In a reserved-phase column, a solute was found to have a retention time of 31.3min while an unretained species required 0.48min to elute when the mobile phase was 30% (by volume) methanol and 70% water.
b) a water/methanol composition that will make the k' value to be 5.
To obtain Pˊ for the mobile phase we substitute polarity indexes for methanol and water from to give
P ˊ = polarity index.
P ˊ = ØAPA ˊ+ ØBPB
P ˊ =0.30×5.1+0.70×10.2=8.7
substitution of this result into Equation
REVERSE-phase separation=K"2 /K"1=10(P2-P1)/2
Taking the log of both sides of equation gives P2=6.5
Letting x be the volume fraction methanol in the new solvent mixture and substituting again into Equation we find 6.5= x ×5.1+(1-x) 10.2 x =0.73 or 73%
Thus, a 73% methanol / 27% water mixture should provide the desired value of k
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