Optimum practical gas velocity

Internal diameter, dc(mm) f MIN (mm) Column efficiency (Nm ) He flow-rate (ml min ) at /iopT 

The separating capability and efficiency of a column is a function of the distribution ratio, K, and the retention ratio, k, of each component, each increasing as the retention time increases. Column efficiency, N, therefore varies with the retention characteristics of each component. 

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Operation at the optimum practical gas velocity (found by determination of column performance in terms of HEPT which in turn is a function of gas flow-rate). 

Figure 5.6 Optimum practical gas velocity (OPGV) approximate values for a 25 m x 0.25 mm dimethylsiloxane WCOT column using He carrier gas.

Optimum practical gas velocity, OPGV average carrier gas velocity in WCOT columns that gives the best separation for a particular sample in the shortest analysis time whilst maintaining satisfactory resolution between peaks. OPGV 1.5-2.0 OGV. 

Outside diameter Optimum practical gas velocity Pressure Gas constant... 

Figure 8 van Deemter plot (H versus 0) showing effects of individual terms A, B/u, and Cu. The most efficient conditions giving minimum plate height (Hmin) at optimum flow velocity (uopt) occur at the minimum of the van Deemter cun/e. Optimum practical gas velocity (OPGV) is located at 2 x tfept. 

The optimum linear velocity for a capillary column depends on the pressure in the column because Wopt is proportional to the average diffusion coefficient, which varies inversely with pressure. Operation of a short wide-bore column at vacuum outlet conditions results in a significantly faster analysis than would occur if the same column was used under atmospheric outlet pressures. Mass spectrometry (MS) has made vacuum GC very easy to implement, since the mass spectrometer provides both detection and a source of vacuum. Vacuum GC can be achieved practically by incorporating a restriction at the inlet end of a wide-bore capillary column, and interfacing the terminal end of the column directly into the MS. The function of the restriction is to deliver an optimal helium flow for the mass spectrometer, and it can be as simple as a short section of 20 pm i.d. capillary (or a longer section of 100-150 pm i.d. capillary). An optimal carrier gas velocity of 90-100cms can be expected for a 10 m x 50 pm column with a restriction at the inlet, and a speed gain of a factor of 3-5 times can easily be obtained. 

The relation is univo only for a given packing, gas velocity, and superficial velocity of foe liquid. Having in mind foe small number of suitable packings and that the packing should operate at its optimum ps and liquid superficial velocities, the restrictions mentioned above are not very significant from a practical point of view. 

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