Chemical Ionization Spectra Research Articles

Chemical ionization mass spectra of urethanes

Chemical ionisation mass spectra using methane as the reagent gas are reported for 33 urethanes of general structure RNHCO2C2H5[R = H, CnH2n+1(n= 1–8), CH2CHCH2, cyclo-C6H11, Ph, PhCH2, PhCH2CH2, and Ph(CH3)CH] and R2NCO2C2H5[R = CnH2n+1(n= 1–4)]. Abundant MH+ ions are present in all the spectra, accompanied by satellite peaks corresponding to [M+ C2H5]+ and [M+ C3H5]+. Four classes of fragment ions are of general importance in the spectra. Two of these, [MH – C2H4]+ and [MH – C2H5OH]+, are associated with the CO2C2H5 group. The other two, corresponding to alkane and alkene elimination from MH+, arise from the RNH or R2N function. The mechanisms whereby these fragment ions are formed are discussed and their analytical utility is illustrated by reference to the spectra of the four isomeric C4H9NHCO2C2H5 and the eight isomeric C5H11NHCO2C2H5 compounds. The results of 2H-labelling studies are presented and a comparison is made between the methane and ammonia chemical ionisation spectra of selected urethanes.

Microbial synthesis of l-[ 15N]leucine l-[ 15N]isoleucine, and l-[3- 13C]- and l-[3′- 13C]isoleucines studied by nuclear magnetic resonance and gas chromatography-mass spectrometry

The preparation of leucine and isoleucine labeled with 15N and of site-specific 13C-labeled isoleucines is described. This method is based on the induction of the biosynthetic pathways specific for branched chain amino acids in glutamic acid producing bacteria, and controlled provision of stable isotope labeled precursors. Corynebacterium glutamicum (ATCC 13032), a glutamic acid overproducer, was incubated in leucine production medium which consisted of a basal medium supplemented with [ 15N]ammonium sulfate, glucose, and sodium α-ketoisocaproate. Production of l-[ 15N]leucine reached 138 μmol/ml at an isotopic efficiency of 90%. It was purified and checked by proton NMR and GC-MS. The electron impact (EI) spectrum showed 95 atom% enrichment. The cultivation of C. glutamicum in a similar medium containing α-ketobutyrate yielded l-[ 15N]isoleucine at a concentration of 120 μmol/ml. The GC-MS EI and chemical ionization (CI) spectra confirmed enrichment of 96 atom% 15N as that of the labeled precursors. The biosynthesis of l-[ 13C]isoleucine was carried out by induced cells which were transferred to a similar medium in which [2- 13C]- or [3- 13C]pyruvic acid replaced glucose. 13C NMR of the product isoleucine revealed single-site enrichment at C-3 or at C-3′ respective to the precursor [ 13C]pyruvate; i.e., C-3 was labeled from [2- 13C]pyruvate and C-3′ from [3- 13C]-pyruvate. Mass spectrometric analysis confirmed that all molecules were labeled only in one carbon. This site-specific incorporation of [ 13C]pyruvate is contrasted with the labeling pattern obtained when producing cells were supplied with [2- 13C]acetate, instead of pyruvate, when most label was incorporated into carbons 3 and 3′ of the same isoleucine molecule.

Mass spectral characteristics of derivatized metabolites of benzo [A] pyrene.

The electron impact (EI) mass spectra of the permethyl, peracetyl and per(trifluoroacetyl) derivatives of hydroxylated benzo[a]pyrene (B[a]P) metabolites were determined and the fragmentation chemistry producing the spectra elucidated. The metabolites investigated were: 3-hydroxy-B[a]P; 7,8-dihydro-7,8-dihydroxy-B[a]P; 7,8,9,10-tetrahydro-7,8,9-trihydroxy-B[a]P; and 7,8,9,10-tetrahydro-7,8,9,10-tetrahydroxy-B[a]P. In addition, the positive and negative methane chemical ionization spectra were determined for the derivatives of the BP-tetrol. The EI fragmentation patterns of the methylated metabolites that contained partially saturated rings were complex and, in the case of the di- and trimethoxy compounds, included apparent violations of the even-electron rule. The permethylated triol and tetrol cleaved through a retro-Diels-Alder reaction. The EI spectra of the peracetates were dominated by losses of acetic acid and ketene. The per(trifluoroacetyl) species fragmented by losing elements of trifluoroacetic acid, trifluoracetate radical and trifluoroacetyl. The spectra obtained from the permethylated tetrol permitted accurate prediction of the corresponding permethylated derivatives of tetrol metabolites of chrysene and benz[a]anthracene. The ability to predict spectra may be useful in trace analysis of hydrocarbon metabolites in biological samples.

