Chemical ionization methane

Hedin, P.A. and Phillips, V.A., Chemical ionization (methane) mass spectrometry of sugars and their derivatives, J. Agric. Food Ghent., 39, 1106, 1991. 

The compound represented by an electron-impact spectrum and a chemical-ionization (methane) spectrum is an ester of a long-chain, aliphatic alcohol. Interpret the spectra and identify the compound. 

Figure 3. Chemical ionization (methane) GC/MS of the acetylated final product derived from periodate oxidation of the myoinositol ring in PSL-I. (a) Total ion chromatogram of co-injected mixture of the unknown dideuterated alcohol product and the authentic erythritol. (b) Chemical ionization spectrum of peak indicated by an arrow in (a). Inset diagrams depict the fragmentation.

Mass spectrum cimetidine- chemical ionization- methane reactant gas. 

The mechanism of the formation of [MH-30]+ ions upon chemical ionization (methane) of 6-nitroindazole has been studied [1338], The absence of metastable peaks corresponding to this abstraction as well as comparison of the mass spectra of coimpact-activated ions [MH-30]+ arisen from 6-nitroindazoles with MH+ ions... 

Table I. Practical quantitation limits (PQL, ng injected), correlation coefficients (R2 and quadratic regression parameters (a and b) of 21 compounds with electron impact, and positive and negative chemical ionization (methane) Particle Beam mass spectrometry, direct flow injection with full scan mode...

Three reagent gases are mainly used in positive chemical ionization methane, isobutane, and ammonia. Their proton affinities (in kilojoules per mol) are methane. 

Because this chemical reaction occurs between the and M species, the original methane (CH4) is called a reagent gas, the CH5+ species are reagent gas ions, and the process is known as chemical ionization (Cl). 

Although the conventional mass spectra of the five C- nitro derivatives of indazole are nearly identical, the corresponding metastable peak shapes associated with the loss of NO-can be used to differentiate the five isomers (790MS114). The protonation and ethylation occurring in a methane chemical ionization source have been studied for a variety of aromatic amines, including indazoles (80OMS144). As in solution (Section 4.04.2.1.3), the N-2 atom is the more basic and the more nucleophilic (Scheme 5). 

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

In brief, the method consists of introducing small amounts (partial pressures of 10 3-10 4 torr) of the substance to be investigated into the ionization chamber of a mass spectrometer which contains a high pressure (1 torr) of methane, the reactant gas. Ionization is effected by electron impact, and because the methane is present in such an overwhelming preponderance, all but a negligibly small amount of the initial ionization occurs in the methane. The methane ions then undergo ion-molecule reactions to produce a set of ions which serve as reactant ions in the chemical ionization process. The important reactant ions formed from... 

Thus the reactant ions for chemical ionization formed in the methane plasma consists of approximately equal amounts of a strong gaseous Bronsted acid (CH5+) and ions which can act either as Lewis acids or Bronsted acids (C2H5+ + C3H5+). These reactant ions will effect the chemical ionization with an added substance by proton transfer or hydride ion transfer, both of which may be accompanied by fragmentation of the ion initially formed. 

We have previously shown (8) that the chemical ionization spectra using methane as reactant are generated by the combination of dissociative proton transfer from CH5 + and hydride ion abstraction and alkyl ion... 

Figure 3.2 Piocesses occurring in chemical ionization mass spectrometry using methane as the reagent gas.

Representative mass spectral conditions (negative chemical ionization) ion source temperature, 150°C ionizing current, 0.20 mamp electron energy, 70 eV methane reagent gas (source pressure 0.5 to 1 torr). 

What are the three commonly used chemical ionization reagent gases (methane, isobutane, water, ammonia, methanol, hydrogen, oxygen, nitrogen, etc.). 

It appears that gas-phase basicity of nitro compounds has been studied only scarcely. Thus, only the values of the parent compounds, nitromethane (179.2 kcalmol-1) and nitrobenzene (193.4 kcalmoD1), are found in the comprehensive listing given in Reference 39. The rather high PA values for nitro compounds suggest protonation by common chemical ionization reagent systems, such as hydrogen (H3+) and methane (CH5+). 

Chemical ionization MS of 2-aryl-3-nitro-2//-chromenes and 4-hydroxy-3-nitroflavans have been studied using methane and ammonia as reagent gases102. The behaviour of 2-aryl-3-nitro-2//-chromenes was found to resemble that of aromatic nitro compounds. Thus, the methane spectra are characterized by the MH+ ions, whereas the ammonia... 

The chemical ionization MS of a selection of substitued nitrobenzenes have been studied106. The H2 chemical ionization MS appeared significantly more useful for characterization of, e.g., isomeric compounds than the corresponding methane spectra, apparently due to the higher internal energy deposited in the protonated molecule by the reaction with H3+, and consequently in the more extensive fragmentation106. 

Field, F.H. Munson, M.S.B. Reactions of Gaseous Ions. XIV. Mass Spectrometric Studies of Methane at Pressures to 2 Torr. J. Am. Chem. Soc. 1965, 87, 3289-3294. Hunt, D.F. Ryan, J.F.I. Chemical Ionization Mass Spectrometry Studies. I. Identification of Alcohols. Tetrahedron Lett. 1971,47,4535-4538. 

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