Carrier gas and flow regulation

The pressure at the head of the column (several tens to hundreds of kPa) is stabilized either mechanically or through an electronic pressure control (EPC) in order that the flow rate remains constant at its optimal value. This device is valuable because if the analysis is performed with temperature programming, the viscosity of the stationary phase and by consequence the loss of charge in the column, increase with temperature. Therefore to maintain the carrier gas flow constant, the pressure must be finely tuned to compensate this effect. The result is a faster analysis and a longer life for the column. 

The injector and the detector have dead volumes (hold-up volumes) which are counted in the total retention volume. In GC, since the mobile phase is a gas, the flow rate measured at the outlet of the column should be corrected by a compression factor /, which compensates for the higher pressure at the head of the column (cf. expression 2.1). 

If a chromatogram contains a peak for a compound that is not retained on the stationary phase, it is possible to calculate the average linear velocity of the progression, u, of the carrier gas. Elsewhere by installing a flow meter at the outlet of the instrument (atmospheric pressure Pq) and knowing the diameter of the column the velocity Uq of the carrier gas, at the end of the column, can be deduced. The ratio between these two velocities is equal to /, the compression factor, which is linked to the relative pressure P/Pq P the pressure at the head of the column)  

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