Isobutane mass spectrum

Figure 5. Particle Beam NCI (isobutane) mass spectrum of the major peak resolved by anion exchange chromatography of an aqueous remainder fraction from a Stringfellow leachate sample.

Figure 10. Particle Beam PCI (isobutane) mass spectrum of the second peak (R, 5.2 min) from the anion exchange chromatogram of a lyophilized drinking water sample from Santa Clara shown in Figure 9.

Example In an overdose case where evidence was available for the ingestion of Percodan (a mixture of several common drugs) the isobutane-CI mass spectrum of the gastric extract was obtained (Fig. 7.8). [29] All drugs give rise to an [Mh-H] ion. Due to the low exothermicity of protonation by the tert-C Hi) ion, most [Mh-H]" ions do not show fragmentation. Solely that of aspirin shows intense... 

Fig. 7.8. Isobutane Cl mass spectrum of gastric contents in an overdose case. Reproduced from Ref. [29] by permission. American Chemical Society, 1970.

Example The extraordinary stable trityl ion, PhsC, m/z 243, tends to dominate mass spectra (Chap. 6.6.2). Thus, neither the El spectrum of chlorotriphenyl-methane nor that of its impurity triphenylmethanol show molecular ions (Fig. 8.11). An isobutane PICI spectrum also shows the trityl ion almost exclusively, although some hint is obtained from the Ph2COH ion, m/z 183, that cannot be explained as a fragment of a chlorotriphenylmethane ion. Only FD reveals the presence of the alcohol by its molecular ion at m/z 260 while that of the chloride is detected at m/z 278. Both molecular ions undergo some OH or Cl loss, respectively, to yield the Ph3C fragment ion of minor intensity. 

Figure 10. Chemical-ionization mass spectrum (obtained using isobutane) 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 following discussion will be concerned primarily with applications of the ms/ms technique in the synfuel area. Attempts will be made to illustrate the unique capabilities of the ms/ms analysis with examples taken from our work on coal liquefaction products. Figure 5 shows the positive ion chemical ionization (PCI) mass spectrum of the coal liquid in question (SRC II mid heavy distillate, total bottoms). This spectrum is actually the normalized sum of approximately 500 individual mass spectra taken while the SRC II was thermally vaporized from a solids probe into the source of a mass spectrometer, and represents the molecular weight profile of this distillate fraction. Since isobutane Cl gives to a first approximation only protonated molecular ions (and no fragment ions), the peaks represent the individual components in the SRC II arranged incrementally by molecular weight. 

For a straight-chain, or normal, alkane, a peak corresponding to the molecular ion can be observed, as in the mass spectra of butane (Fig. 8.5) and octane (Fig. 8.6). As the carbon skeleton becomes more highly branched, the intensity of the molecular ion peak decreases. You will see this effect easily if you compare the mass spectrum of butane with that of isobutane (Fig. 8.7). The mol-... 

The El mass spectrum (Figure 5.3a) shows only peaks due to ion fragmentation in the ion source, whereas molecular ions are absent. The isobutane Cl spectrum shows instead the presence of protonated molecular ions at m/z 131 and m/z 261, due to the cyclic monomer and dimer, respectively. The isobutane DCI spectrum shows furthermore the presence of protonated molecular ions corresponding to cyclic trimer and tetramer at m/z 391 and m/z 521, respectively. ... 

Isobutane Cl mass spectrum of pyrolysis compounds evolved in the DPMS of a PC/Nylon 6 (1 /I, mol/mol) blend sample, at 385°C. (Reprinted from Ref. 103.)... 

Example The NICI mass spectrum of tetraiodoethene, I2C=CI2, has been obtained using isobutane reagent gas (Fig. 7.12). The negative molecular ion, NT, at m/z 531.6 has a relative intensity of just 0.15%, while the product of nucleophilic addition, [M+I]", tn/z 658.5, yields the base peak [77], Losses of T and I2 from M"" are also observed. The series of peaks at m/z 126.9, 253.8, and 380.7 corresponds to traces of iodine present as impurity of tetraiodoethene. The iodine is also ionized by both electron capture (EC, next paragraph) and iodide addition. The spectrum nicely exemplifies the superimposition of mass spectra of two components of a mixture. It is not always simple to tell the corresponding peaks apart accurate mass measurements or tandem mass spectrometry may be required. It is worth noting the mass defect introduced by the iodine and the C isotopic peak of merely 2% due to only two carbon atoms present. 

Fig. 7.12. NICI mass spectrum of tetraiodoethene (isobutane reagent gas, electron emission 300 pA at 200 eV, and ion source temperature of 200°C). Adapted from Ref. [77] with permission. Wiley-VCH, Weinheim, 2007.

The main advantage of Cl-MS is the selective production of intact qnasi-molecnlar ions [(M + H)" ]. Figure 3.4 shows the mass spectrum of butyl methacrylate acquired under different ionization conditions. The molecnlar ion (miz = 142) is barely visible in the El-MS, but the (M + H)" ion (miz = 143) is prominent in the CI-MS spectra. The CI-MS acqnired nsing isobutane has... 

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