Deck 14: Gas Chromatography

Question 1

Why are the following procedures invalid?
(a) To minimize the time required for an analysis, the temperature of a GC instrument is increased above the boiling points of the components in the mixture.
(b) To increase the retention times and thus maximize the separation between peaks, the gas flow is adjusted to a very low value.

Answer 1

(a) The gaseous compounds will not reach a liquid-vapor equilibrium with the liquid adsor- bent on the inert packing of the column.
(b) The peaks would becomevery broad by diffusion so tthat no advantage is gained.

Question 2

Alcohols have very long retention times on Carbowax columns. Why? (Hint: Consider the polarity of alcohols.)

Answer 2

Alcohols can form hydrogen bounds with the oxygen atoms in a Carbowax.

Question 3

Suppose an organic compound has a higher heat conductivity than the carrier gas. How would its GC signal appear on a chromatogram run on an instrument with
(a) a thermal conductivity detector?
(b) a flame ionization detector?

Answer 3

(a) The signal will appear as a dip from the baseline instead of a peak rising from the baseline because the sample arm becomes cooler (instead of hotter) relative to the feference arm in the Wheatstone bridge.
(b) The signal will appear as a normal peak because a flame ionizaiton detector is not affected by thermal conductivity.

Question 4

The RT for cyclohexane on a GC instrument is 0.75 min. The temperature of the GC is 70?C and the gas flow rate is 60 mL / min.
(a) Would the RT be higher or lower if the column temperature is raised to 100?C?
(b) Would the RT be higher or lower if the flow rate is changed to 30 mL / min?

Question 5

Between sample injections, some students clean a microsyringe with acetone rather than with the mixture to be analyzed. Why is this a poor technique?

Question 6

Helium is the carrier gas of choice for a gas chromatograph containing a thermal conductivity detector, and nitrogen gas is preferred for a gas chromatograph having a flame ionization detector. Suggest a reason for this.

Question 7

Suppose that, on a GC column, methyl hexanoate eluted in 1.87 minutes, methyl heptanoate eluted in 2.35 minutes, and methyl octanoate eluted in 3.0 minutes. What is the structure of an unknown methyl ester, known to belong to the same homologous series, that elutes at 4.7 min- utes?

Question 8

A student at another university wants to duplicate a GC separation that you did in lab. What information about the instrument and column conditions should you send him?

Question 9

Consider the following compounds:
If a mixture of these compounds is run on a GC using an OV-101 column, which compound will have the lowest RT? the highest?

Question 10

Consider the following compounds:
If a mixture of these compounds is run on a GC using a DEGS column, which compound will have the lowest RT? the highest?