NICI electron-capture technique

The MCI spectrum of psoralen (Fig. 9.25) indicates that resonance capture is occurring so that most of the ion current is carried hy the molecular ion at m/z 186. The associated GC trace indicates that ionisation is ca 10 times more efficient for the same amount of psoralen in comparison with El. In addition, since most of the ion current is channelled into the molecular ion, an analytical method which selectively monitored the molecular ion of psoralen would be ca 40 times more sensitive if NICI conditions were used rather than El. The technique only works for compounds which are electron capturing. The small ion at m/z 218 indicates that the psoralen has also formed an adduct with traces of oxygen present in the instrument, i.e. true chemical ionisation. 

Careful selection of the derivatization technique may also be employed to enhance the detection limit of an analyte. The conversion of amphetamines to tetrafluorophthaloyl derivatives yields compounds possessing high electron capture cross-sections and the sensitivity toward such compounds, using negative ion chemical ionization (NICI) can be better than that achieved using positive ion formation by two orders of magnitude. Employing NICI in conjunction with SIM, the detection of amphetamine derivatives in quantities as small as 10 fg (SOamol) may be effected. 

Electron capture EC Not a Cl technique in the strict sense, but ion source operation is equal to NICI... 

In the NICI technique (1), methane was used as a buffer gas to produce thermal electrons, which were preferentially captured by an analyte species to form anions. In the case of PAH, it was observed that the ionization behavior of these compounds under NICI conditions was related to the electron affinity (EA) of the compound. Compounds with EA values greater than about 0.5 eV captured electrons to form molecular anions, whereas PAH with lower EA values did not ionize. This difference in ionization behavior... 

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