Mass spectra of methane and

The mass spectrum of a compound is usually presented as a bar grap. with masses (m/z values) on the x axis and intensity (number of ions of given m/z striking the detector) on the y axis. The tallest peak, called ih-. base peak, is arbitrarily assigned an intensity of 100%. Figure 12,2 show mass spectra of methane and propane. ... 

Foner and Hudson (25), however, detected only CH3 in the mass spectra of methane-oxygen flames. 

The effect of temperature on the relative molecular ion abundances in the mass spectra of CH4 and CD4 has been studied [493]. Isotope effects have been determined in the charge exchange mass spectra of partially deuterated methanes [682]. 

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. 

Corina and coworkers24b also gave the ten ion listings for the El mass spectra of CH3S02CH2C0R (R = —CH2CH2COOCH3, —(CH2) COCH2Cl or —(CH2)14CH3) in context of their studies on the GC and GC/MS of various methane thiolsulphonates. 

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. 

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.

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 ... 

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>. 

A Finnigan MAT TSQ mass spectrometer was used to record the mass spectra of brinzolamide. The conditions were 0.3 mA current, 1100-volt acceleration voltage and 10 preamp sensitivity. The ionization voltage was 70 eV for El (electron impact) ionization mode. For Cl (chemical ionization) mode, a positive ionization voltage of 100 eV and a pressure of 0.3 torr of methane were used. The El and Cl spectra are shown in Figures 12 and 13, respectively. Peak assignments for the El and Cl spectra are listed in Table 6. Both the Cl and El spectra have an MH peak at m/z = 384. 

El (top), methane Cl (middle) and isobutane Cl (bottom) mass spectra of butyl methacrylate. The ionization techniques (El vs Cl) and the reagent gases (methane vs isobutane) influence the amount of fragmentation and the prominence of the protonated molecular ions detected at 143 Th. 

---