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When a compound of total mass m_T is introduced onto the column, it separates into two quantities: m_M, the mass in the mobile phase, and m_S, 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 m_T:

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The retention factor k, also known as the capacity factor, is a very important parameter in chromatography for defining column performance. Though it does not vary with the flow rate or the column length, k is not a constant, as it depends upon the experimental conditions. k is dependent via K (Nernst distribution law) on the intensity of solute–stationary phase interactions, and via β on the column’s design. For this reason, it is sometimes designated by k′ rather than by k alone.

This parameter takes into account the ability, great or small, of the column to retain each compound (capacity). When separations are being developed, k should not exceed 10. Ideally, k should be around five, otherwise the time of analysis is unduly long.

The expression capacity factor presents a possible confusion with the capacity of a column, which is the maximum solute mass the column may retain without being saturated. This factor is given by manufacturers when describing a column.