Why should one not carry out a distillation process at a very rapid rate? If at

Question

Why should one not carry out a distillation process at a very rapid rate? If at a rapid rate, we must decrease the temperature with a rapid rate the pint is likely to be acurate and the A sample consisting of 1-bromobutane and 1-chlorobutane is injected into a gas chromatograph equipped with a column whose stationary phase is Apiezon L. Which compound will have the shorter retention time? Explain your answer. Which one of the above two compounds will have the shorter retention time if it was injected into a column whose stationary phase is DC 710? Indicate whether the retention times of a compound on a GC column would either increase or decrease or remain unchanged if following changes were made? (Insert the letter corresponding to the correct choice in the blank provided to the left of the increase decrease remain unchanged from a DC 200 stationary phase to an Apiezon N stationary phase Helium (He) gas flow rate from 15.Ib/in^2 to 10 lb/in^2 column length from 20 m to 30 m

Explanation / Answer

4. Apeizon L is a non-polar stationary phase used in GLC. The elution of a compound by GC depends on the interaction of the compound with the stationary phase as well as the mobile phase. The “like dissolves like” rule of thumb is useful here. 1-chlorobutane is more polar as compared to 1-bromobutane because of the higher electronegativity of Cl atom; the C-Cl bond is more polar. 1-chlorobutane is retained less on the Apeizon L stationary phase while 1-bromobutane spends more time interacting with the particles of the stationary phase. Thus, 1-chlorobutane is eluted first followed by 1-bromobutane.

DC 710 stationary phase is an intermediate stationary phase which means that it borders on being somewhat polar. Since 1-chlorobutane is more polar, this compound will be retained more in the stationary phase. Thus, 1-bromobutane will be eluted faster, i.e, it will have a lower retention time.

5 (1). The retention time will depend on the nature, more importantly polarity of the compound. Since DC 200 stationary phase is used (which is somewhat polar, intermediate at best), it may be assumed that the compound is most likely polar. Now Apeizon N is a non-polar stationary phase. When the stationary phase is changed from DC 200 to Apeizon N, the retention time for a polar compound will decrease because there is lower interaction between the solute molecules and the stationary phase molecules.

Ans: (B) decrease

(2) Increasing the carrier gas flow rate will reduce the retention time. This is because the carrier gas will push the compound through the stationary phase without allowing sufficient time for the compound to interact with the stationary phase molecules. The increase in flow rate decreases the resolution of a chromatographic separation as well.

Ans: (B) decrease

(3) Increasing the column length of a GC column will increase the retention time because the compound will take more time to travel through the column.

Ans: (A) increase

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