Question

What is the difference between retention time and retention volume? What are some of the advantages and disadvantages of each? Is using the retention time valid when using a pressure-flow controller?

Answer 1

Retention time (tR) is the time it takes a solute to travel through the column. The retention time is assigned to the corresponding solute peak. The retention time is a measure of the amount of time a solute spends in a column rather than in the mobile phase. It is the sum of the time spent in the stationary phase and the mobile phase.

The total retention volume, VR, is the volume of eluent carrier gas admitted to the column between the injection of the sample and the emergence of the peak maximum of the specified component. It includes the hold-up volume. In gas chromatography, the volume of carrier gas is specified at the outlet pressure and temperature of the column.

Advantages of retention time:

1. Retention time (RT) is a measure of the time taken for a solute to pass through a chromatography column. It is calculated as the time from injection to detection.

2. Used in qualitative analysis by comparing the retention times of the peaks in an unknown sample with those of known standards. If their retention times match, then a positive identification can be made.

2. The use of the relative retention time (RRT) is the one which reduces the effects of some of the variables that can affect the retention time. RRT is equal to the sample’s retention time relative to the standard’s retention time.

RRT = Standard RT / Sample RT

To measure RRT, a sample matrix is made up by mixing the sample with an internal standard (IS).

Disadvantages of retention time:

The retention time for a compound is not fixed as many factors can influence it even if the same GC and column are used. These include:

1. The gas flow rate

2. Temperature differences in the oven and column

3. The exact composition of the solvent.

4. Column degradation

5. Column length

6. The pressure used (because that affects the flow rate of the solvent)

7. The nature of the stationary phase (not only what material it is made of, but also particle size)

These factors can make it difficult to compare retention times.

Advantages of retention volume:

1. V_r: retention volume is proportional to retention time, based on the constant flow rate of a chromatography system. If the retention time is 5 min and the volume flow rate (u_v) is 5 mL/min, then V_r is 25 mL. V_m: mobile phase volume (also known as void volume)

2. The net retention volume per gram of liquid phase is an absolute retention parameter sufficient to measure the retention power.

3. The specific retention volume of a solute is the corrected retention volume of the solute per unit mass of stationary phase. This can be calculated from a column carrying a known weight of stationary phase and is measured at a carefully controlled known temperature. It is the difference between the solute retention volume and the dead volume.

Disadvantages of retention volume:

1. The gas flow rate

2. Temperature differences in the oven and column

3. The exact composition of the solvent.

4. Column length

5. The pressure used

These factors can make it difficult to measure the retention volume. Thatis why corrected retention volume is introduced.

Yes, the retention time is valid when using a pressure-flow controller because the pressure used which inturn affects the flow rate of the solvent.