307 

 spectrum. These are presumed to originate from either wall reactions, or the 

 presence of an impurity such as water. A closer inspection of the low mass range 

 (figure A-4) reveals the presence of HiO"^' and H3O*, at m/z 18 and m/z 19, 

 respectively. The ions at m/z 56, 60, 72, and 76 are generated by CO2 cluster ions 

 of (C02)C*', (CO^)©^*, (C02)C0**, and (C02)02+', respectively [128]. The ions at 

 m/z 30, 34, 45, 46, 47, 63, 89, and 90 are presumed to result from impurities adding 

 hydrogen to the system, either from water, or from surface-bound radicals on the ion 

 source walls [121]. 



Experiments were conducted involving the addition of water to the system in 

 both positive and negative ion modes. The mass spectra were acquired in the 

 absence of calibration gas. The direct insertion probe was inserted with an empty 

 vial as the blank; the second series contained water in the probe vial. The mass 

 spectrum of the blank in positive ion mode is shown in figure A-5. The relative 

 intensities of the water ions at m/z 18 and m/z 19 should be noted, as well as the 

 ratio of m/z 32 (02~*) to m/z 28 (CO~'). Additionally, the intensity of the m/z 45 

 ion is approximately 30% of the m/z 44 base peak. The m/z 88 ion is at about 5% 

 RA, with an ion at m/z 89 not observed in this spectrum. 



This experiment was repeated with water introduced via the direct insertion 

 probe vial. The mass spectrum in figure A-6 clearly shows the presence of water in 

 the system (compared to figure A-5). The intensity of m/z 18 and m/z 19 are 

 increased to almost the intensity of the base peak. There is also an increase in the 



