Electron Impact Mass Spectra Research Articles

Development of a gas chromatography–mass spectrometry method for the analysis of aminoglycoside antibiotics using experimental design for the optimisation of the derivatisation reactions

A packed column GC–electron-capture detection method for the analysis of the aminoglycoside antibiotics kanamycin and gentamicin was adapted to capillary GC–MS. The analytes were derivatised using a two-step procedure involving trimethylsilylation of the hydroxyl groups with trimethylsilylimidazole and acylation of the amino groups with heptafluoro-butyrylimidazole. Electron impact mass spectra of the resulting derivatives of kanamycin A and gentamicins C 1, C 1a and C 2 are given and interpreted. The derivatisation procedure was optimised using experimental design. This chemometrical approach considers main effects as well as interactions of the influential parameters, thus conducting a more thorough investigation of the method than the common step-by-step approach. Optimisation using fractional factorial and Box Behnken Designs produced a derivatisation method featuring better yield than previously published methods while in many cases requiring less reagents and shorter reaction times.

Micro-scale Synthesis and In-Situ Spectroscopic Characterization of Some Chemical Weapons Related Organophosphonate Compounds

Inspection protocols for the Chemical Weapons Treaty, which went into effect on April 29, 1997 require on-site analytical procedures which include a comprehensive mass spectral database of hundreds of scheduled compounds. A number of these compounds were prepared on a nanogram scale via a micro-scale synthetic approach using GC-IRD/MSD analytical instrumentation to purify and identify the resulting products. The collected electron impact mass spectra will be made available for use in relevant databases.

Mass spectra of some new heterocycles: cyclobutapyrimidines and their Diels-Alder adducts with C60

The electron impact mass spectra of new compounds, 2,4-dialkyl and 2,4-diaryl substituted-5,6-dihydrocyclobuta[d]pyrimidines, are reported. The main fragmentation pathway leads to the formation of cyclobutadiene [C4H4]·+ m/z = 52 as the base peak. The FAB spectra of the adducts of cyclobutapyrimidines with C60 have also been recorded. In these spectra the dominant peaks are formed by a retro-Diels–Alder reaction and by the apparent loss of the substituents attached at the pyrimidine, ring forming the ions m/z = 720 and m/z = 825, respectively. © 1998 John Wiley & Sons, Ltd.

Mass spectrometry study on five‐substituted pyrroles and the related N‐methylene‐ 1,3,4‐thiodiazoles

Electron impact mass spectra of nine new substituted pyrroles are reported. Apart from compounds 1a and 1b, the other seven compounds produce R1--Ph--C≡O+ ions as base peaks. Except for compounds 2a–c, each compound may produce [M−EtOH]∔, [M−OEt]+ and other fragment ions. The proposed fragmentation mechanisms of the six new compounds giving M∔ ions have been supported by mass analysed ion kinetic energy spectrometry and accurate mass data. No molecular ion peaks of compounds 2a–c are observed under electron impact conditions. However, very intense protonated molecule peaks of compounds 2a–c were observed under fast atom bombardment conditions together with [M+H − EtOH]+ ions. By D-exchange experiments, it was verified that the proton is localized on the N atom between the carbonyl and thiocarbonyl groups during the formation of protonated molecules. © 1998 John Wiley & Sons, Ltd.

Identification of conjugated fatty acids by gas chromatography‐mass spectrometry of 4‐methyl‐1,2,4‐triazoline‐3,5‐dione adducts

Abstract4‐Methyl‐1,2,4‐triazoline‐3,5‐dione was reacted with conjugated fatty acid methyl esters to form Diels‐Alder cycloaddition products. The electron impact mass spectra of conjugated octadecadienoates and 9,11,13‐octadecatrienoate were simple and informative and allowed the positions of the double bonds to be determined. The dienes gave single adducts whereas the triene formed four products that corresponded to two stereoisomers of the adducts of the 9,11‐diene system and two of the 11,13‐diene system. The method can be used to complement other methods for identifying conjugated fatty acids.

