314 

 m/z 19 ion relative to m/z 18 indicating increased Cl-like high-pressure proton 

 transfer reactions. There is a significant increase in the production of m/z 32 (02"^') 

 as well as m/z 45 and m/z 62, the (C02)H"^ and (C02)H20** ions, respectively. The 

 presence of an ion at m/z 89 from (C02)2H^ is also noticeable in this mass spectrum. 

 The mass spectra for equivalent experiments with negative ions are in figures A-7 

 (normal background) and A-8 (with water added). The addition of water to the 

 system does not seem to generate an appreciable difference, except for the increased 

 abundance of the m/z 16 (0~') and m/z 17 (OH~) ions. Although water may not 

 affect the background ions of CO2 significantly, the 0~* and OH~ ions present may 

 lead to conventional CI proton transfer reactions, generating unexpected fragment 

 ions [129,130]. 



Selected Examples 



Impurities such as water, as stated previously, are not the only cause for 

 unexpected results in a negative ion CI mass spectrum. An example of the presence 

 of both wall reactions and self-CI from high ion source pressure can be seen (in 

 figure A-9) for lactic acid. The predominant ions at m/z 179 and m/z 251 are 

 characteristic ions found in mass spectra of lactic acid under saturated ion source 

 conditions (see Chapter 3). These ions are attributable to self-CI reactions. The 

 region of interest pertaining to suspected wall reactions is in the m/z 87 to m/z 90 



