In partition chromatography, the change in the retention factor for a given anal
ID: 952203 • Letter: I
Question
In partition chromatography, the change in the retention factor for a given analyte when the composition of the mobile phase changes can be calculated using the following equations, For RPLC log(k_1/k_2) = (P_tot1 - P_tot2)/2 For NPLC log(k_1/k_2) = (P_tot2 - P_tot1)/2 where k_1 and k_2 are the retention factors for the analyte when two different mobile phases are used, which have solvent polarity indexes of P_tot1 and Ptot_2. A list of solvent polarity indexes for various liquids can be found here. When a 29.0% ethyl acetate/71.0% water mobile phase is used, an analyte elutes from a reversed-phase column with a retention factor of 14.7. For the same analyte, what will the retention factor be if the mobile phase is changed to 14.5% ethyl acetate/85.5% water? k = If instead you want the analyte to elute with a retention factor of 3.7, what mixture of ethyl acetate and water is needed? ethyl acetate waterExplanation / Answer
The relation between dielectric constand and mixtures is given by
The value of the dielectric constant for a mixture is obtained by multiplying the volume fraction of each solvent times its dielectric constant and summing.
Also the dielectric constant () of a compound is an index of its polarity.
So we can assume that polarity index of a mixture can be calculated in a similar way
Polarity index of Ethyl Acetate is 4.4 and that of Water is 10.2
so when we have 29% ethyl acetate and 71% water the overall polarity of the mobile phase is
0.29 x 4.4 + 0.71 x 10.2 = 8.52
14.5% ethyl acetat and 85.5% water will be
0.145 x 4.4 + 0.855 x 10.2 = 9.36
log(k1/k2) = (Ptot1-Ptot2)/2
log(14.7/k2) = (8.52-9.36)/2
log(14.7/k2) = -0.419
14.7/k2 = 10-0.419
14.7/k2 = 0.38
k2 = 14.7/0.38
k2 = 38.62
Is the retention factor in the new solvent.
log(k1/k2) = (Ptot1-Ptot2)/2
log(14.7/3.7) = (8.52-Ptot2)/2
log (3.97) = (8.52-Ptot2)/2
0.599 x 2 = (8.52-Ptot2)
1.19 = (8.52-Ptot2)
Ptot2 = 8.52 - 1.19
Ptot2 = 7.32
so the polarity index of this solvent should be 7.32
x1 x 4.4 + 1-x1 x 10.2 = 7.32
4.4x1 + 10.2 -10.2x1 = 7.32
-5.8x1 = 7.32-10.2
-5.8x1 = -2.88
x1 = 2.88/5.8
x1 = 0.49
So we need a mixture have 49% ethyl acetat and 51% water.