Identification of a biliary metabolite of cisapride.

Radiolabeled cisapride was administered orally to male Wistar rats. The drug was metabolized extensively, resulting in the formation of a large number of urinary and faecal metabolites. In bile-cannulated rats a major metabolite was excreted with the bile whose structure could not be elucidated with the aid of the registered electron impact and desorption chemical ionization spectra. Therefore the biliary metabolite was subjected to extensive analytical procedures combining fast atom bombardment mass spectrometry, thermospray liquid chromatography/mass spectrometry, nuclear magnetic resonance and ultraviolet analysis. The results of this study allowed the identification of the biliary metabolite as the O-sulphate of metabolically formed 3'-hydroxy-cisapride.

Fragment–molecule adduct ion formation in positive methane chemical ionization mass spectrometry1

AbstractFragment–molecule (F+M) adduct ions of a variety of heterocyclic systems have been observed in positive methane chemical ionization spectra at relatively low sample concentrations, i.e. in the microgram range. The relative abundance of these adduct ions increases with sample concentration. Possible mechanisms of formation of F+M are discussed. Formation of such ions should be recognized as an artifact, otherwise such F+M adduct ions in chemical and electron ionization may be mistaken for impurities present in a sample.

Primary thermal fragmentation processes in poly(ethylene oxalate) investigated by mass spectrometry

The primary thermal decomposition mechanism of poly(ethylene oxalate) (PEO) has been investigated by pyrolysis-mass spectrometry. Several mass spectrometric techniques have been used in order to identify compounds present in the pyrolysis mixture: comparison of electron impact and chemical ionisation spectra, high resolution accurate mass measurements and tandem mass spectrometry (daughter and parent ion spectra). The results obtained indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation processes yielding cyclic oligomers up to tetramer. No other pyrolysis products were detectable under our experimental conditions. PEO, prepared by condensation polymerisation, was shown by 1H-NMR to contain up to 7% of diethyleneglycol (DEG) units along the polymer chain. A small amount of cyclic oligomers containing DEG units was detected among the pyrolysis products from PEO.

Identification of Explosives' Mixtures by Tandem Mass Spectrometry (MS/MS)

ABSTRACTThe detection and identification of the explosive components in post-explosion residues are often complicated by the complexity of the matrix in which they are found. Tandem mass spectrometry (MS/MS)—a method shown to be very useful in detecting and identifying compounds in complex mixtures—has been used to identify the explosive components in several military explosives. The method consists of recording the chemical ionization (CI) spectrum of the mixture and the MS/MS collision induced dissociation (CID) spectra of typical ions from the CI spectrum. These CI spectra are then compared with the CID spectra of parallel ions in standard explosives. Identical CID spectra positively confirmed the identify of the explosive components in the analyzed explosives' mixtures.

A mass spectrometric approach to the identification of conjugated drug metabolites.

For the identification of intact underivatized drug conjugates, the mass spectrometric technique of choice is fast atom bombardment (FAB); the combined use of both positive and negative ion FAB usually provides information on the molecular mass and nature of conjugates under study, while the number of exchangeable hydrogen atoms can be determined using trideuterated glycerol as the FAB-matrix. Electron impact and desorption chemical ionization spectra can be used to study the aglycone part of the conjugated metabolites. With this approach metabolites conjugated with glucuronic acid, sulphuric acid and amino acids have been identified. The identification was supported by analysis of reference compounds, prepared by chemical synthesis. The examples given are selected from current metabolism studies on drug candidates in development within Organon's research.

Packed capillary liquid chromatography-mass spectrometry using both direct-coupling and moving-belt interfaces

Packed fused-silica columns of 0.25-mm I.D. have been interfaced to a quadrupole mass spectrometer by direct coupling and to a magnetic mass spectrometer with a moving-belt interface. Electron impact (EI) and chemical ionization (CI) spectra were obtained by both techniques. Using the direct-coupling interface, we have successfully analyzed and obtained molecular ions for a number of compounds that are too thermally labile for analysis by gas chromatography-mass spectrometry. However, extremely thermally labile compounds, such as sulfonylureas, did not give MH+ ions in their CI spectra due to thermal degradation in the capillary interface line. With the moving-belt interface, MH+ ions were obtained for sulfonylureas in the CI mode. Better quality chromatograms and spectra, and better EI sensitivity were obtained with the moving-belt interface than by direct coupling.

