Chromatographic Retention

Retention parameters are measured in terms of chart distances or times, mobile phase volumes or retention factors (k) (previously called capacity factors, k'). With a constant recorder speed, chart distances are directly proportional to times. Likewise if the flow rate is constant, the volumes are proportional to times, e.g. tR (time) is analagous to VR (volume). In GC with a compressible carrier gas, VM, VR and VR represent volumes under column outlet pressure.

Figure 1 The general elution problem.

If Fc is used in their calculation these correspond to volumes at column temperature.

When a mixture is chromatographed, the time taken for a component to be eluted from the column, the (total) retention time (tR), is measured from the moment of injection to the appearance of the peak maximum. This, together with the width of the peak measured at the baseline (w) or at half peak height (wh), and the elution of an 'unretained peak', are important parameters in chromatography. These are illustrated in Figure 2, which represents the separation of a two-component mixture.

The retention volume (VM) of an unretained peak (where VM = FxtM) is also called the gas hold-up volume or dead volume, and is equal to the volume (both inter- and intra-particle) available to the mobile phase in the column. The corrected gas hold-up volume (VM) is corrected for gas compressibility where VM = Vm j.

Injection techniques in GC, where the sample is held at the head of the column before it starts moving through the column, have necessitated the introduction of additional terms. These are the peak time/ volume (tR, VR), where the time/volume is measured from the start of elution rather than time of injection, and the adjusted retention time/volume (tR/VR), which is the total elution time/volume minus the gas hold-up time/volume:

The corrected retention time/volume (tR/VR) is the total retention time/volume corrected for carrier gas compressibility:

The net retention time/volume (tN, VN) is the adjusted retention time/volume corrected for carrier gas

Time

Figure 2 Separation of a two-component mixture showing retention parameters.

Time

Figure 2 Separation of a two-component mixture showing retention parameters.

compressibility:

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment