Chromatography is another technique used to separate mixtures into their individual components. The most common chromatographic techniques are paper chromatography, gas-liquid chromatography (also called gas chromatography), thin layer chromatography, and high pressure (or high performance) liquid chromatography (HPLC). All methods of chromatography share common characteristics. The process is based on the principle that different chemical compounds will stick to a solid surface, or dissolve in a film of liquid, to different degrees. Chromatography involves a sample (or sample extract) being dissolved in a mobile phase (which may be a gas, a liquid, or a supercritical fluid). The mobile phase is then forced through an immobile, immiscible stationary phase. The phases are chosen in such a manner that components of the sample have differing solubilities in each phase. The least soluble component is separated first, and as the separation process continues, the components are separated by increasing solubility.