Studies in chemical ionization mass spectrometry: Aryl Ketones

The CI mass spectra of aryl ketones, πCOR, were studied and found to give primarily [M + 29]⁺, [M + 1]⁺, [M ? 1]⁺, [πCO]⁺ and [RCO]⁺ ions. The major change in the spectra with increasing length of the aliphatic side chain was an increase in the [M ? 1]⁺/[M + 1]⁺ ratio. Increasing sample size was reflected primarily in the formation of [2M + 1]⁺ ions and a decrease in [M + 1]⁺ ions. Small amounts of water in the reactant gas reduced the extent of fragmentation action.     

Studies in chemical ionization mass spectrometry: 17-hydroxy steroids

The chemical ionization mass spectra of several hydroxy steroids were obtained using methane as the reactant gas. The spectra are much less complex than the electron ionization spectra and little fragmentation of the steroid nucleus is observed. The major fragment ions involve the loss of water from [M + H]⁺. A 3-keto group in the steroids was characterized by an abundant [M + C₂H₅]⁺ ion. 5α- and 5β-Dihydrotestosterone could be distinguished by their spectra, with H₂ as the reactant gas by marked differences in amounts of [M + H]⁺, [M + H ? H₂O]⁺ and [M + H ? 2H₂O]⁺. Substituted 3α-X-, 17 β-ol compounds, (X = Cl, Br) were also studied to obtain relative amounts of protonation at these sites.     

Studies in chemical ionization mass spectrometry of some flavonoids

Formation of ions in chemical ionization mass spectrometry of flavonoid compounds has been studied. Production of adduct ions and fragment ions as a function of ring substituents and of reagent gas has been observed. Pressure and repeller field dependence of ions has been found as a function of ring substituents.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane

Mixtures 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.     

Negative chemical ionization mass spectrometry of explosives

The negative chemical ionization mass spectra of nitrobenzene, ethylene glycol dinitrate and nitroglycerine have been obtained using various reagent ions. For nitrobenzene, [OH]^? gives the [M ? H]^?, together with [M]^?˙ ions formed by electron capture, but other reagent ions gave relatively low intensity adduct peaks. Ethylene glycol dinitrate and nitroglycerine gave abundant [M + X]^? ions (X = NO₂, NO₃, Cl, Br, I), together with ions arising from the thermal decomposition of the samples in the heated inlet system. The rate of anion attachment to these compounds is much greater than that to related compounds having only one functional group, and it is suggested that this is due to the participation of the adjacent groups in the bonding between the substrate and anion.     

Hydrogen chemical ionization mass spectrometry of metalloporphyrins

The hydrogen chemical ionization (H₂ CI) mass spectra of a range of metal(II) (Ni, Cu, Co, Pt), metal (III) (Al, Mn, Ga, Fe (bearing a single axial ligand)) and metal(IV) (Si, Ge, Sn (bearing two axial ligands) and V (as V?O²⁺)) porphyrins have been determined, The spectra are highly dependent on the coordinated metal, rather than the axial ligand(s) (where present). Ni(II), Cu(II), Mn(II or III), Ga(III), Ge(IV), Fe(III) and Sn(IV) porphyrins fragment via hydrogenation and demetallation, followed by cleavage of the resulting porphyrinogens at the meso(bridge) positions to give mono- and di-pyrrolic fragments. Tripyrrolic fragments are also observed in the case of Ni(II), Cu(II) and Sn(IV). Fragmentations of this type are similar to those observed for free-base porphyrins. In the case of Pt(II), Co(II), Al(III), Si(IV) and V(IV) (as vanadyl), the dipyrrolic fragment ions are either very weak or completely absent; hence their H₂CI spectra contain limited structural information. This variable CI behaviour may be related to the relative stabilities of the metalloporphyrins together with the multiple stable valency states exhibited by several metals.     

The chemical ionization mass spectrometry of RDX

The fragmentation pathways of RDX in chemical ionization mass spectrometry have been rationalized, using data from different reagent gases, including CD₄ and iso-C₄D₁₀. The dependence of spectra taken with different gases on the acid strength of the reactant ions in the gases is accounted for.     

Methane chemical ionization mass spectrometry of flavonoids

The chemical ionization mass spectra of a representative selection of flavones, flavonols, flavanones, and flavanols have been examined, using methane as the reagent gas. The flavones and flavonols showed no significant fragmentation under the conditions employed, but the flavanones and flavanols showed characteristic fragmentation which could be of use in structural elucidation of these compounds.     

