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.     

Electron impact and chemical ionization mass spectra of some tetra-acetylated anomeric glycosides

Electron impact and chemical ionization (CH₄, iso-C₄H₁₀ and NH₃) mass spectra of some tetra-acetylated anomeric glycosides have been examined with a view to the characterization of anomeric pairs. Minor differences observed in the relative intensities of common ions in the anomeric pairs in the electron impact mass spectra are found to be enhanced in the methane and isobutane chemical ionization mass spectra. In the absence of thermal decomposition, the β-anomers show greater ion abundances of common glycosyl ions than the α-anomers. Ammonia chemical ionization mass spectra show complementary behaviour indicating strong adduct ions and practically no fragmentation.     

The mass spectrometry of polychlorinated dibenzo-p-dioxins

The negative ion chemical ionization mass spectra of polychlorinated dibenzo-p-dioxins using oxygen, methane and methane/oxygen are reported together with their methane positive ion chemical ionization mass spectra and conventional electron impact spectra. The methane/oxygen negative ion chemical ionization mass spectra proved to be the most useful of the negative ion spectra for structure determination.     

Accurate mass measurements of positive ions in the desorption chemical ionization mode

Desorption chemical ionization mass spectrometry employing ammonia as the reagent gas has been extensively used to obtain molecular mass and structural information on a wide variety of compounds. Mass-deficient reference standards normally used for calibration purposes in mass spectrometry do not provide adequate mass spectra under ammonia chemical ionization conditions. In order to overcome this problem a mixture of ammonia and methane as reagent gases was employed. In high-resolution accurate mass measurement experiments, this gas mixture allows the simultaneous detection of mass spectra of perfluorokerosene adequate for calibration purposes and spectra containing molecular mass information of the analyte. A needle valve system was used to control the composition of the gas mixture introduced into the ion source. For positive-ion accurate mass measurements of higher masses (up to m/z = 2300), Fomblin 18/8 oil was successfully used as a reference standard under ammonia, methane and isobutane desorption chemical ionization conditions.     

In situ chemical ionization in ion trap mass spectrometry--the beneficial influence of isobutane as a reagent gas

We report a comparison of the ionization yields provided by the most common reagents (methane, ammonia, methanol, acetonitrile, isobutane) performing in situ chemical ionization with an ion trap mass spectrometer. Four molecules were chosen in the medical field to illustrate experimental results: alprazolam, diazepam, flunitrazepam and acetaminophen. Under usual operational conditions, relative abundances of protonated ions appreciably depend on the reagents. The greatest abundance of MH(+) ions was obtained with isobutane while observed intensities for MH(+) ions varied from 73% for methanol and ammonia to about 23% for acetonitrile and methane. Results were temptatively rationalized comparing energies of formation of the reagent ions and storage efficiency in the trapping field.     

Electron capture negative ion and positive ion chemical ionization mass spectrometry of polychlorinated phenoxyanisoles

Several polychlorinated phenoxyphenols with three to nine chlorine atoms were examined as their methyl ethers by electron capture negative ion and positive ion chemical ionization and electron impact mass spectrometry. In chemical ionization studies methane, hydrogen, nitrogen, helium and argon were used as reagent gases. Selected compounds were also examined with deuteriomethane, ammonia and deuterioammonia as reagent gases. Utilization of chemical ionization spectra in conjuction with electron impact spectra provides substantial structural information about these compounds. Chemical ionization spectra provide information about chlorine atom substitution. The position of phenoxy substitution can be established from electron capture negative ion and positive ion spectra.     

Chemical ionization mass spectrometery utilizing and isotopically labeled reagent gas

The use of a mixture of 5% ammonia in methane, where the ammonia is 50% ¹⁵N-labeled, provides a very useful reagent gas for chemical ionization mass spectrometry. We find that this combination gives spectra very much like pure ammonia reagent gas except that all of the adduct ions are clearly labeled.     

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.     

Reaction of analyte ions with neutral chemical ionization gas

Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Ion-molecule reactions of neutral methane with analyte ions under normal methane chemical ionization conditions are discussed. Reactant ions can be generated by direct electron ionization (EI) fragmentation, chemical ionization (CI) fragmentation, or collision-induced dissociation (CID). Examples in which products of such reactions appear in mass spectra in both conventional CI sources in “beam” instruments and low pressure CI in a quadrupole ion trap are presented. Also shown is an example in which MS/MS product ions react with neutral methane used for CI in an ion trap. It is shown that it is relatively straightforward to recognize such reactions in a quadrupole ion trap and in certain cases to minimize or preclude them. Effects of various operating parameters have been investigated and are discussed.     

