296 

 Charge exchange reagent gases provide a means for controHing the extent of 

 fragmentation. The mass spectra produced are similar to EI spectra; however, due 

 to the average energy deposition being lower for CE than for EI, the abundance of 

 M^* will be greater. In the case of carbon dioxide, there are several species which 

 can interact with sample molecules. The COj^* ion at m/z 44 is the most obvious 

 charge exchange ion. The (C02)2** cluster ion (RE unlisted) at m/z 88 and the 02"^' 

 ion (RE of 9.7-17.0 eV) at m/z 32 may also react. These species, however, will only 

 have a minor impact on the overall fragmentation pattern due to the low abundance 

 of these ions with respect to COj"^'. 



One concern involved with the choice of a charge exchange gas is the 

 production of secondary ion species which may also be involved in the charge 

 exchange process. There is also a greater variation, compared to conventional CI, 

 in major ion abundances of CE reagent gases as a function of ion source pressure. 

 Therefore, optimization and more precise control of the pressure is necessary such 

 that the predominant charge exchange ion is the reagent ion of interest [112]. Water 

 is one of the most detrimental impurities. The presence of [M+H]"^ ions from a 

 reagent gas incapable of proton transfer may be due to self-CI or from reaction with 

 ions produced from water, e.g. (H30)'^ at m/z 19, (CHO)^ at m/z 29, and 

 (C0-H20)* at m/z 46. 



