46 

 of the septum and to prevent premature migration of volatiles past the intended 



point of cryo-focusing on the column. 



Before increasing the helium head pressure, liquid nitrogen was placed in a 

 12 oz. styrofoam cup. The cup was placed in the oven such that approximately 8 cm 

 of column could be looped in the cup about 15 cm below the point where the 

 column passes from injection port to oven. The helium pressure was then increased 

 to 20 psig and the initial desorption phase started. This entailed loading a program 

 to ramp the injection port from 25°C to 250°C over 7.5 min and holding at 250°C for 

 2.5 min. Throughout the cryo-focusing phase, the GC oven was set at 25''C and the 

 transfer line set 40°C. Liquid nitrogen was added to the cup as necessary during this 

 10 min cryo-focusing phase. 



Subsequent to the completion of the cryo-focusing, a new program was loaded 

 from the TSQ70 to the Varian 3400 GC. Prior to running this program and 

 concurrently acquiring data, the cup containing liquid nitrogen was removed. Tlie 

 GC oven ramp consisted of an initial 1.0 min hold at 25°C, a 12 min ramp at 

 15°C/min up to 210°C, and then a hold at 210°C for 5.0 min. The transfer line was 

 concurrently ramped, after a 1.0 min hold at 40''C, up to 225°C at 15°C/min, and held 

 5.0 min at 225°C. Experiments were performed with either an 18 m x 0.18 mm i.d. 

 DB-5 column (df=0.25 p,m) or a 20 m x 0.25 mm i.d. Carbowax column (df=0.25 

 Jim). 



The mass spectrometer was operated in NCI mode, positive and negative ion 

 mode (PPINICI), or EI mode. For NCI and PPINICI experiments the reagent gas 