Atmospheric pressure chemical ionization of fluorinated phenols in atmospheric pressure chemical ionization mass spectrometry, tandem mass spectrometry, and ion mobility spectrometry

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) for fluorinated phenols (C6H5-xFxOH Where x = 0-5) in nitrogen with Cl- as the reagent ion yielded product ions of M Cl- through ion associations or (M-H)- through proton abstractions. Proton abstraction was controllable by potentials on the orifice and first lens, suggesting that some proton abstraction occurs through collision induced dissociation (CID) in the interface region. This was proven using CID of adduct ions (M Cl-) with Q2 studies where adduct ions were dissociated to Cl- or proton abstracted to (M-H)-. The extent of proton abstraction depended upon ion energy and structure in order of calculated acidities: pentafluorophenol > tetrafluorophenol > trifluorophenol > difluorophenol. Little or no proton abstraction occurred for fluorophenol, phenol, or benzyl alcohol analogs. Ion mobility spectrometry was used to determine if proton abstraction reactions passed through an adduct intermediate with thermalized ions and mobility spectra for all chemicals were obtained from 25 to 200 degrees C. Proton abstraction from M Cl- was not observed at any temperature for phenol, monofluorophenol, or difluorophenol. Mobility spectra for trifluorophenol revealed the kinetic transformations to (M-H)- either from M Cl- or from M2 Cl- directly. Proton abstraction was the predominant reaction for tetra- and penta-fluorophenols. Consequently, the evidence suggests that proton abstraction occurs from an adduct ion where the reaction barrier is reduced with increasing acidity of the O-H bond in C6H5-xFxOH.     

Low pressure chemical ionization in ion trap mass spectrometry

This article presents the specificities of low pressure chemical ionization in ion trap mass spectrometry. One main feature is the ability to perform chemical ionization with liquid reagents as readily as with "conventional" gases (methane, isobutane and ammonia). The reactivities and analytical applications of gas and liquid reagents are summarized from literature data and are compared when possible.     

Factors affecting reactivity in ammonia chemical ionization mass spectrometry

Several factors affecting reactivity in ammonia chemical ionization mass spectrometry (NH₃ CI) have been examined. These include the sample proton affinity, the preferred site of protonation and [NH₄]⁺ attachment, and substituent effects. In general, compounds having proton affinities ?787 kJ mol^?1 do not yield analytically useful intensities of the [M·NH₄]⁺ adduct ion. Substituted aromatic compounds in which the ring is the most basic site yield little (if any) [M·NH₄]⁺ ion even if the proton affinity of the compound is greater than 787 kJ mol^?1. On the other hand, some aromatic compounds in which the substituent is the most basic site yield relatively abundant adduct ions. The spectra of compounds possessing a good leaving group (X) exhibit only weak [M·NH₄]⁺ ions, but intense [M·NH₄ ? HX]⁺ and [M ? X]⁺ ions formed by substitution and elimination reactions. Electronic effects strongly influence these processes. Several examples are presented in which isomers are readily differentiated because of different reactivities under ammonia chemical ionization conditions.     

Pyrolysis chemical ionization mass spectrometry of cellulose ethers

Pyrolysis ammonia chemical ionization (PyCI) mass spectrometry was performed on hy-droxyethyl-, hydroxypropyl-,methyl-, hydroxypropylmethyl-, and ethylhydroxyethyl cel-luloses. The mass peaks in the PyCI mass spectra of these cellulose ethers could be assigned to the ions of pyrolytic dissociation products which form via the [2 + 2 + 2] cycloreversion and the E_i elimination pyrolysis pathway. Structural information about the residual amount of nonderivatized cellulose, the relative chain length distributions of the substituents in hydroxyalkyl celluloses, and the end-capping of hydroxyalkyl substituents by alkyl groups in the mixed cellulose ethers is obtained. Interference of secondary pyrolysis products in the PyCI mass spectra is found to be of minor importance, especially in the lower mass regions. © 1995 John Wiley & Sons, Inc.     

Pyrolysis/chemical ionization mass spectrometry of polymers

Pyrolysis/mass spectrometric studies have been made on polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) with electron ionization (EI) and chemical ionization (CI) mass spectrometry and a variable temperature probe for direct insertion into the source of the mass spectrometer. Similar results obtained with EI and CI mass spectrometry are in agreement with previous experiments. Advantages of the simplification of spectra in the CI made, as well as the advantages of using both techniques for identification of pyrolysis products, are discussed.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with ethers

