Remember the following facts about ecd 1 electrons

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Remember the following facts about ECD: 1. Electrons from a -source ionize the carrier gas (nitrogen) 2. Organic molecules containing electronegative atoms capture electrons and decrease current 3. Simple and reliable 4. Sensitive (10 -15 g/s) to electronegative groups (halogens) 5. Largely non-destructive 6. Insensitive to amines, alcohols and hydrocarbons 7. Limited dynamic range (10 2 ) 8. Mass sensitive detector
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62 Gas Chromatographic Columns and Stationary Phases Packed Columns These columns are fabricated from glass, stainless steel, copper, or other suitable tubes. Stainless steel is the most common tubing used with internal diameters from 1-4 mm. The column is packed with finely divided particles (<100-300 m diameter), which is coated with stationary phase. However, glass tubes are also used for large- scale separations.
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70 Much more efficient separations can be achieved with capillary columns, as compared to packed columns, due to the following reasons: 1. Very long capillary columns can be used which increases efficiency 2. Thinner stationary phase films can be used with capillary columns 3. No eddy diffusion term (multiple paths effect) is observed in capillary columns
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72 Temperature Programming Gas chromatographs are usually capable of performing what is known as temperature programming gas chromatography (TPGC). The temperature of the column is changed according to a preset temperature isotherm. TPGC is a very important procedure, which is used for the attainment of excellent looking chromatograms in the least time possible. For example, assume a chromatogram obtained using isothermal GC at 80 oC, as shown below:
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73 Programmed 30 to 180° Isothermal at 145° Isothermal at 45°
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74
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76
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77 The General Elution Problem Look at the chromatogram below in which six components are to be separated by an elution process using isothermal conditions at for example 120 o C:
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78 It is clear from the figure that the separation is optimized for the elution of the first two components. However, the last two components have very long retention and appear as broad peaks. Using isothermal conditions at high temperature (say for example 200 o C) can optimize the elution of the last two compounds but, unfortunately, results in bad resolution of the earlier eluting compounds as shown in the figure below where the first two components are coeluted while the resolution of the second two components becomes too bad:
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80 One can also optimize the separation of the middle too components by adjusting the isothermal conditions (for example at say 160 o C). In this case, a chromatogram like the one below can be obtained:
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81 However, in chromatographic separations we are interested in fully separating all components in an acceptable resolution. Therefore, it is not acceptable to optimize the separation for a single component while disregarding the others. The solution of this problem can be achieved by consecutive optimization of individual components as the separation proceeds. In this case, temperature should be changed during the separation process. This is called temperature programming gas chromatography (TPGC)
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