Gas Chromatographic--Mass Spectrometric and High-Performance Liquid Chromatographic Analyses of the Bromination Products of the Regioisomeric Dimethoxyphenethylamines: Differentiation of Nexus from Five Positional Isomers

The brominated products from all six positional isomers of dimethoxyphenethylamine are prepared, and their analytical properties are evaluated. The major bromination product from 3,5-dimethoxyphenethylamine is the 2,6-dibromo isomer; all other regioisomers of dimethoxyphenethylamine yield a monobromo species as the major product. The mass spectra divide these compounds into two distinct groups: one group showing a strong m/z 180 ion via loss of bromine from the molecular ion (M-Br)+ and a second group showing no significant m/z 180 ion. The three compounds that do not show the m/z 180 ion in their electronimpact mass spectra are brominated 2,4-; 2,5-; and 2,6dimethoxyphenethylamine. These compounds are well-resolved by reversed-phase liquid chromatographic methods using a Hypersil Elite C 18 stationary phase and a mobile phase of phosphate buffer (pH 3) and methanol-acetonitrile.

Electron impact mass spectrometry of some thienopyrimidone derivatives

The electron impact mass spectra of some previously synthesized thienopyrimidones have been recorded and the identity of various ions in the mass spectra established.

On the McLafferty and Double Hydrogen Rearrangements in Cycloalkylacetones.

In the electron impact mass spectra of a series of cycloalkylacetones with ring sizes varying from cyclopropane to cycloheptane, the intensity of the McLafferty rearrangement ion at m/z 58 increases with the larger, less strained rings. The larger ring ketones give surprisingly intense “McLafferty plus one” ions at m/z 59, which is the base peak in the mass spectrum of cyclohexylacetone. This double hydrogen McLafferty rearrangement is favored over the single hydrogen rearrangement at both high (70 eV) and low (14 eV) ionizing voltages. Deuterium labelling confirms the expected considerable hydrogen migration in the cycloalkyl ring, occurring following hydrogen transfer to the carbonyl oxygen and prior to C-C bond cleavage.

Electron impact and multiphoton ionization and fragmentation of molybdenum-cyclopentadienyl-dicarbonyl-nitrosyl at 351, 248 and 193 nm

Electron impact (EI) and multiphoton ionization (MPI) mass spectra of CpMo(CO) 2NO (Cp  C 5H 5, cyclopentadienyl ligand) are reported. The ionization energy (8.20 eV) and appearance energies of major fragment ions are determined. Ion dissociation energies and formation enthalpies are derived from these data. On EI ionization molecular ions and large fragment ions are formed. A primary loss of CO ligands from molecular ions is favoured over NO or Cp elimination. Mo + ions are generated in low abundance only. On multiphoton excitation at the excimer laser wavelengths 351 and 248 nm and low laser intensities MoO + ions and, with lower abundance, Mo + ions are detected. At increasing laser intensities Mo + ions are becoming predominant. At 193 nm and low laser intensities CpMoNO + and CpMo + ions are formed. At higher laser intensities an increasing yield of smaller ions, particularly Mo +, are found. The number of absorbed photons for the formation of prominent ions is determined and compared with EI appearance energies. The MPI results at 351 and 248 nm are interpreted with a dominant dissociation of the neutral parent compound followed by multiphoton ionization of the fragments. At 193 nm, however, a resonant multiphoton absorption leads to intermediate molecular ionization and subsequent fragmentation.

Novel β-Distonic Radical Cations [CnH2n+2S]•+ (n = 2, 3) Formed upon Decarbonylation of Ionized S-Alkyl Thioformates: A Mass Spectrometric and ab Initio Study

Decarbonylation is commonly observed in the 70 eV electron impact mass spectra of a series of S-alkyl thioformates, H−C(O)SR 1−7. Using a combination of collisional activation (CA) and neutralization−reionization (NR) mass spectra, it is shown that the spectra of the so-formed [M−CO]•+ ions differ from the isomeric alkane thiol ions (RSH•+), if R > CH3. These fragment ions are therefore distonic radical cations, and given the fact that the α-distonic •CH2−S+H2 ions are not present under our experimental conditions, it is proposed that they actually are β-distonic ions. Strong evidence for the distonic nature of the fragment ions derived from 2−7•+ has been obtained owing to the use of a new hybrid tandem mass spectrometer presenting a sector−quadrupole−sector configuration. In the case of ethyl derivatives (R = C2H5), both the distonic •CH2CH2S+H2 (2b) and conventional CH3CH2SH•+ (2a) ions are observed. The nonclassical structure 2b reacts indeed with nitric oxide (NO•) giving the production of H2S+−NO io...

