+ ion peaks were observed in FAB and ESI spectra. MALDI also yielded [M + H]+ ion peaks in good agreement with FAB and ESI studies.     

Characterization of polysiloxanes with different functional groups by time-of-flight secondary ion mass spectrometry

Polydimethylsiloxane (PDMS), polyhydromethylsiloxane (PHMS), and polymethylphenylsiloxane (PMPhS) have been studied by TOF-SIMS to investigate effects of functional group changes on polymer fragmentation mechanisms. Cyclic fragments are observed in the low mass range spectra of PDMS and PHMS, but not in the spectrum of PMPhS. Effects of functional group substitution on the fragmentation mechanisms of polysiloxanes are evident in the high mass range spectra (>1000 Da). Peaks of oligomers cationized by silver dominate the high mass range of the spectra of all low molecular weight polysiloxanes. However, fragmentation patterns of these samples are different. Neutral cyclic fragments cationized by silver are identified in the high mass range of the spectra of PDMS and PHMS, but not in the spectrum of PMPhS. The major fragments of PHMS and PMPhS are [oligomer-14+Ag]⁺. The PHMS spectrum also shows peaks [oligomer-28+Ag]⁺. These distinctive fragmentation patterns can be used to identify the polysiloxanes.