retention factor for a solute =
= ((time for solute of interest to elute) – (time for an unretained solute to elute)) / (time for an unretained solute to elute)
selectivity factor for a column =
= (retention factor for solute B) / (retention factor for solute A)
polarity index for a given solvent mixture =
= ((polarity index for solvent 1) * (volume fraction of solvent 1 in mix)) + ((polarity index for solvent 2) * (volume fraction of solvent 2 in mix))
where:
• The polarity indices for various solvents is given in Table 28-2 on page 743.
For a normal phase separation:
(retention factor under mix 2) / (retention factor under mix 1) =
= 10 ^ (0.5 * ((polarity index for solvent mixture 1) – (polarity index for solvent mixture 2)))
For a reversed-phase separation:
(retention factor under mix 2) / (retention factor under mix 1) =
= 10 ^ (0.5 * ((polarity index for solvent mixture 2) – (polarity index for solvent mixture 1)))
Steps:
(1) Determine the retention factor for the solute under initial conditions.
(2) Determine the polarity index for the initial solvent mixture.
(3) Decide on the target retention factor.
(4) Calculate the polarity index needed to achieve the target retention factor.
(5) Use the equation for the polarity index for the new solvent mix to calculate the different volume fractions of each solvent.
For Step 1: Use the retention factor equation based on the initial conditions.
For Step 2: Use the equation for the polarity index for a given solvent mixture (above).
For Step 4:
polarity index for target mixture in a normal phase system =
= (polarity index for initial mixture) – (2 * LOG10 ((target retention factor) / (initial retention factor)))
polarity index for target mixture in a reversed phase system =
= (polarity index for initial mixture) + (2 * LOG10 ((target retention factor) / (initial retention factor)))
For Step 5:
volume fraction of solvent 1 =
= ((polarity index for new solvent mixture) – (polarity index for solvent 2)) / ((polarity index for solvent 1) – (polarity index for solvent 2))
volume fraction of solvent 2 =
= 1 – (volume fraction of solvent 1)