Electron Impact Mass Spectra of Phenol Blocked Isocyanates

The electron impact mass spectra of a series of substituted phenol blocked toluene diisocyanate crosslinkers were analyzed and a suitable dissociative pathway proposed.

Study of sinapinyl but-3-enylglucosinolate (boreavan A) and related compounds by mass spectrometry

Electron impact (EI), positive and negative ion fast atom bombardment (FAB) combined with tandem mass spectrometry was used for the structural elucidation of sinapinyl but-3-enylglucosinolate (boreavan A). The EI mass spectra showed the presence of ions due to the sinapic acid moiety. Protonated and deprotonated quasi-molecules and some structurally significant fragment ions due to sulphate were observed in the FAB mass spectra. The tandem mass spectra gave information for the identification of the main building blocks and the sequence ions of the sinapinium cation in boreavan A, and also showed that the number of side chains present in the glucosinolate moiety has a potential influence on the location of the anion part of boreavan A. © 1997 John Wiley & Sons, Ltd.

Stereospecific Fragmentation of 3-Dimethylaminocyclohexanols upon Electron Impact Ionization

Stereoisomeric cis- and trans-1-butyl-3-dimethylaminocyclohexanols have been previously reported to exhibit different electron impact (EI) mass spectra. The m/z 100 [C6H14N]+ ion is obtained only from the cis-isomer. The results of a collision-induced dissociation study are inconsistent with the previously proposed protonated dimethylaminocyclobutane structure (ion a) and suggest the N,N-dimethyl-1-butaneimmonium (CH3CH2CH2CH=N+(CH3)2) structure (ion b) for this ion. The mechanistic pathway proposed for this highly stereospecific process involves initial hydrogen migration from the hydroxy group to the radical site at the charged amino group as the stereospecific step, this being possible only for the cis-amino alcohol. The EI mass spectra of the corresponding stereoisomeric methyl ethers exhibit preferential elimination of formaldehyde from the cis-isomer, which is explained by initial hydrogen migration from the methoxy group to the N atom. The unsubstituted cis- and trans-1-methoxy-3-dimethylaminocyclohexanes do not show any stereospecificity in their behavior under EI. © 1997 by John Wiley & Sons, Ltd.

Determination of Sulfides and Thiols in Petroleum Distillates Using Solid-Phase Extraction and Derivatization with Pentafluorobenzoyl Chloride

An improved method for differentiating sulfides, thiophenes, and thiols in petroleum distillates is presented. Thiophenes are separated from sulfides and thiols via solid-phase extraction. Reaction with pentafluorobenzoyl chloride is employed to form thiol esters, while sulfides do not react. Thiol elution times increase sufficiently after derivatization to provide separation from sulfides during gas chromatographic analysis. In addition, electron impact mass spectra of derivatized thiols exhibit an intense, characteristic 195 fragment ion, which aids in their identification.

Capillary gas chromatography–mass spectrometry of lower oxyethylenated aliphatic alcohols

This paper deals with the capillary gas chromatography–mass spectrometric (CGC–MS) analysis of alkylpoly(ethylene glycol) ethers in the products of the oxyethylenation of the individual lower aliphatic alcohols C4–C10, both in synthetic mixtures and as residues in water after their biodegradation. MS has been used for the acquisition of spectra and for the recording of retention times and the areas of eluted components. Particular alkylpoly(ethylene glycol) ethers have been identified by a combination of interpreting their electron impact (EI) mass spectra and by correlating their relative retention times with the structure of the molecules. Using these correlations, the length of both the poly(ethylene glycol) part and the alkyl part of a molecule were determined. Retention times of the eluted components were used for calculation of the relative retention characteristics. The peak areas were used to determine the concentration decreases of individual components as a function of the degradation time. Alkylpoly(ethylene glycol) ethers were analysed in both the non-derivatized form and after their conversion to trimethylsilyl derivatives and acetates.

