297 

 Electron Capture Negative Ion Chemical Ionization 



Electron capture NCI was addressed in Chapter 5; a brief overview is 

 included here to provide continuity of this appendix. Negative ions are produced 

 from electron capture by three processes [33,72,74,108]. The least common process 

 is that of ion-pair production, whereby the electron is captured, exciting the sample 

 molecule, and then re-ejected at a lower kinetic energy, leaving the molecule 

 dissociated into a positive ion and a negative ion. The more common processes are 

 those of associative resonance electron capture (equation A-2) and dissociative 

 resonance electron capture (equation A-3) of thermal electrons (Ct^"): 



e,h~ + MX ^ MX— A-2 



e^h" + MX ^ M + X-' A-3 



In order for a molecule to capture an electron, the electron affinity (EA) of the 

 molecule must be positive. Additionally, the electron attached species must be long- 

 lived enough for collisional quenching (MX~) or dissociation (M-f X~) to occur 

 [72,74,108,114-116]. 



The appeal of ECNCI, for compounds amenable to this process, is due to the 

 higher efficiency of electron/molecule reactions compared to conventional CI 

 ion/molecule reactions [72,74,108,117]. Unfortunately, ECNCI is not applicable to 

 a majority of compounds; however, this selectivity is generally considered an 

 advantage rather than a disadvantage. Although the electron capture processes are 

 fairly straightforward, mass spectra obtained by ECNCI are not always simple to 



