The corrected retention volume (V'r) is the volume of mobile phase passed through the column between the dead point and the peak maximum. It will also be the retention volume minus the dead volume.

Thus, V'r = Vr - Vo = Q(tr - to) where (Q) is the flow rate in ml/min.

The peak height (h) is the distance between the peak maximum and the base line geometrically produced beneath the peak.

The peak width (w) is the distance between each side of a peak measured at 0.6065 of the peak height (ca 0.607h). The peak width measured at this height is equivalent to two standard deviations (2s) of the Gaussian curve and, thus, has significance when dealing with chromatography theory.

The peak width at half height (w0.5) is the distance between each side of a peak measured at half the peak height. The peak width measured at half height has no significance with respect to chromatography theory.

The peak width at the base (wB) is the distance between the intersections of the tangents drawn to the sides of the peak and the peak base geometrically produced. The peak width at the base is equivalent to four standard deviations (4s) of the Gaussian curve and thus also has significance when dealing with chromatography theory.

Factors Controlling Retention

The equation for the retention volume (Vr), as derived from the Plate theory (see Book 6 The Plate Theory and Extensions) is as follows,

Vr = Vm + KVS

or V'r = KVS (1)

where (Vm)	is the volume of mobile phase in the column
(VS)	is the volume of stationary phase in the column,
(K)	is the distribution coefficient of the solute between the phases,
and (V'r)	is the corrected retention volume i.e., (Vr - Vm)

From equation (1) it is seen that the corrected retention volume is controlled by two parameters: firstly the distribution coefficient of the solute between the two phases and secondly, the amount of stationary phase that is available to the solute.

Consequently, the magnitude of (V'r) is determined by (K) or (Vs) or both.

From equation (1) the conditions necessary to separate two solutes (A) and (B) can be deduced.

To separate solutes (A) and (B), V'r(A)V'r(B),

which can be achieved by making either K(A)< >K(B)

or VS(A) < > VS(B) or an appropriate combination of both.

Thus, to separate a mixture, either the values of (K) for all components, or the amount of stationary phase (VS), available to each component, must be made to differ or, again, appropriate adjustments must be made to both.

Prior to discussing the parameters that determine the magnitude of (K) and (Vs) and how they can be changed, it is useful to develop the thermodynamic approach to the problem of solute retention in chromatographic separations.