Question

Explain the differences between GC (gas chromatography) and LC (liquid chromatography) [principally and technically] Please give me a detail answer at least two paragraphs.

Answer 1

Gas chromatography and Liquid Chromatography are both separation techniques. Liquid Chromatography has found favour in applications covering pharmaceuticals, foods, life sciences and polymers whereas Gas Chromatography has significant applications in petroleum and petrochemical industry, flavours and fragrances and, environmental air monitoring,

Mobile phase

As the name suggests Liquid Chromatography uses a liquid mobile phase and gas chromatography uses a gas as the carrier. Liquids are generally mixtures of solvents of compatible polarities whereas in gas chromatography the mobile phase is a single high purity gas.

Operating temperature

LC separations are mostly carried out at ambient temperatures whereas Gas Chromatography separations are carried out at elevated temperatures which can be held at a constant value (isothermal) or variable as decided by the temperature program. Newer packingshave extended temperature limits of Liquid Chromatography operation as well.

Nature of compounds

Gas chromatography separations are mainly carried out on compounds ranging in molecular weights up to a few hundreds. Such compounds separate on differences in their volatilities and remain stable at high temperatures. On the other hand compounds separated on LC have higher molecular weights ranging from a few hundreds to several millions for large polymers and biomolecules. Such compounds can be analysed at room temperature only because at elevated temperatures they tend to degrade.

Column Dimensions

Liquids used as carrier in LC generally have higher viscosity in comparison to gases used in Gas Chromatography. This results in increased column back pressures in LC. It is for this reason that Liquid Chromatography columns are much shorter and have wider diameters in comparison to GC columns which can be much longer and narrower. Increased column length improves resolution between closely spaced peaks. As the trend is towards faster analysis columns used for LC are as short as 1 cm in length.

Column Packings

Column packings offer greater resistance to flow of liquids in comparison to gases. Gases also have a higher permeability of solid supports than liquids. The retention mode of LC columns depends on polarity differences or molecular sizes whereas GC separations are based on differences in volatilities of compounds

Detection principles

LC detection is commonly based on nondestructive detection such as UV, RI, photodiode array detectors, conductivity and laser detection. On the other hand Gas Chromatography detection is based largely on destructive principles such as a FID, NPD and FPD. Mass spectrometry detectors common to both LC and GC are destructive in nature.

Cost of operation

LC solvents are costly in comparison to gases used for GC analysis. In addition to cost of solvents the maintenance cost of Liquid Chromatography systems is also higher due to high pressures developed in pumps and columns. GC analysis in comparison is less costly and there is lesser maintenance cost.

Both LC and GC are versatile and have contributed to to the ever increasing scope of applications. Tandem techniques such as LC – MS – MS and GC – MS – MS have expanded the limits of detection to new frontiers of detection and automation has also contributed to increased laboratory throughputs.