Electron-impact Spectra

Electron-impact Spectra.—A review of electron-scattering spectroscopy has appeared.265 Electron-impact spectra have been reported of helium,263 H and He,267 Li,258 Ba,259 Hg,280 H2,261 CO,282 Rbl and KI,283 NO and N20,284 water,286 ammonia and methane,2866 N02,288 and C02.287 The study on NOa yields the interesting result that the 2B2 state is asymmetric, possessing different equilibrium bond lengths between the N atom and each atom this can explain the results of recent microwave-optical double-resonance experiments.288 

Yamakawa, K. Ezumi, Y. Mizuno, and T. Kubota, Bull. Chem. Soc. Japan, 1974, 47, 2982. 

Llewellyn, J. Quant. Spectroscopy Radiative Transfer, 1975, 15, 513. 

Ishibashi, Chem. Phys. Letters, 1974, 26, 586 . C. Smyth, 

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The appearance and reproducibility of chemical ionization mass spectra depends on the ionizing conditions, principally the source temperature and presstire and the purity of the reagent gas. Chemical ionization mass spectra are generally not as reproducible as electron impact spectra. 

Based on a new technology, particle beam enhanced liquid chromatography-mass spectrometry expands a chemist s ability to analyse a vast variety of substances. Electron impact spectra from the system are reproducible and can be searched against standard or custom libraries for positive compound identification. Chemical ionization spectra can also be produced. Simplicity is a key feature. A simple adjustment to the particle beam interface is all it takes. 

This technique is complementary to the thermospray technique. Relative advances of the particles beam technique over thermospray include library searchable electron impact spectra, improved reproducibility, easier use and increased predictability over a broad range of compounds. But since a particle beam requires same sample volatility, very large and polar compounds such as proteins may not provide satisfactory results using particle beam liquid chromatography-mass spectrometry. Additionally, certain classes of compounds such as preformed ions, azo dyes and complex sugars may not yield satisfactory electron impact spectra, but can be run on thermospray. In other words, both liquid chromatography-mass spectrometry techniques complement each other s limitations and the analyst may want to add both to address a broader range of samples. 

Consequently, the MS investigations of C-nitroso compounds are frequently hampered by superimposed spectra due to dimerization/rearrangement of the initial compound. Further, electron impact spectra of larger non-aromatic C-nitroso compounds in general carry little information due to extensive fragmentation118. 

GC-MS analysis used a Finnigan 4000 quadrupole EI/CI mass spectrometer. Electron Impact spectra were recorded continually using an Incos Nova 4 data system. Ion source temperature was 250°C and the ionisation energy 70 eV. 

Traditionally, conventional electron impact techniques have been used to effect sample ionization. The often complex fragmentation pattern which results can provide invaluable information for elucidating the structure of unknown compounds. Vast computer libraries of electron impact spectra are available to assist the analyst with identifications. However, electron impact is an ionization technique that is somewhat limited in scope. By contrast, sample ionization by chemical ionization techniques offers the potential for selectivity, increased sensitivity, complimentary fragmentation, and confirmation or determination of molecular weight. (7). In this paper we will describe several examples in which the versatility of the chemical ionization technique has simplified or improved an analysis. 

Figure 13. Electron impact spectra (70 eV) of aldehyde o-methyl oximes...

Ions are produced in a standard electron impact ion source (VG 70-70). The only modification necessary to obtain true electron impact spectra was to widen the LC inlet hole and uncover the other three inlet ports. 

A 500-L solution containing 2 mg/L of free chlorine residual in distilled water was pumped onto the four-column system the columns were eluted and the eluants were processed as described earlier. This chlorine blank and resin eluant blanks were analyzed by GC-MS by using a Finnigan 4023 with the I NCOS data system and a 31,000-compound National Bureau of Standards library. Electron impact spectra were obtained by using an electron energy of 70 eV and a scan time of 1 s for the mass range 33-550 amu. A 30-m WCOT SE-54 fused-silica capillary column (J W Scientific) was used for separations. Injections were made with the oven at 40 °C and the door open, the injector at 220 °C, and the interface at 270 °C. Two minutes after injection, the door was closed and the temperature was raised ballistically to 60 °C, ramped at 4 °C/min to 280 °C, and held there for 4 min. The split and septum purge valves were closed for injection and opened after 1 min. 

