215 

 then react with a neutral sample molecule via associative or dissociative resonance 



capture. Therefore, it is necessary that a suitable reagent gas is used to produce 



thermal electrons for ECNCI to take place. 



It is estimated that the rate of capture of electrons by compounds amenable 

 to EC is approximately 400 times greater than for CI ion/molecule reactions; thus, 

 enhanced sensitivity is observed for these compounds [72]. The compounds 

 amenable to ECNCI are generally oxidizing or alkylating agents [107]. The 

 molecular anions of these compounds were found to be stable if a "low-lying" 

 unoccupied molecular orbital is available, or if the molecule can be stabilized 

 through TT-system resonance [72]. It is best stated that the more "molecular debris" 

 and less MX or [M-H]~, the less resonance stabilization is available to the 

 molecule. Generally, the best systems or molecules for EC are extended 7r-systems 

 with strong electron withdrawing groups. 



There are additional constraints as well as differences between the 

 characteristics of negative ions and positive ions in the gas phase [72]. Each positive 

 ion fragment can carry a charge, whereas the EA must be greater than zero for 

 negative ion fragments to be present as an anion. The probability of anion formation 

 is approximately a linear function of the EA of the neutral species. In positive ion 

 analysis, the fact that the filament does not produce monoenergetic electrons nor 

 does it significantly alter the fragmentation within the standard operation regime of 

 filament electron energies (70 eV). The lack of monoenergetic electrons in the 

 negative mode allows for multiple reaction processes to occur. The autodetachment 



