Isobutane chemical ionization

Two examples of chemical ionization (isobutane) spectra. The top spectrum is that of a pure compound. The bottom spectrum is that of a mixture of two compounds with masses 261 and 270. They correspond respectively to the loss of hydrogen cyanide and water. 

Ms. Mass spectroscopy has been used to characterize pyrrolizidine alkaloids since the mid 1960 s and the major fragmentation patterns have been determined (32-34). We have found that the rather low-intensity molecular ions observed in electron Impact (El) spectra can be dramatically enhanced by use of chemical ionization (isobutane) where intense ions corresponding to the molecular ion plus hydrogen are observed. The El and Cl spectra of lycopsamine shown in Figure 8 illustrate this phenomenon. 

Figure 8. Mass spectra of ly cops amine electron impact (a) chemical ionization (isobutane) (b).

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

McGuire, J.M. Munson, B. Comparison of Isopentane and Isobutane as Chemical Ionization Reagent Gases. Anal. Chem. 1985, 57, 680-683. 

The electron impact (Figure 9) and chemical ionization (figures 10 and 11) spectra of benzoic acid were obtained using a Finnigan Mat 8200 mass spectrometer. Methane and isobutane were used as the reactants for the Cl methods. The studies were performed at 220°C(EI) and 130-140°C (Cl), 0.05 mA, and 70 eV. The assignment of the main fragments is presented in Table 6. 

Figure 10. Chemical-ionization mass spectrum (obtained using isobutane) of benzoic acid.

Chemical ionization produces less fragmentation than electron ionization. For chemical ionization, the ionization source is filled with a reagent gas such as methane, isobutane, or ammonia, at a pressure of 1 mbar. Energetic electrons (100-200 eV) convert CH4 into a variety of reactive products ... 

R. Saferstein, J.M. Chao J.J. Manura, Isobutane Chemical Ionization Mass Spectro-graphic Examination of Explosives , JAssoc of... 

Another conventional ionization technique termed chemical ionization (Cl), utilizes a reagent gas (such as isobutane, methane, ammonia) to form reagent ions (Rif) which can undergo ion-molecule reactions with the compound of interest to form protonated molecules. 

Using chemical Ionization with Isobutane and selected Ion monitoring of the M + H peak, the detection limit of the methyl ester of N-trifluoroacetyl-melphalan was less than 500 pg [41]. 

M. T. Gilbert and C. J. W. Brooks, Characterization of diastereo-meric and enantiomeric ephedrines by gas chromatography combined with electron-impact mass spectrometry and isobutane chemical-ionization mass spectrometry, Biomed. Mass Spectrom., 4 226(1977). 

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. 

The detection of chiral recognition with a mass spectrometer was reported first in 1977 by Fales and Wright.357 Their study showed that the chirality of dialkytartrates (T) strongly influences the stability of their diastereomeric proton-bound dimers, generated by isobutane chemical ionization (CIMS) of their racemic mixtures.358,359 In order to differentiate the protonated species of the homochiral self-dimers... 

Spectrum of the isobutane plasma under chemical ionization conditions at 200 pbar. 

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