299 

 and relatively inert gases) [115]. One additional feature of this reagent gas merits 

 mentioning. Due to the absence of hydrogen in the system and the low proton 

 affinity of CO2 (548 kJ/mol), formation of the [M-H]"" ion from samples would be 

 a rare occurrence with this reagent gas. As with CO2/CE, impurities present in the 

 ion source need to be minimized to prevent unexpected or unwanted reaction 

 pathways. Production of an [M-H]~ ion from this reagent gas is a likely indication 

 that an impurity is present, or that self-CI is occurring. Self-CI reactions were found 

 to be the most significant cause, in the work for this dissertation, for the inability to 

 perform electron capture. The elution of high concentrations of acids into the ion 

 source, as stated previously, generated mass spectra comparable to that of methane 

 NCI. 



Instrument Optimization and Background Ions 



The optimization for negative ion analysis was carried out by examining 

 characteristic PFTBA negative ions at m/z 264 and m/z 414, as well as the m/z 127 

 ion (I~) found in the background on the TSQ70. The optimization plot is presented 

 in figure A-1. Formation of ions requires indicated pressures greater than 500 mtorr 

 CO2 in the ion source. Examination of the profiles demonstrates the need for 

 precise control over the ion source pressure to maximize sample ion generation 

 efficiency. The optimum indicated pressure of 1200 mtorr is most likely a combined 



