Variation of the electric field IKE technique

These energy spectra have been extensively utilized. Termed IKE spectra, they are veritable fingerprints of compounds [105b,c,l 18,119a,b], thus leading to the otherwise impossible characterization of isomeric compounds. 

Isomers occasionally generate identical IKE spectra [120,121]. For instance, the various methyl indole isomers cannot be distinguished. This is explained by the possibility of isomerization of the M molecular ions (produced in El at 70 eV) into the same isomeric ion. This disadvantage may be eliminated by using other ionization techniques which can be softer than electron impact. 

Gallegos [116] published an example of the differentiation among alkene isomers. In the case of mixtures, this technique may be combined with GC, as in GC/MS. GC/IKE leads to a more satisfactory characterization of a mixture when GC/MS is insufficient. 

In this way, all the decomposition ions, m j, of the parent ion are obtained. It should be noted here that if a metastable transition m - n 2. i) also exists in the first FFR such that it verifies Eqn. 9, 

The measurement of MIKE spectra to elucidate ion structures and to study fragmentation mechanisms remains the method of choice, providing they lead to numerous fragmentations, which is not always the case when soft methods, such as Cl, FD, etc., are utilized. It is necessary to use an ionization which transmits a higher energy to the molecular ions. A larger number of fragments can be observed for the ions which decompose in the second FFR. 

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