GC, MDGC

What is Retention Time?

Jul 31 2014 Comments 0

Retention time is the amount of time a compound spends on the column after it has been injected. If a sample containing several compounds, each compound in the sample will spend a different amount of time on the column according to its chemical composition i.e. each will have a different retention time. Retention times are usually quoted in units of seconds or minutes.

Equilibrium Constant

A components retention time is determined by the equilibrium constant (K) if all other factors are kept constant. In GC, specifically gas-liquid chromatography, there are two phases namely the:

  • Mobile phase – usually a gas such as helium
  • Stationary phase – a high boiling point liquid adsorbed onto a solid

A vaporised sample is injected into the head of the GC column, which contains a liquid stationary phase, adsorbed onto the surface of an inert solid. The inert solid support (usually diatomaceous earth or clay) is necessary to keep the liquid phase stationary in the column. The speed with which a particular compound travels through the column depends on how much of its time is spent moving with the gas as opposed to being attached to the liquid. Materials that prefer the stationary phase have longer retention times than those that prefer the mobile phase.

The equilibrium constant, K, is defined as the molar concentration of analyte in the stationary phase divided by the molar concentration of the analyte in the mobile phase. A high value of K means the compound is more soluble in the liquid phase than in the gas phase. K is temperature dependent.

Polar or Non-Polar Stationary Phase

One of the key factors when setting up a GC method is to choose the polarity of the stationary phase. The polarity is chosen using knowledge of the sample matrix and what separation is required. If the polarity of the target compound and the stationary phase are similar, then there is likely to be a greater interaction between the two. Consequently, the retention time will be longer for polar compounds on polar stationary phases and shorter on non-polar stationary phases.

What Other Factors Affect RT?

Boiling point

  • If a component has a low boiling point, then it is likely to spend more time in the gas phase. Therefore its retention time will be lower than a compound with a higher boiling point. A compound’s boiling point can be related to its polarity.

Column temperature

  • A high column temperature will give shorter retention times, as more components stay in the gas phase but this can result in poor separation. For better separation, the components have to interact with the stationary phase.

Carrier gas flow-rate

  • A high flow rate lowers retention times but also yields a poor separation.

Column length

  • A longer column will produce longer retention times but better separation. Unfortunately, if a component has too long a transit time in the column, there can be a diffusive effect that causes the peak width to broaden.

All these factors must be considered to determine the GC parameters that will produce the best separation in a reasonable time. For an in-depth discussion of the factors affecting retention time and separation refer to the article: Optimisation of Column Parameters in GC.

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