GC & HPLC revision

In gas chromatography, the time taken for a substance to travel through the column is called

• Reference time

• Retention time

• Separation time

• Travelling time

A low R_t (retention time) means the component has a 

• greater affinity to the stationary phase

• greater affinity for the mobile phase

What is a full name of FID detector?

• Free induction decay

• Flame ionization detector

• Fuel injection diagnostic

• First input delay

C-8 and C-18 columns are examples of:

• packed columns for GC

• capillary columns for GC

• normal phase HPLC columns

• reverse-phase HPLC columns

A mixture is injected into a HPLC instrument. The stationary phase used is non-polar and the mobile phase is polar. The chromatogram shown to the left was obtained.

Select the correct conclusion from the following alternatives.

• There are three components only in the sample

• Compound A is the most polar

• Compound C is the most polar

• The concentration of each component is similar

Which of the following can be used to find the concentration of a substance in HPLC

• Retention factor, R_f value

• Retention time, R_t value

• peak area
• peak width

A mixture of propan-1-ol and butan-1-ol is injected into an HPLC column that is using a polar solvent. The print-out shown to the left is obtained.

A second sample is passed through the same column and produces only one peak with a retention time of 9.4 minutes and an area of 1200 units.

The second sample contains

• propan-1-ol and butan-1-ol at lower concentrations than the original solution.

• propan-1-ol only, with a concentration that is one-third of the original solution.

• butan-1-ol only, with a concentration that is one-third of the original solution.

• butan-1-ol only, with a concentration that is three times that of the original solution.

A mixture of organic compounds was analysed using high-performance liquid chromatography (HPLC) and produced the result shown in the diagram to the left.

How were the peaks identified?

• A standard database with retention times of all organic compounds was consulted.

• The heights of the peaks indicated the molecular structure of the organic compounds.

• Known compounds were analysed on the same column under identical conditions.

• The distances between the peaks revealed which organic molecules were present.

During a HPLC run, the mobile phase started at 50% methanol in water and gradually increased to 100% methanol after 2 minutes.

This is known as:

• Isocratic elution

• Gradient elution

HPLC was carried out on a reverse-phase column with a mobile phase of methanol:water (95:5). The analyte had a retention time of 2 min. What would happen to the retention time if a mobile phase of methanol:water (50:50) was used instead?

• Retention time would be < 2 min

• Retention time would still be 2 min

• Retention time would be > 2 min