Question

What types of analytes can be analyzed using gas chromatography? What types of analytes are unsuitable for analysis with this technique? Which would be expected to elute first on a polydimethyl sioxane column, butane, or butanol? Why? What factors determine relative retention time with gas chromatography?

What is the basis for separation in gas chromatography? (I.e. adsorption, partition, size exclusion, ion exchange, or affinity?)

Answer 1

Chromatography is an analytical technique used for the separation, isolation, purification and identification of components of a mixture. Chromatographic separations can be carried out by the distribution of components in a mixture between a fixed moving phase. The fixed phase is known as stationary phase, while the moving phase is known as mobile phase. Separation between two substances begins to take place when one is held more strongly by the stationary phase than the other, which tends to move on faster in the mobile phase. The chromatographic techniques can be classified by the state of aggregation of the phase used. In gas chromatography (GC) the stationary phase can be liquid or solid and mobile phase is gas. According to the state of the stationary phase, gas chromatography can be classified into gas-solid chromatography (GSC), where the stationary phase is solid and gas-liquid chromatography (GLC), where the stationary phase is liquid. GLC is widely used than GSC.

Separation of the different components depends upon the retention time of the component. Retention time is the measure of the time taken for a solute to pass through the column. It is determined by several factors.

• Boiling temperature of the component

The boiling temperature of a compound is related to its polarity. If the compound is more polar then its boiling point will be higher and thus it will spend less time in gas phase (more time in stationary phase). That means its retention time will be higher.

• Column temperature

Higher temperature of column will give less retention time but the separation will be pure. So for better separation the column temperature has to adjust according the nature of sample.

• Flow rate of the mobile phase

Higher flow rate will give less retention time but the separation will be pure. So for better separation the flow rate has to adjust according the nature of sample.

• Column length

Shorter column will give less retention time.

• Polarity of the stationary phase

If the polarity of the component and stationary phase are similar retention time will increase