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injection devices are encouraged to use them. This may eliminate many of the problems mentioned 

 above. 



5.4.1.7 Carrier gas: Suitable carrier gases for electron capture detection are nitrogen, 

 hydrogen and helium. The maximum separation efficiency is slightly higher for N2 than for H2 

 and He, but the corresponding average linear gas velocity is considerably lower for N2. Similar 

 resolution can be obtained when H2 or He are used resulting in much shorter GC runs. As the 

 change in resolution with variations in gas velocity is lowest for H2, it is the preferred carrier gas. 

 Care must be taken to prevent H2 from entering the GC oven at high flow rates, therefore, tests for 

 leaks are a mandatory pan of operation. This can be done by including a flow controller in the 

 carrier gas supply immediately after the regulator. Non-zero gas flow, with carrier gas supply at 

 the GC being closed, indicates a leak somewhere in the system. The use of electronic flow rate 

 controllers to check carrier gas flow and emergency shut-off valves are highly recommended. 



The quality of the carrier gas can be checked by keeping the column at room temperature at 

 normal flow rates overnight and carrying out a normal temperature programmed run. The presence 

 of contaminants in the carrier gas will show up in the form of extraneous peaks and/or unstable and 

 increased baseline. The problem may be resolved by regeneration and/or replacement of the 

 external and internal (GC mounted) molecular sieve gas traps. 



5.4.1.8 GC conditions and optimization: Resolution and time required for a 

 chromatographic run depend essentially on the interaction of six parameters: column internal 

 diameter, O; film thickness, d; column length, L; carrier gas type and average velocity, V and 

 temperature, T. 



A smaller O results in increased capacity ratios, and thus, in better separations at the cost of 

 longer analysis times. Alternatively, time can be unchanged with a corresponding smaller L and/or 

 higher V and T. The selection of column properties specified in section 5.4.1.1 represents a useful 

 compromise between resolution and analysis time. Changes in O and d have considerably more 

 effects on separation efficiency than column length (the opposite is true with packed columns). 

 With O, d and L fixed by column selection, and H2 selected as the carrier gas, V and T have to be 

 optimised for the separation problem at hand. There are many variations of temperature 

 programming rates, etc. for each type of compound to be analyzed. Appropriate chromatographic 

 conditions for the analysis of pesticide CB's are the following: 



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