CHEM 232 Experiment 7

What is gas chromatography

An analytical technique of separation and analysis with a mobile and stationary phase.

What is the basis of GC

Volatility or boiling point

What is GC limited to

compounds that can exist in the gas phase

What can not be separated by GC?

Large biological molecules

Is GC more of an analytical tool or a preparatory one and why?

Analytical, because it is destructive and the sample can not be retrieved at the end

In a GC, where is the stationary phase?

Coated on the walls

In GC, where is the mobile phase and what is it

It is flowering and is an inert gas such as helium in our lab. It pushes the sample along the column

Is the sample in the gas phase or liquid phase, or both?

It spends part of the time in the gas phase being pushed along the column by the mobile phase and part of the time interacting with the stationary phase being retained in the column

A compound with a lower boiling point needs ___ energy in order to get to the gas phase?

Less

What is the stationary phase in our lab composed of?

95% silicone oil and 5% phenyl methyl silicone oil which has a very high boiling point and is relatively non polar

How can we change the time and reslution in GC

By changing the flow rate of the mobile phase gas and the temperature in the column

Increasing the temperature...

gives compunds more energy and they spend more time in the gas phase, shortening the analysis time but DECREASING the reoslution

Increasing the flow rate...

will push hte samples faster through the colun giving the mixture less time to separate

Can we change the column length in our lab?

No

When is the best separation usually achieved

When the temperature is held constant and it is in between the boiling point of a two-component mixture

What happens once the sample exits the column?

It goes to a flame ionization detector (FID)

When must we use a response factor

when comparing the quantities of samples that do not have the same molecular formula

The response factor serves as

an equalizing factor

The ratio of the peak areas on a gas chromatogram most accurately reflects which of the following ratios between the two compounds?

Mole ratio

The factor that most directly affects retention time of a compound is

boiling point

which of the following runs would be analyzed to determine the response factor

equal moles

the mobile phase in the gas-chromatograph is

an inert gas

Imagine that you run your original mixture through GC and you find two large peaks. ne peak has a retention time of 2.52 minutes and the other peak has a retention time of 5.30 minutes. what is likely the identitiy of the compound responsible for the first peak?

Peantane

Imagine that you run your original mixture through GC and you find two large peaks. ne peak has a retention time of 2.52 minutes and the other peak has a retention time of 5.30 minutes. what is likely the identitiy of the compound responsible for the second peak?

Cyclohexane

Imagine that you run your original mixture through GC and you find two large peaks. ne peak has a retention time of 2.52 minutes and the other peak has a retention time of 5.30 minutes. Which sample would you run to check your answers for pentane?

Pure pentane

Imagine that you run your original mixture through GC and you find two large peaks. ne peak has a retention time of 2.52 minutes and the other peak has a retention time of 5.30 minutes. Which sample would you run to check your answers for cyclohexane

Pure cyclohexane

Imagine that you run your original mixture through GC and you find two large peaks. ne peak has a retention time of 2.52 minutes and the other peak has a retention time of 5.30 minutes. Which peak will most likely have the largest area for the early fraction distillation sample?

Peak 1

Imagine that you run your original mixture through GC and you find two large peaks. ne peak has a retention time of 2.52 minutes and the other peak has a retention time of 5.30 minutes. Which peak will most likely have the largest area for the late fraction distillation sample?

Peak 2

Imagine that you run your original mixture through GC and you find two large peaks. ne peak has a retention time of 2.52 minutes and the other peak has a retention time of 5.30 minutes. Which of the runs would be the response factor run?

Equimola run

True of false: the retention time for a specific compound will be the same on any GC

False

True or false: the response factor for a specific GC is always the same, regardless of the column used

False

True or false: The response factor "corrects" the areas of peaks on a GC to make the area ratio of the peaks better resemble the molar ratio of the compounds

True

True or false: The presence of an impurity in a mixture cannot be detected by GC analysis

False

True or false: a mixture of two constituional isomers does not require the use of a response factor

True