Methane mass spectra

Figure 9. Dependence of the ratio of CH + ions to total ions in the methane mass spectrum on the pressure of methane in the mass spectrometer ion source for different values of source repeller voltage...

Recently, the CH4+-CH4 reaction has been investigated (9) by measuring the CH4 + disappearance cross-section rather than CH5 + formation cross-sections. Results of this work are shown in Figure 9. Two mechanisms cause a loss of CH4 + ions from the total ion yield in the methane mass spectrum. There are loss processes in the ion source which generate new ions, CH5 +, and possibly other products. Other loss... 

Figure 7 TIC, mass chromatogram (at m/z 344), and Cl (methane) mass spectrum from a silylated extract of debris. The peak emerging after 138 seconds was identified as the TMS derivaive of pentaerythritol trinitrate.

Similarly, methane Cl spectrum of 18a was found to be dominated by the (C6H5C= CC6H5 + H)+ ion. A distinct molecular ion species at m/e value corresponding to (M + H)+ was observed in the methane mass spectra of this thiirene oxide (26% 2 40). Furthermore, the relative intensity of the (M +H)+ peak of 18a was shown to increase substantially in the isobutane and dimethyl amine Cl mass spectra91. 

If the sample consists of atoms of one element, the mass spectrum gives the isotopic distribution of the sample. The relative molar masses of the isotopes can be determined by comparison with atoms of carbon-12. If the sample is a compound, the formula and structure of the compound can be determined by studying the fragments. For example, the + 1 ions that CH4 could produce are CH4, CH3+, CH, CFI4, C+, and H4. Some of the particles that strike the detector are those that result when the molecule simply loses an electron (for example, to produce Cl I4+ from methane) ... 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

The peak obtained in the spectrum is referred to as M+ peak and the intensity of this highest peak is called the base peak and it corresponds to the molecular weight of the substance and the intensities of all other peaks are expressed relative to the base peak. The base peak is caused by the fragment ion which is most stable and whose formation requires least energy. Mass spectrum of methane is given on page 268. 

In addition to this there are some additional peaks which is unusual in the typical organic compounds, The mass spectrum of methane shows m/e values of 14, 13, 12, 2 and 1. This is explained as due to the formation of cationic fragments as follows ... 

A simple example is the mass spectrum of methane, CH4 (Figure 10.18). A methane molecule breaks into the following fragments (in order of relative abundance) CH, CH, CH, CH+, H+, C+, and 13CH4. In the figure, each peak is identified as being due to one of these fragments. Notice that one of the peaks... 

Figure 10.24 shows the mass spectrum of acetone. Based on the mass-to-charge ratio of each line, write a formula for as many of the fragments as you can, as was done for methane in Figure 10.18. 

Fig. 2.12.6. Identification of esterquat compounds FIA-APCI-MS-MS(+) (CID) product ion mass spectrum of selected [M — RCO]+ base peak ion of cationic surfactant blend of di-hydrogenated tallowethyl hydroxyethyl ammonium methane sulfate type (mlz 692 general formula (R(C0)0CH2CH2)2-N (CH3)-CH2CH2(0H)CH30S03) fragmentation behaviour under CID...

Fig. 2.12.9. (a) FIA-ESI-MSC+) overview spectrum of cationic gemini surfactant lV,lV,lV,lV,-tetramethyl-lV,iV,-dicloclecyle-l,3-propane-cliyle-diammonium dibromide C12H25-N (CH3)2-CH2-CH2-CH2-N (CH3)2-C12H25 2Br- (b) FIA-ESI-MS(+) mass spectrum of cationic gemini surfactant as in (a) in the presence of methane sulfonic... 

In the mass spectrum of methane (Fig. 1.2), there is a tiny peak at m/z 17 that has not been mentioned in the introduction. As one can infer from Table 3.1 this should result from the content of natural carbon which belongs to the X-rl elements in our classification. 

