Question

(a) In what order do you expect the following species to be eluted in normal-phase LC? Indicate with numbers and provide brief rationale: 0₂N NO₂ NO2 trinitrotoluene 1-heptene heptane heptane phenol toluene (b) Fraction of which component of the mobile phase should be increased when a hexane/acetone gradient is employed to facilitate elution of phenol and trinitrotoluene? Provide brief rationale

Answer 1

a) In the normal phase chromatography, the stationary phase is polar and hence interacts strongly with the polar compounds. This results in a longer duration to get eluted from the column for the polar compounds.
Whereas, non-polar compounds are less interacting with the stationary phase and that results in faster elution of the non-polar compounds.
Overall, as the polarity of compound increases the elution time increases.

In the given series of compounds, the polarity order is:
Phenol > trinitrotoluene > toluene > 1-heptene > heptane
Thus the order of elution is:
1) Heptane 2) 1-heptene 3) toluene 4) trinitrotoluene 5) phenol

b) Phenol and trinitrotoluene are polar compounds and hence to elute them faster or facilitate the elution polar component of the mobile phase should be increased. In the given gradient mobile system hexane is a non-polar component and acetone is the polar component. Hence We need to increase acetone component to facilitate the elution of polar compounds like phenol and trinitrotoluene.