Reagent gases, mass spectra

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.

If the substrate (M) is more basic than NHj, then proton transfer occurs, but if it is less basic, then addition of NH4 occurs. Sometimes the basicity of M is such that both reactions occur, and the mass spectrum contains ions corresponding to both [M + H]+ and [M + NH4]. Sometimes the reagent gas ions can form quasi-molecular ions in which a proton has been removed from, rather than added to, the molecule (M), as shown in Figure 1.5c. In these cases, the quasi-molecular ions have one mass unit less than the true molecular mass. 

These arise either by an analogous process to that described above for Cl, i.e. the adduction of a negatively charged species such as Cl , and the abstraction of a proton to generate an (M — H) ion, or by electron attachment to generate an M ion. The ions observed in the mass spectrum are dependent on the species generated by the reagent gas and the relative reactivities of these with each other and with the analyte molecule. 

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>. 

Two isomeric complexes, 60 and 61, have been analyzed by Cl MS (reagent gas CH4), producing [M -b I] and [M -b 29]+ ions. Electron ionization mass spectrometry was applied in the analysis of the product of the reaction between Zn(CF3)Br-2CH3CN (62) and 4-(Af,Af-dimethylamino)pyridine (DMAP). The El (20 eV) mass spectrum of the product, Zn(CF3)Br DMAP (63), was recorded at 280 °C and consisted mainly of [C6H3BrF2NZn]+, [ZnBr2]+ and [ZnBr]+ ions. At lower temperatures, this compound did not yield any Zn-containing ions, and the spectra were dominated by the peaks of the [DMAPJ+ and [C2HgN]+ ions . 

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.

Gillis et al (Ref 79), using H2 reagent gas in a chemical ionization mass spectrometry system, suggest that the RDX mass spectrum can be rationalized by the following scheme ... 

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. 

As each mixture component elutes and appears in the ion source, it is normally ionized either by an electron beam (see Chapter 3, Electron Ionization ) or by a reagent gas (see Chapter I, Chemical Ionization ), and the resulting ions are analyzed by the mass spectrometer to give a mass spectrum (Figure 36.4). 

The product ions generated by chemical ionization are stable, even-electron species, with relatively little excess energy compared to those generated by electron impact. The chemical ionization mass spectrum, therefore, is characterized by the presence of a few intense molecular ion adducts with very little further fragmentation from which the sample molecular mass is readily identified from the m/z value of the molecular ion adducts. The type of molecular ion adduct formed depends mainly on the sample composition and the identity of the reagent gas. 

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