On the other hand, Gas Chromatography, highly volatile substances such as alcohols are being separated in a mixture, therefore a very high pressure is not necessary because in GC the components are volatile and they do not have to be forced to interact with the mobile phase and the stationary phase because the components themselves are reactive; instead components travel through the stationary phase with capillary action. Heat is being used instead in GC to turn the mixture into gas form in order for the gaseous mobile phase to carry it along the column; the temperature can be controlled by a thermostatically controlled oven; the HPLC is done at room temperature conditions. Also in GC the mobile phase is a gas such as nitrogen or helium; in HPLC, the mobile phase is in a liquid state and is usually polar such as the mixture of water and methanol.
UV light is absorbed by components after they are separated from the mixture in HPLC and the amount of light they absorb is detected by sensors in the UV Light Spectrometer. Another main type of detector used in HPLC is a mass spectrometer; which detects the individual molecular weight of the component. The peaks on the display screen of the mass spectrometer indicate the atom or molecule that has been separated from the entire compound. It helps to identify them. Some other common HPLC detectors are conductivity detectors, fluorescence detector and evaporative light detectors.
A special detector used in GC is the flame ionization detector. This detector senses the individual components in the mixture as it uses hydrogen gas that burns in air.
In GC, the separation of the components in the mixture typically depends on how volatile they are, the most volatile component leaves the column first. Whereas, in HPLC polarity affects the separation of the components; usually non-polar stationary phases are used, and more polar mobile phases carry the more polar component out of the column and into the detector at a faster rate. The non-polar components will take more time to exit the column as they will spend more time attached to the non-polar stationary phase.