When a compound of total mass mT
is introduced onto the column, it separates into two quantities:mM, the mass in the mobile phase and m, the mass in the stationary phase. During the solute’s migration down the column, these two quantities remain constant. Their ratio, called the retention factor k, is constant and independent of mT:S
is introduced onto the column, it separates into two quantities:mM, the mass in the mobile phase and m, the mass in the stationary phase. During the solute’s migration down the column, these two quantities remain constant. Their ratio, called the retention factor k, is constant and independent of mT:S
The retention factor, also known as the capacity factor k, is a very important parameter in chromatography for defining column performances. Though it does not vary with the flow rate or the column length, k is it not a constant as it depends upon the experimental conditions. For this reason it is sometimes designated by k
rather than k alone. This parameter takes into account the ability, great or small, of the column to
retain each compound. Ideally, k should be superior to one but less than five, otherwise the time of analysis is unduly elongated.
An experimental approach of k can be as follows: Suppose the migration of a compound in the column. Recalling Craig’s model, each molecule is considered as passing alternately from the mobile phase (in which it progresses down the column), to the stationary phase (in which it is immobilized). The average speed of the progression down the column is slowed if the time periods spent in the stationary phase are long. Extrapolate now to a case which supposes n molecules of this same compound (a sample of mass m. If we accept that at each instant, the ratio of the n molecules fixed upon the stationary phase (mass mS and of the nMS
molecules present in the mobile phase (mass mM, is the same as that of the times tSand t
spent in each phase for a single molecule, the three ratios will therefore have the same value:
rather than k alone. This parameter takes into account the ability, great or small, of the column to
retain each compound. Ideally, k should be superior to one but less than five, otherwise the time of analysis is unduly elongated.
An experimental approach of k can be as follows: Suppose the migration of a compound in the column. Recalling Craig’s model, each molecule is considered as passing alternately from the mobile phase (in which it progresses down the column), to the stationary phase (in which it is immobilized). The average speed of the progression down the column is slowed if the time periods spent in the stationary phase are long. Extrapolate now to a case which supposes n molecules of this same compound (a sample of mass m. If we accept that at each instant, the ratio of the n molecules fixed upon the stationary phase (mass mS and of the nMS
molecules present in the mobile phase (mass mM, is the same as that of the times tSand t
spent in each phase for a single molecule, the three ratios will therefore have the same value:
Knowing that the retention time of a compound tR=tM+tS
is such that tR, the value of k is therefore accessible from the chromatogram tS=t'R; see Figure 1.7:
is such that tR, the value of k is therefore accessible from the chromatogram tS=t'R; see Figure 1.7:
Bearing in mind the relations (1.16) and (1.18), the retention volume VR of a solute can be written :
This final expression linking the experimental parameters to the thermodynamic coefficient of distribution K, is valid for the ideal chromatography.
This final expression linking the experimental parameters to the thermodynamic coefficient of distribution K, is valid for the ideal chromatography.