# Plate Theory and Extensions - The Column Dead Volume > Page 31

Now the
retention volume (V_{r}_{(A)})
of a solute (A) is

V_{r}_{(A)} = V_{o} +
K_{A}V_{S } (36)

Equation (36) is correct but, if accurate retention measurements are important, then second order effects must be taken into account and equation (35) shows that equation (36) is grossly over simplified. From equation (35), a more accurate expression for solute retention would be

V_{r} = V_{I(m)} + KV_{I(s)} + K_{1}V_{p}_{(1)} +
K_{2}V_{p}_{(2)} + K_{3}V_{S(A)} (37)

where, (K) | is the distribution coefficient of the solute between the moving phase and the static portion of the interstitial volume, |

(K_{1}) |
is the distribution coefficient of the solute between the moving phase and the
static pore contents, (V_{p}_{(1)}), |

(K_{2}) |
is the distribution coefficient of the solute between the moving phase and the
static pore contents, (V_{p}_{(2)}), |

and (K_{3}) |
is the distribution coefficient of the solute between the moving phase and the available
stationary phase (V_{S(A)}). |