Mass spectra of 3-nitro-2H-chromenes and 3-nitrochromans

The 4kV, 70 eV electron impact mass spectra (e.i.-m.s.) of eight 2-aryl-3-nitro-2H-chromenes and ten 2-aryl-3-nitrochromans of diverse functionality are reported and discussed. The peak due to the molecular ion is always appreciable and often intense in these spectra. Loss of NO2 from M+˙ is responsible for the base peak in the nitrochromene spectra. In contrast, [M– HNO2]+˙ is more abundant than [M– NO2]+ in the spectra of the nitrochromans; furthermore, this unusual HNO2 loss persists even for metastable molecular ions. A diagnostically important ion, of nominal structure [ArCH2]+, accounts for the base peak in the chroman spectra; this ion, which defines the formula of the 2-aryl subsituent, corresponds to elimination of benzofuran from [M– NO2]+.All the compounds studied exhibited [M]–˙ ions in their electron capture negative ion chemical ionisation (c.i.) spectra. Apart from [M]–˙ and the base peak at [M– NO2]–, few ions were of considerable abundance in these spectra, although [NO2]– ions were present in the nitrochroman series.

Electron impact and chemical ionization mass spectra of 2,2‐diphenyl‐3‐arylcyclobutanone oximes. Formation of ion m/z 167

AbstractThe electron impact (EI) and chemical ionization (CI) spectra of 2,2‐diphenyl‐3‐aryl cyclobutanone oximes (1–5) are reported. Formation of diphenylmethyl cation at m/z 167 is a major fragmentation process in both EI and CI spectra. Labelling studies established that the hydrogen involved in this rearrangement transfers from the NOH group and not from cyclobutane ring positions. The [M + 3]+ ions are formed under CI conditions as a result of CN double bond reduction. An interesting secondary kinetic isotope effect is observed in the formation of ion e at m/z 183 in both EI and CI spectra. Other characteristic fragmentation pathways occurring in the EI and CI spectra of these compounds are outlined.

Mass spectra of piperidine dicarboxylic acids and their esters

Electron impact mass spectra of the epimers of 2,4-, 2,5-, and 2,6-piperidine dicarboxylic acids and their methyl esters and N-acyl derivatives have been studied. Data from DADI spectra have been used to establish the basic fragmentation paths. Chemical ionization spectra of positive and negative ions of 2,4- and 2,5-piperidine dicarboxylic acids are described.

Electron impact and ammonia chemical ionization mass spectra of n‐azabicyclo[m.2.0]alkan‐(n + 1)‐ones (m = 3–6, n = 6–9)

AbstractElectron impact induced fragmentation of the title compounds obeys a route where the lactam moiety, OCNH, is cleaved first, with the accompanying formation of a cycloalkene ion. This can be verified by low‐resolution, high‐resolution, B/E and B2/E spectra as well as by collisional activation spectra of, for example, the ions m/z 82 and 67 from 7‐azabicyclo[4.2.0]octan‐8‐one and from cyclohexene. The only, and fairly weak, fragment ions including O and N are [C3H3O]+, [CkH2k‐2N]+ (k = 5–8) and [C3H6N]+. The ammonia chemical ionization spectra are also characteristic for all four lactams and show the same dominant ions in all cases, namely [M + 1]+, [M + 1 + NH3]+˙ and [2 M + 1]+˙.

14] Isolation and identification of vitamin D metabolites

Publisher Summary This chapter describes the isolation and identification of vitamin D metabolites. A successful isolation and identification of a vitamin D metabolite usually consists of extraction, prepurification (optional), several high-performance liquid chromatography (HPLC) procedures, ultraviolet (UV) absorbance spectroscopy, mass spectrometry (MS), chemical derivatization, and may include proton nuclear magnetic resonance (NMR) spectrometry. HPLC is the best method known for the resolution and purification of vitamin D metabolites. A number of dissimilar vitamin D derivatives comigrate in a single HPLC system. This emphasizes the need for more than one system in determining coidentity or for comparing metabolites from biological sources to those obtained in vitro from known precursors. The UV spectra of vitamin D and its metabolites having the 5,6- cis geometry obtained in ethanol are smooth curves with one absorbance maximum at 265 nm and a molar absorptivity, ɛ , of 18,200 for D 3 , and 19,200 for D 2 . Vitamin D metabolites produce strong positive ions in the chemical ionization mode. The nature of the positive ion chemical ionization (PCI) spectra is dependent on the reagent gas. NMR spectroscopy can be a powerful aid in identifying new metabolites but is not used frequently because it requires larger amounts of sample (10–500 μ g) than MS (25–200 ng) and because the mass spectra of metabolites and their derivatives usually provide conclusive structural evidence. Nevertheless, the NMR spectra of vitamin D compounds have distinctive signals that are diagnostic of the alkene protons, the side chain substitution, and the A-ring substitution.

