Effect of Gas Compressibility on Retention in GC

If the sample size and thus the nonlinearity is not extreme, then neither is the tailing. Modest tailing is common in chromatographic peaks. The reduction of solute load is frequently a cure for tailing. 

Tailing sometimes has a kinetic (rather than equilibrium) origin, stemming from slow desorption processes [28]. Thus the observation of tailing does not prove that the column is overloaded and consequently behaving nonlinearly (see Section 5.9). When kinetic factors are involved, the reduction of solute load will not eliminate tailing. 

The calculation of retention time tr is simple when the flow velocity in the column is uniform throughout the column length and has a known value v which can be substituted into Eq. 10.2. However in gas chromatography, where the mobile phase is highly compressible, v varies substantially with the level of compression, which changes through the column. This requires special consideration. 

The retention time tr can no longer be calculated as L/Rv (Eq. 10.2) because v is not constant. Below we write v as v(x) to show that it varies with distance x down the column. To get tr we must sum up (integrate) all the small time elements dt needed for a component to pass through each thin slice dx of column. We have 

This integrates readily if we change our variable of integration to y 

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