302 

 tradeoff between COj efficiency of electron thermalization and collisional quenching 

 of sample ions by this gas. 



The negative ion mass spectrum of PFTBA, acquired at an indicated pressure 

 of 1200 mtorr CO2 reagent gas, is shown in figure A-2. All ions found in the mass 

 spectrum, with the exception of m/z 60, are characteristic ions seen with this 

 instrument when methane is employed as the electron moderating gas for NCI. The 

 m/z 60 ion is presumed to be COj"'. This ion has been examined previously due to 

 concerns with modeling the ionized atmosphere above the earth, and for instability 

 problems with CO2 lasers [127]. Based on this literature source, it was proposed that 

 the C03^* ion is formed via collisions of COj with 0~'. Later studies support the 

 attachment of an electron to clusters of (C02)n, whereby stepwise elimination of 

 neutral CO2 or CO occurs from the solvated ion to eventually yield CO3 [128]. 

 Additional negative ion clusters are absent from the background spectra shown here 

 due to the low pressures employed in this study; the optimization for EC reactions 

 occurred at a low pressure, i.e. 1200 mtorr. At pressures below 2000 mtorr, the 

 CO3 and O ions predominate. The appearance of (C02)2 at m/z 88, or higher 

 cluster ions, will not occur until pressures of 2000 mtorr or greater are reached [128]. 



Ions produced from cluster reactions in the positive ion mode are more 

 abundant in the pressure regime of these studies (figure A-3). Major ions present 

 in the positive ion background are at m/z 32 (02'^'), m/z 44 (C02'^*), and m/z 88, the 

 (C02)2^' ion. Additionally, ions at m/z 45 and m/z 46 are visible in the mass 