The most intensive peaks in the El mass spectrum of methane are the molecular ion, m/z 16 and the fragment ion at m/z 15 (Fig. 6.1). Explicitly writing the electrons helps to understand the subsequent dissociations of CH4 to yield CH3, m/z 15, by H loss (oi) or by CH3 loss (02), respectively. In general, it is more convenient to write the molecular ion in one of the equivalent forms. The charge and radical state are then attached to the brackets (often abbreviated as 1) enclosing the molecule. 

Fig. 6.1. El mass spectrum of methane. Used by permission of NIST. NIST 2002.

Examples To rationalize the mass spectrum of methane, reactions 6.2-6.6 were proposed. They all obey the mle. You should check the mass spectra and fragmentation schemes throughout this chapter for additional examples of the nitrogen rule ... 

The El mass spectrum of methane has already been discussed (Chap. 6.1). Rising the partial pressure of methane from the standard value in El of about 10 Pa to 10 Pa significantly alters the resulting mass spectrum. [1] The molecular ion, CH/ , m/z 16, almost vanishes and a new species, CHs", is detected at m/z 17 instead. [16] In addition some ions at higher mass occur, the most prominent of which may be assigned as C2H5, m/z 29, [17,18] and CsHs", m/z 41 (Fig. 7.2). The positive ion Cl spectrum of methane can be explained as the result of competing and consecutive bimolecular reactions in the ion source [4,6,10]... 

Fig. 7.5. Comparison of (a) 70 eV El spectrum and (b) methane reagent gas Cl spectrum of the amino acid methionine. Fragmentation is strongly reduced in the d mass spectrum.

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

A molecular ion is seen in the electron impact mass spectrum of compound (6) but the base peak is that resulting from the loss of nitrogen [M — 28] <91AG(E)1476>. The electron impact mass spectra of sydnones show the loss of NO and CO, either consecutively or simultaneously <84CHEC-I(6)365). A study of the Cl mass spectra of protonated sydnones, with methane as the reagent gas, shows that HNO and CO are the fragments lost <89H(29)185>. The electron impact mass spectrum of compound (19) shows an intense molecular ion which loses NO and then HCN <85JCS(Pi)2439>. 

Figure 11. Chemical-ionization mass spectrum (obtained using methane) of benzoic acid.

Figure 16.17—Chemical ionisation. This figure shows reactions occurring when methane is used as a reagent gas. The last equation represents the reaction that occurs when isobutane is used. Because of the high pressure used, the intensity of ions from the reagent gas is high, thus the mass spectrum is not scanned below 50 Da.

The chemical ionisation (Cl) mass spectrum Fig. 3, was recorded on a Finnigan 4000 Mass Spectrometer with ion source pressure 0.3 Torr, ion source temperature 150°C, emission current 300 yA, electron energy 100 eV using methane as a reagent gas. The electron impact (El) mass spectrum Fig. 4, was recorded on Varian MAT 311 Spectrometer, with an ion source pressure 10 6 Torr, ion source temperature 180, emission current 300 yA and electron energy of 70 eV. 

The chemical ionization mass spectrum of cimetidine and the major fragmentation ions are presented in Figure 8 and Table 4. The spectrum was obtained using a Finnigan lYbdel 3200 quadrupole mass spectrometer fitted with a chemical ionization source. The sample, applied to the probe from an acetone solution, was introduced via the direct inlet system. Methane was used as the reactant gas. 

Mass spectrum cimetidine- chemical ionization- methane reactant gas. 

The electron impact (El) mass spectrum at 70 eV recorded on Varian Mat 311 mass spectrometer and the methane derived chemical ionization (Cl) mass spectrum obtained with Finnigan 4000 mass spectrometer are shown in Figures 1 and 2 respectively. The (El) spectrum Fig. 

The carrier gas was argon/methane, and the source temperature was 240°C. A PZ179 column was used in the gas chromatograph. This mass spectrum was obtained while the compound was in the source, as indicated in Figure 2. 

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