Incorporation of2H from2H2O into ABA in Tomato Shoots: Evidence for a Large Pool of Precursors

Incorporation of²H from²H₂O into ABA in Tomato Shoots: Evidence for a Large Pool of Precursors

Chemical ionization (NO) spectra of n‐alkenoic acids and their esters

AbstractIt is shown that the position of one double bond in long‐chain unsaturated fatty acids and their esters can be determined by chemical ionization (NO) mass spectra. Homoconjugated dienoic acids (as linoleic acid) do not give a characteristic framentation pattern, in contrast to those which contain three or more homoconjugated double bonds.

Fast ATOM Bombardment Mass Spectra of Nucleosides. Comparison with Electron Impact and Chemical Ionization Mass Spectra

Abstract The fast atom bombardment (FAB) mass spectra of the eight major nucleosides found in RNA and DNA, pseudouridine and 2′,3′-O-isopropylidene adenosine are described and compared to El, CI, and desorption chemical ionization (DCI) spectra reported in the literature or obtained in this laboratory. Bcty, cocltl nun FAB spectra are reported. The FAB spectra are simple and provide unambiguous molecular weight information along with structurally significant fragment ions. Mechanisms of ion formation are thought to closely parallel those suggested earlier to be operating in the CI mode. Advantages and disadvantages of FAB relative to the standard ionization modes are discussed.

Photodissociation of transition metal β-diketonates

The laser photodissociation mass spectra of various metal β-diketonates were obtained. Molecular ions were mass selected and photodissociated at 488 nm. The photofragment ions were analyzed using ion kinetic energy (IKE) analysis. Generally, the loss of an intact ligand was observed for the trivalent metal chelates whereas the loss of a β-diketone terminal substituent was observed for the divalent metal chelate ions. In contrast, non-transition metal β-diketonates did not yield photofragments. The spectroscopic results are compared qualitatively with the electron ionization, chemical ionization, and electronic absorption spectra of the β-diketonates.

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane

AbstractMixtures of 50% tetramethylsilane (TMS) and methane have been found to give [M+73]+ adduct ions and structurally useful fragment ions for many oxygen‐ and nitrogen‐containing organic compounds. All of the reagent ions in TMS react with polar compounds. The high‐pressure TMS chemical ionization spectra of many simple oxygenated compounds are in agreement with predictions from ion chemistry of (CH3)3Si+ obtained by ion cyclotron resonance experiments at very low pressures, but differences are noted. Sensitivities for oxygen‐, nitrogen‐, and sulfur‐containing compounds with TMS as the reagent gas appear to be approximately the same.

Chemical ionization over a wide range of pressures. 1. Mass spectra of dimethyl esters of maleic and fumaric acids

By using chemical ionization over a wide range of pressures, from 0.01 torr to atmospheric pressure, and also by selecting the reagent gas, different mass spectra of isomers can be obtained, which are suitable for their reliable identification. Under ionization conditions at atmospheric pressure in helium (reagent ions [H(H2O)n]+), peaks of cluster ions [MGH]+ and [2M+1]+ are observed in the spectrum of dimethyl fumarate, which are absent in the case of the cis-isomer. Under the conditions of chemical ionization at a normal pressure (0.4-0.2 torr) of the reagent gases Me3CH, n-C7H16 and at an ionic source temperature of 50°C, a stereospecific fragmentation of dimethyl maleate [MH]+-MeOH is observed, which is absent in the case of the the trans-isomer. In the chemical ionization spectra at reduced pressure of the reagent gases MeOH, EtOH, i-PrOH (0.01 torr), a peak of the cluster ion [MGH]+ is observed for dimethyl fumarate, which is absent in the spectra of the cis-isomer.