Reagent isobutane

Polarization also occurs in coupling and disproportionation reactions of Grignard reagents with alkyl halides. The vinyl protons of isobutene produced in the reaction of t-butylmagnesium chloride with t-butyl bromide show A/E polarization as do the methyl protons of isobutane (Ward et al., 1970). Similar results arise in the reaction of diethyl-magnesium with organic halides (Kasukhin et al., 1972). 

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

Isobutane is an especially versatile reagent gas, because i) it provides low-fragmentation PICI spectra of all but the most unpolar analytes, ii) gives almost exclusively one well-defined adduct ([Mh-C4H9], [M+57]) if any (Fig. 7.7), and iii) can also be employed for electron capture (Chap. 7.4). 

Note Resulting from the large excess of the reagent gas, its spectrum is of much higher intensity than that of the analyte. Therefore, Cl spectra are usually acquired starting above the m/z range occupied by reagent ions, e.g., above m/z 50 for methane or above m/z 70 for isobutane. 

However, other processes, in particular proton transfer, are prevailing with methane, isobutane, and ammonia, for example. Reagent gases suitable for CE should exhibit abundant molecular ions even under the conditions of Cl, whereas potentially protonating species have to be absent or at least of minor abundance. 

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

This reagent was obtained either from Aldrich Chemical Company, Inc., or Lithium Corporation of America, Bessemer City, NC. A technical data sheet is available from the suppliers. Solutions of ca. 2 M were titrimetrically analyzed for active alkyllithium by the tosylhydrazone method. It is advisable to make certain that the organolithium reagent to be used was prepared in pentane solution. This evaluation can be easily accomplished by the gas chromatographic analysis of the organic layer obtained from the hydrolysis, under a nitrogen atmosphere, of the tert-butyllithium solution to be used. Isobutane and pentane should comprise essentially all of the... 

In the PICI mode a reagent gas is continuously introduced into the ion source, e.g. methane (isobutane and ammonia are also used). The gas interacts with electrons produced by the filament to produce a series of ions shown in Figure 9.22. 

Thus by choice of reagent gas, we can control the tendency of the Cl-produced M + H+ ion to fragment. For example, when methane is the carrier gas, dioctyl phthalate shows its M + H+ peak (m/z 391) as the base peak more importantly, the fragment peaks (e.g., m/z 113 and 149) are 30-60% of the intensity of the base peak. When isobutane is used, the M + H+ peak is large and the fragment peaks are only roughly 5% as intense as the M + H+ peak. 

Chemical ionisation results from the gas-phase collision between the analyte and species formed from the reagent gas introduced concomitantly in the ion source and bombarded by electrons. Methane, ammonium or isobutane are often used as reagent gases (Fig. 16.17). The reagent gas is introduced into the ion source at a pressure of a few hundred pascals, which reduces the mean free path and favours collision. Chemical ionisation produces positively and negatively charged species. 

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.

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 ... 

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