The partition process in a chromatographic separation

Once the sample reaches the chromatographic column, the separation process starts. The time necessary for a component injected into the chromatographic column to elute is called the absoiute retention time tR. The separation is based on different retention times of the components of the mixture. These retention times are different because the partition of each analyte between the two phases, the gas phase in motion and the stationary phase, are different. Hydrogen, helium, and nitrogen are common gases used as mobile phase. Two basic types of columns are known packed columns containing solid support particles coated with the stationary phase, and open-tubular columns with the stationary phase as a film on the inner wall (capillary columns). Because the retention time tpfi) of the analyte i is temperature dependent, the chromatographic column of any (GC) is put in an oven with temperature control capability. 

The retention of an analyte in the stationary phase can be characterized with several parameters, one of them being the partition coefficient (or distribution constant) K (or Kio). This is defined for a component i by the relation  

The isotherm is a straight line when K is a true constant, but it is a (non-linear) curve when the distribution constant Ki varies with Ci s and Cj.g. Depending on the isotherm type, the chromatographic process can be classified as linear or nonlinear. 

Because the elution appears as a broadened zone, the retention time tR must be measured when the analyte reaches the maximum of its Gaussian elution peak. The absolute retention time tp is the sum of the (average) time spent by a component in the mobile phase (U) and the (average) time spent in the stationary phase (tR-)  

The time spent by a component in the stationary phase tR is compound specific (the index i indicating analyte i was omitted), but the time spent in the mobile phase is the same for any component and depends on the linear flow rate u of the gas, and the column length L, where 

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