Gas chromatography, otherwise known as Gas-liquid chromatography, is for separating, identifying, and quantifying the components in organic compound mixtures. This technique works by selectively partitioning the organic compounds’ components between mobile and stationary phases inside a column. Gas chromatography is suitable for separating compounds with high volatility, low molecular weights, and thermal stability.
3 types of gases employed in GC are carrier gas, fuel gas and zero air. There are different components involved in GC chromatography, namely, mobile phase, sample injector, column, column oven, detector and data system. The main types of GC are Gas-Liquid Chromatography or GLC and Gas-Solid Chromatography or GSC. Either liquid or solid can be used as the stationary phase in these methods, where gas is the mobile phase.
HPLC and GC techniques
GLC is the technique used to separate the molecules or ions dissolved in a solvent. In GSC, retention happens because of physical absorption.
It works on the principle that when contact occurs between the sample solution and a liquid or solid phase, the solutes present in these components will start to interact with other phases present there.
Then separation of different components may happen due to the ion-exchanges, different absorption rates, varied sizes, change in interaction pattern, etc. After passing through the column at different rates, the components in the compound will be separated.
Gas chromatography is an analytical instrument that helps in saving time. It is user-friendly, which requires minimum operations.
High-Performance Liquid Chromatography is like GC in many ways. They work on the same principles. But, there are specific differences between these two techniques. The primary difference is that HPLC uses liquid as the carrier while GC uses gas.
Unlike GC, HPLC is the analytical technique used for testing thermally labile compounds with higher molecular weights. The separation happens in HPLC, with molecules’ distribution between the mobile liquid phase and the stationary solid phase inside a column.
A clear understanding of the difference in characteristics of these two composer separation methods will help choose the proper technique for separation.
Key differences between HPLC and GC
1. The mobile phase in HPLC is liquid, and it is a gas in GC. There will be a pump in HPLC to drive the liquid through the column. There will be high viscosity and density of liquids in this method.
2. The nature of analytes is another factor that is different in both methods. The samples used in the HPLC method are thermally labile, and they will degrade at high temperatures. In GC, the samples can resist and withstand high temperatures, even up to 400-degree Celsius. The volatile nature of solutes, the capacity to handle liquids, dissolved solids, and even gases make GC different from HPLC. Samples for HPLC are always liquids, either normal ones or solutions with dissolved solids. The samples with high molecular weights are used in HPLC while those with lower molecular weights are used in GC.
3. Column size is generally short and broad in HPLC due to the higher density and viscosity of liquids. It ranges from 10 cm to 25 cm with about 0.46 mm id. Longer columns are used in GC, with narrow bores. It is because the gases are less dense and with low levels of viscosity. The length of the capillary columns can be several meters.
4. Operating temperatures for both techniques are different. Due to the thermally labile nature, HPLC experiments are always conducted at ambient temperatures. It may extend from 40 degrees Celsius to 60 degrees. On the other hand, GC operations can be conducted up to 400-degree Celsius, as the samples are irresistible to high temperatures.
5. Operating pressures are also different in these two analytical methods. Liquids require high pressures of about 5000-6000 psi for analytical separations because of the higher levels of density and viscosity. HPLC operations can even afford a range of 15,000 to 18,000 psi. Only lower pressures are required in GC, that range from 150 to 200 psi.
6. The standard detectors used in HPLC are UV detector, photodiode array detector, Fluorescence detector, Refractive index detector, and Conductivity detector. Detectors used in GC are Flame ionization detector, Thermal conductivity detector, Electron capture detector, Nitrogen phosphorus detector, Flame photometric detector, and Photoionization detector.
The use of mass selective detectors in both GC and HPLC helps in achieving outstanding results. Though similar in many ways, the differences in both the GC and HPLC features make them unique in their way, and the awareness about these differences will enable the user to select the correct analytical method to fulfill their purpose.