Solutions, Chromatography(II)

11. Predict the elution order of the following in reverse phase and in normal phase chromatography: (a) n-heptane, heptanol, and toluene.
(b) nitrobenzene, benzene and phenol

In normal phase least polar elutes first:

a) heptane, toluene, hepatnol

b) benzene, nitrobenzene, phenol.

    Order is opposite for reverse phase.

    14. In a normal phase LC separation, nitrobenzene had a retention time of 28.0 min, while an unretained compound eluted in 0.9 min. The mobile phase was a 50:50 mixture of chloroform and hexane. What mixture of chloroform and hexane will reduce the k’ to 9.0.

    Initial k'1 = (28.0-0.9)/0.9 = 30

    P Hexane = 0.1, P toluene = 2.4, from table 4.5

    P of initial eluent = 2.4 x 0.5 + 0.1 x 0.5 = 1.25 = PI

    Calculate P2 (Eq 4.21)

    ,      and   P2 = 2.29

    Let x = fraction of Toluene

    2.29 = 2.4(x) + 0.1(1-x)

    x = 0.95, so mixture should be 95% toluene, 5% hexane.

    15. The following data were obtained for a separation using a 0.5 mm id, 10 meter open tubular column:

    Retention time (min) Peak base width (min)
    Methane2.0
    Cyclohexane8.00.65
    Methylcyclohexane8.170.72
    Toluene10.20.95

    a) Draw the chromatogram and label the peaks

    b) Calculate the number of peaks for each compound

    c) Calculate the capacity factor for toluene

  1. Calculate the length of the column needed to separate cyclohexane and methylcyclohexane at a resolution of 1.6.

    2. b) for cyclohexane, 1175 for methylcyclohexane and 1192 for toluene

  2. for cyclohexane, and methylcyclohexane

                and k' =3

       

N = 98000. Current column is 1200, so need a much more efficient column or one of similar efficiency which is 80 times longer.

16. In a reverse phase separation of chlorinated phenols, trichlorophenol has a retention time of 15.0 min, when 30:70 acetonitrile-water mixture is used as the mobile phase. What mixture of these two solvents will reduce the retention time to 12.0.

P1 = (5.8 x 0.3) + (10.2 x 0.7) = 8.88

Since to is not given, we assume it is small and negligible with respect to tr and use the retention time ratios in place of k'.

and P2 = 8.69

Let x = fraction of acetonitrile

8.69 = 5.8 x + 10.2(1-x)

x = 0.34, 34% acetonitrile, 65.7% water

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