Ionisation Chimique de Composés Aromatiques Nitrés

The collision induced dissociations of [MH – 30]⁺ ions observed in the chemical ionization (methane) mass spectra of some nitro aromatic compounds show that these ions are formed by reduction in the ion source with subsequent protonation and not by the previously reported losses of NO^· from the protonated molecular ions.     

Differentiation of diastereomeric conduramine derivatives under electron ionization and chemical ionization mass spectral conditions

Diastereomeric conduramine derivatives, i.e., (1R,2S,3R/S,6S)-6-(N-carbomethoxyamino) 1,2-O-isopropylidenecyclohex-4-ene-1,2,3-triol (1 and 2) and their O-acetyl derivatives (3 and 4), were studied using gas chromatography (GC) with electron ionization (EI) and chemical ionization (CI). The EI mass spectra of diastereomeric pairs show consistent differences in the relative abundances of characteristic ions. The EI fragmentation patterns are based on precursor/product ion spectra, high-resolution mass spectrometry (HRMS) and deuterium labelling. The CI spectra show differences from the EI spectra, and the isobutane/CI spectra are much simpler than the methane/CI spectra. The differences shown in the CI spectra are similar to those shown in the product ion spectra of [M+H](+) ions generated under electrospray ionization (ESI) conditions. Theoretical calculations are performed to understand the observed differences. The differences in the relative stabilities of molecular ions, or protonated molecules at different sites, can explain the observed differences in the spectra.     

Electron ionization and chemical ionization mass spectra of pyridinium and isoquinolinium ylides

Electron ionization (EI) spectra and both positive and negative chemical ionization (CI) spectra have been obtained for four isoquinolinium ylides and two pyridinium ylides. Electron transfer reactions dominate the CI mass specra. The base peak in negative chemical ionization is the [M]^?· ion, formed by electron capture. In the positive methane CI spectra the molecular ion, [M]^+·, is relatively more intense than [MH]⁺ showing electron transfer to be the main positive ionization process. In the positive ammonia CI spectra, proton transfer to give [MH]⁺ is the main ionization process, but electron transfer is also observed. The EI spectra show fragmentations in which the aromatic nitrogen moiety retains the charge and fragmentation is by loss of radicals or small neutral molecules from the side-chains. Radical driven reactions are proposed to explain these spectra.     

An evaluation of atmospheric pressure ionization techniques for the analysis of N-Methyl carbamate pesticides by liquid chromatography mass spectrometry

Atmospheric pressure ionization (API) techniques are evaluated for the mass spectral analysis of N-methyl carbamate pesticides. Atmospheric pressure chemical ionization (APCI) using a heated nebulizer interface provided both protonated molecules and abundant, characteristic fragment ions. With ion spray (ISP; pneumatically assisted electrospray ionization), which utilizes a milder “ion evaporation” process, primarily protonated molecules were obtained, although fragment ions similar to those observed in APCI could be induced by variation of the API orifice voltage. Product ion spectra of ISP-derived protonated molecules, generated by tandem mass spectrometry using collision-induced dissociation, are also presented. The APCI and ISP spectra of the carbamates are compared to those obtained with a thermospray interface and also to their electron ionization and methane CI spectra obtained with a particle beam interface. For all four interfaces, combined liquid chromatography mass spectrometry methods using conventional (4.6 mm i.d.) columns are described for the separation and detection of pesticide mixtures. These methods are applied to the confirmatory analysis of three representative carbamate pesticides, spiked at the 0.1-ppm level in green peppers. For those carbamates amenable to gas chromatography mass spectrometry, comparative results are presented.     

High-pressure collisional activation mass spectrometry of aromatic halides

The high-pressure collisional activation mass spectra with methane as the reagent/collision gas are reported for five aromatic halides. The major decomposition of the protonated aromatic halides is hydrogen halide elimination. The energy-resolved mass spectra and the chemical reactivities of fragment ions with the methane collision gas are used to establish dissociation pathways and structures of fragment ions. The high-pressure collisional activation mass spectra are compared with conventional collisionally induced dissociation and chemical ionization mass spectra.     

Some stereochemical effects in gaseous ion decompositions

Stereochemical effects were observed in the electron ionization, methane, isobutane and ammonia positive ion chemical ionization and hydroxy negative ion chemical ionization behaviour of some isomeric triaryl nitrocyclohexenes. cis-Eliminations leading to loss of HNO₂, H₂NO₂ and arene were found to be stereochemically preferred in gaseous ions.     

