213 

 electron with a methane molecule to form a positive ion removes approximately 30 



eV of the kinetic energy from the incident electron [105,106]. At high enough 



methane pressures, multiple collisions lead to a population of thermal electrons 



(e,h~) for reaction with sample molecules by electron capture negative ion chemical 



ionization (ECNCI) [33,70,72,74,76,105,107,108]. The possible negative ion mode 



electron capture (EC) reactions are as follows: 



e(|,~" + MX -> MX~' associative resonance capture 5-10 



e,h~ + MX ^» M + X~' dissociative resonance capture 5-11 



ep~ -I- MX ^ M"" -I- X~' -I- e~ ion-pair production 5-12 



The common denominator of these processes is the capture of an electron by 

 a species (MX). Ion-pair production (Eq 5-12) is a special case and will be 

 explained later. In order for electron capture to occur, the electron affinity (EA) of 

 MX must be positive. Provided the species does not rapidly undergo 

 autodetachment of an electron, it can proceed through one or more of the processes 

 given in equations 5-10 through 5-12. 



Associative resonance capture (Eq 5-10) involves the capture of a thermal 

 electron, forming an excited species in which the excess energy can be distributed 

 over the molecule without bond rupture [72]. Studies have shown associative 

 resonance capture to be observed when electron energies are in the range of to 2 

 eV [72,76,108]. 



Dissociative electron capture (Eq 5-11) occurs when the electron captured 

 species contains sufficient excess internal energy to cause the rupture of one of more 