Since an absorption coefficient of 10,100 cm-1 is equivalent to an oscillator strength of approximately two, the Johnson-Rice calculated oscillator strength and the observed oscillator strength differ by an order of magnitude. However, the calculated shape of the absorption versus energy agrees relatively well with the optical and electron impact spectra. The slow rise of the ion yield spectrum must still be explained as being caused by radiationless transitions. 

The stereospecificity of the RDA reaction can be used to produce unstable species and to determine their thermodynamic properties with the mass spectrometer. Such a project was undertaken by Turecek and collaborators571 to distinguish the Z- and -form of the dienol c given in Scheme 17. The 3-ew-vinylbicyclo[2.2.1]hept-5-en-2-ol is flash-pyrolized in the inlet system of a mass spectrometer and ionized within a few ms after decomposition. The mass spectra and the ionization energies of E-c and Z-c were determined. The electron impact spectra are similar, although some reproducible differences can be seen the CID spectra are identical. The same ions are produced by dissociative ionization of the precursors a and b. Their CID spectra are identical, however different from the isomeric aldehyde572. The ionization energies of the neutral dienols were measured as IIi(/i - c) = 8.51 0.03 and IE(Z - c) = 8.47 0.03 eV, respectively. Since the activation... 

Vainiotalo and collaborators585 measured the electron impact spectra of several either di-exo- or di-mfo-norbornane/ene-fused 2-lV-phenyl iminoperhydro-l,3-oxazines. Whereas the stereoisomeric unsaturated compounds cannot be distinguished on the basis of their electron impact spectra, the spectra of the stereoisomeric saturated compounds are sufficiently different to allow differentiation of the stereoisomers. The N-substituents were either H or Me. The difference is due to the fact that the unsaturated compounds fragment mainly by two RDA reactions with first a loss of cyclopentadiene and then yielding... 

The specifications approved for the GC/MS instrument allow the selection of an instrument with performance criteria of a standard GC/MS bench top system. The requirements specified in relation to blinding and transportability are substantial and require significant adaptations to any system currently available on the market. However, they do not limit analysis performance in terms of sensitivity or selectivity. The current system specifications do not include chemical ionization and limit analysis to electron impact spectra. 

In addition to preserving structure, a soft ionization technique such as chemical ionization (Cl) has a further advantage in ms /ms. By minimizing the number of ions generated from each molecular species the complexity of the (ionic) mixture which has to be separated is minimized. It is for this reason that electron impact ionization is seldom a good choice for ms/ms, although for compounds such as the polyaromatic hydrocarbons which give predominantly one ion in their electron impact spectra this complication is minimized. 

Lastly, it has been shown that negative ion electron-impact spectra of some compounds contain no skeletal reaiTangement ions, in contrast to the corresponding positive ion spectra (Bowie et al., 1969a). 

In 1974 Scott et al. [16] developed a transport system for transferring the solute from the LC column to the mass spectrometer. This interface allowed electron impact spectra to be obtained and so all the expected ion fragments were obtained to facilitate interpretation of the spectra. A diagram of the transport LC/MS interface is shown in figure 15. 

Figure 1.6. Electron impact spectra of isomeric (synlanti) oxime-t-butyldimethylsilyi derivatives of 13,14-dihydro-15-ketoPGFz . Both spectra are dominated by loss of the t-butyl radical giving m/e 669 (M-57) as the base peak (reproduced from reference [31])... 

Figure 6.14 Electron impact spectra for Cl(—) from C6H5C1 and simultaneous electron transmission spectra versus the electron energy, replotted from [69].

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