Preparation of Some Toxic Metabolites of Disulfoton, Phorate, and Terbufos, Their Separation by Thin-Layer Chromatography and Confirmation by Electron Impact Mass Spectrometry

Milligram quantities of sulfoxides, sulfones, and oxygen analogue (oxon) sulfones of the insecticides disulfoton, phorate, and terbufos were prepared by selective oxidation. Pure insecticides or acetone extracts of granular formulations served as reactants. Structures of 9 compounds were confirmed by comparing their electron impact mass spectra (EI-MS), obtained by the direct inlet system with published data. Thin-layer chromatography (TLC) on silica gel was used to screen oxidation products and to purify products. Products were detected by spraying plates with PdCl2 reagent and exposing to iodine vapors. An esterase inhibition technique gave low detection limits, which are promising for residue analysis.

The Mass Spectral Fragmentation of Perezone and Related Compounds

Electron impact mass spectra of six isomeric compounds, namely perezone, isoperezone, α-pipitzol, β-pipitzol, β-isopipitzol and dihydroisoperezinone have been recorded. The proposed fragmentation pathways leading to the formation of a number of important daughter ions have been confirmed from the corresponding parent ion spectra by collision-induced dissociation (CID) experiments. The elemental composition of the principal fragment ions was determined by accurate mass measurements. © 1997 John Wiley & Sons, Ltd.

Determination of the absolute configuration of sugar residues using gas chromatography: Method with potential for elimination of references

The absolute configuration of a sugar can be determined by gas–liquid chromatography of the acetylated or trimethylsilylated dithioacetals from 1-phenylethanethiol. The isolation of both enantiomers of 1-phenylethanethiol is also described. Using the acetates and both thiol reagents the absolute configuration of C-2 can be determined, provided it is a hydroxyl group, with great certainty. A new way of determining the absolute configuration of sugars, without references, is thereby provided. The sugars analysed include aldoses, deoxyaldoses, 2-acetamido-2-deoxyaldoses and uronic acids. The analysis is made using columns with non-chiral stationary phase and the electron impact mass spectra of the acetylated and trimethylsilylated bis(1-phenylethyl)dithioacetals are described.

Identification of phenolic acids and inositols in balms and tissues from an Egyptian mummy

A number of samples taken from an Egyptian mummy (ca. 100 B.C.) from the Guimet Museum in Lyon have been analyzed by GC-MS. Derivatives of aromatic acids (hydroxyhydrocinnamic, vanillic, protocatechuic and gallic acids) and inositols (non-methylated and mono- O-methyl) have been found among the constituents of extracts prepared by methanolysis and trimethylsilylation. From the reported electron impact mass spectra, ion sets where proposed for a sensitive and selective profiling of these selected compounds by mass fragmentometry. The source of gallic acid and inositol was found to be a vegetable tannin, an ingredient which was not previously known to be used for mummification in ancient Egypt. The nature and abundance ratios of the detected inositols also appeared to be a promising criterion further investigate the botanical source of the tannin employed.

Mass Spectral Study of Extractable La- and Lu-Containing Fullerenes

Mass spectra of LaxC2n (x = 1,2), well known endohedral metallofullerenes, and Lu2C2n (2n = 76-112), new members of extractable metallofullerenes, were studied. Positive-ion laser desorption/ionization (LDI) and electron impact (EI) mass spectra indicated that lutetium is a special lanthanide that prefers to form dilutetium fullerenes by the are-burning method. However, the signals for Lu2C2n become very weak in negative-ion LDI-MS, this is different from La-2@C-80, which has close relative abundances in positive- and negative-ion MS. The distinction between Lu2C2n and La-2@C-80 in the negative-ion LDI mass spectra may be due to the different structures of La- and Lu-containing fullerenes. (C) 1997 by John Wiley & Sons, Ltd.