Chemical ionization mass spectra of several ethers obtained with He/(CH₃)₄Si mixtures as the reagent gases contain abundant [M + 73]⁺ adduct ions which identify the relative molecular mass. For the di-n-alkyl ethers, these [M + 73]⁺ ions are formed by sample ion/sample molecule reactions of the fragment ions, [M + 73 ? C_nH_2n]⁺ and [M + 73 ? 2CnH_2n]⁺. Small amounts of [M + H]⁺ ions are also formed, predominantly by proton transfer reactions of the [M + 73 ? 2CnH_2n]⁺ or [(CH₃)₃SiOH₂]⁺ ions with the ethers. The di-s-alkyl ethers give no [M + 73] ⁺ ions, but do give [M + H]⁺ ions, which allow the determination of the relative molecular mass. These [M + H]⁺ ions result primarily from proton transfer reactions from the dominant fragment ion, [(CH₃)₃SiOH₂]⁺ with the ether. Methyl phenyl ether gives only [M + 73]⁺ adduct ions, by a bimolecular addition of the trimethylsilyl ion to the ether, not by the two-step process found for the di-n-alkyl ethers. Ethyl phenyl ether gives [M + 73]⁺ by both the two-step process and the bimolecular addition. Although the mass spectra of the alkyl etherr are temperature-dependent, the sensitivities of the di-alkyl ethers and ethyl phenyl ether are independent of temperature. However, the sensitivity for methyl phenyl ether decreases significantly with increasing temperature.     

Negative ion chemical ionization mass spectrometry of some isocyanates

Summary Negative ion mass spectra for 3 aliphatic and 4 aromatic isocyanates have been obtained by low pressure chemical ionization, using CH₄, CO₂ and N₂O as reagent gases. All compounds furnished intense anions at m/z 42. With CH₄, quasi-molecular anions were observed at m/z M+1 for aliphatic and m/z M+1 and M–1 for aromatic isocyanates. With N₂O, anionic substitution products at m/z M+15 and M+30 were observed, and with CO₂ and N₂O, peaks at m/z M–12 could be detected for all aromatic isocyanates. Studies with ¹³CO₂ and C¹⁸O₂ as reagent gases showed that the anions at m/z M–12 and M+15 correspond to [M–CO+O]^– and [M–H+O]^–, respectively.

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Negativionen-Massenspektrometrie mit chemischer Ionisierung von einigen Isocyanaten

Zusammenfassung Die Negativionen-Massenspektren von 3 aliphatischen und 4 aromatischen Isocyanaten wurden mittels chemischer Ionisation bei tiefem Quellendruck aufgenommen, und zwar mit den Reagensgasen CH₄, CO₄ und N₂O. Alle Verbindungen lieferten intensive Anionen mit m/z 42. Mit CH₄ erhielten wir die quasi-molekularen Anionen M+1 für aliphatische sowie M+1 und M–1 für aromatische Isocyanate. Das Reagens N₂O ergab die anionischen Substitutionsprodukte M+15 und M+30. Sowohl CO₂ als auch N₂O führten mit aromatischen Isocyanaten zur Bildung von M–12 Anionen. Versuche mit ¹³CO₂ und mit C¹⁸O₂ als Reagensgase zeigten, daß die Anionen M–12 und M+15 den Ionen [M–CO+O]^– und [M–H+O]^– entsprechen.

     

Internal energy deposition in chemical ionization/tandem mass spectrometry

The efficiency of the collision-induced dissociation (CID) process as a function of the internal energy deposited into the ion during the ionization event was evaluated. (M + H)+ ions of pyrrole, pyrrolidine, pyridine and piperidine (five and six-membered ring heterocyclics) were generated by chemical ionization (CI). The internal energy of the ions was varied by using different reagent gases. Both high-energy (keV) and low-energy (eV) CID were performed on these ions. The experiments showed that the (M + H)+ ions of the five-membered ring compounds, pyrrole and pyrrolidine, have higher fragmentation efficiencies than the six-membered ring compounds, pyridine and piperidine. Fragmentation efficiencies in high-energy CID clearly correlate with the internal energy deposited by the ionization technique. Experiments showed that the low-energy CID process is more sensitive than high-energy CID to changes in internal energy.     

Selective reagents in chemical ionization mass spectrometry: Tetramethylsilane with aliphatic alcohols

Mixtures of tetramethylsilane and helium have been found to form [M + 73]⁺ adducts, hydrated trimethylsilyl ions and alkyl ions with aliphatic alcohols. The adduct ions were found to be formed by displacement of water from the hydrated trimethylsilyl ion. Ratios of the abundances of the adduct ions to the hydrated trimethylsilyl ion can be used to differentiate among primary, secondary and tertiary alcohols. Sensitivities for a number of alcohols with the tetramethylsilane/helium chemical ionization reagent system are approximately equal.