Analysis of triacetone triperoxide by gas chromatography/mass spectrometry and gas chromatography/tandem mass spectrometry by electron and chemical ionization

The explosive triacetone triperoxide (TATP) has been analyzed by gas chromatography/mass spectrometry (GC/MS) and sub-nanogram detection limits are reported by ammonia positive ion chemical ionization (PICI), electron ionization (EI) and methane negative ion chemical ionization (NICI). Analysis by methane PICI and ammonia NICI gave detection limits in the low nanogram range. Analyses were carried out on (linear) quadrupole and ion trap instruments. Analysis of TATP by PICI using ammonia reagent gas is the preferred analytical method, producing low limits of detection as well as an abundant (greater than 60% of base peak) diagnostic adduct ion at m/z 240 corresponding to [TATP + NH4]+. Isolation of the [TATP + NH4]+ ion with subsequent collision-induced dissociation (CID) produces extremely low abundance product ions at m/z values greater than 60, and the m/z 223 ion corresponding to [TATP + H]+ was not observed. Density functional theory (DFT) calculations at the B88LYP/DVZP level indicate that dissociation of the complex to form NH4+ and TATP occurs at energies lower than peroxide bond dissociation, while protonation of TATP leads to cleavage of the ring structure. These results provide a method for pico-gram detection levels of TATP using commercial instrumentation commonly available in forensic laboratories. As a point of comparison, a detection limit of 15 ng was obtained by flame ionization detection.     

Cluster ion structures formed by positive and negative chemical ionization of lactic and other hydroxyacids

Both positive ion and negative ion chemical ionization mass spectra of hydroxycarboxylic acids (hydroxyethanoic acid, 3-hydroxypropanic acid, 2-hydroxypropanoic acid and 1-phenyl-1-hydroxyethanoic acid) show intense oligomeric ions when the samples are evaporated into a chemical ionization source. The observation of oligomeric anionic and cationic species is unusual, and the parallel behavior observed between the positive and negative ion mass spectra is striking. These results are explicable in terms of evaporation of oligomers and their dehydration products from the hot probe, although gas phase clustering reactions of singly charged ions are not excluded. Hydrogen bonding and dehydration provide bonding within each cluster. The structures of the ions have been confirmed by recording the collision induced dissociations of individual cluster ions via their mass analyzed ion kinetic energy spectra. Temperature dependence of the chemical ionization mass spectra provides a method for distinguishing hydrogen bonding from covalent bonding and gives further structural information on the cluster ions.     

Negative-ion mass spectrometry of substituted adenine bases and adenosine nucleosides

The negative-ion chemical ionization (ammonia, 5 Pa source pressure) mass spectra of a series of substituted adenine bases, adenosine nucleosides, and the trimethylsilyl derivatives of the nucleosides are described. Selected ions from these spectra were subject to collisionally activated dissociation with mass-analysed ion kinetic energy (CAD/MIKE) analysis of the products and the spectra assessed for information content. In addition to observing strong peaks due to quasimolecular ions and heterocyclic-base ions, it proved possible to differentiate between 2'-, 3'- and 5'-deoxy and between 2'- and 3'-O-methyl isomers. The negative-ion chemical ionization spectra of four methyladenines are essentially identical, but could be clearly distinguished from each other by CAD/MIKE analysis.     

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.     

Chemical ionization mass spectra of acetals of beta-D-glycopyranosylnitromethanes

O-Isopropylidene and O-benzylidene acetals of common 2, 6-anhydro-1-deoxy-1-nitroalditols (beta-D-glycopyranosylnitromethanes) derived from D-glucose, D-galactose and D-mannose were studied by chemical ionization mass spectrometry (CIMS) using methane, isobutane, ammonia or pyridine as reaction gas. Production of [M+H](+) adduct ions dominates in the case of methane or isobutane possessing proton affinity values PA = 552 or 683 kJ mol(-1), respectively. The collision-induced dissociation time-of-flight product ion spectra of [M+H](+) ions differ characteristically according the stereochemical arrangement of the pyranoid ring. These differences can be helpful when assigning stereochemical arrangements for the pyranoid ring. The dominant process in ammonia (PA = 853 kJ mol(-1)) CIMS for most of the compounds studied is the production of the cluster ions [M+NH(4)](+). The cluster [M+pyridineH](+) ions are observable only for substances possessing the O-benzylidene group (PA of pyridine = 924 kJ mol(-1)). Copyright 2000 John Wiley & Sons, Ltd.