Question

Define and describe GC and HPLC, what are the differences in operation and application?

Answer 1

GC:

Gas chromatography is used to separate organic compounds that are volatile. A gas chromatograph consists of a flowing mobile phase, an injection port, a separation column containing the stationary phase, a detector, and a data recording system.The organic compounds are separated due to differences in their partitioning behavior between the mobile gas phase and the stationary phase in the column.Mobile phases are generally inert gases such as helium, argon, or nitrogen.The injection port consists of a rubber septum through which a syringe needle is inserted to inject the sample.The injection port is maintained at a higher temperature than the boiling point of the least volatile component in the sample mixture. Since the partitioning behavior is dependent on temperature, the separation column is usually contained in a thermostat-controlled oven.Separating components with a wide range of boiling points is accomplished by starting at a low oven temperature and increasing the temperature over time to elute the high-boiling point components.Most columns contain a liquid stationary phase on a solid support. Separation of low-molecular weight gases is accomplished with solid adsorbents.

HPLC:

High-performance liquid chromatography (HPLC; formerly referred to as high-pressure liquid chromatography), is a technique in analytical chemistry used to separate, identify, and quantify each component in a mixture. It relies on pumps to pass a pressurized liquid solvent containing the sample mixture through a column filled with a solid adsorbent material. Each component in the sample interacts slightly differently with the adsorbent material, causing different flow rates for the different components and leading to the separation of the components as they flow out the column.

Differences in operation and applications of GC and HPLC:

Gas Chromatography and High Performance Liquid Chromatography are both separation techniques which have gained immense popularity in both academic and industrial laboratories. High Performance 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,. A general question that comes up in your mind is how our HPLC and GC are different from each other and what factors decide suitability of one over the other.

Mobile phase

As the name suggests High Performance 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

HPLC 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 packings have extended temperature limits of High Performance 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 HPLC 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 HPLC generally have higher viscosity in comparison to gases used in Gas Chromatography. This results in increased column back pressures in HPLC. It is for this reason that High Performance 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 HPLC 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 HPLC columns depends on polarity differences or molecular sizes whereas GC separations are based on differences in volatilities of compounds

Detection principles

HPLC 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

HPLC solvents are costly in comparison to gases used for GC analysis. In addition to cost of solvents the maintenance cost of High Performance 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 HPLC 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.