<Li> P / T (Purge - and - Trap) system; An inert gas is bubbled through an aqueous sample causing insoluble volatile chemicals to be purged from the matrix . The volatiles are' trapped' on an absorbent column (known as a trap or concentrator) at ambient temperature . The trap is then heated and the volatiles are directed into the carrier gas stream . Samples requiring preconcentration or purification can be introduced via such a system, usually hooked up to the S / SL port . </Li> <P> The choice of carrier gas (mobile phase) is important . Hydrogen has a range of flow rates that are comparable to helium in efficiency . However, helium may be more efficient and provide the best separation if flow rates are optimized . Helium is non-flammable and works with a greater number of detectors and older instruments . Therefore, helium is the most common carrier gas used . However, the price of helium has gone up considerably over recent years, causing an increasing number of chromatographers to switch to hydrogen gas . Historical use, rather than rational consideration, may contribute to the continued preferential use of helium . </P> <P> The most commonly used detectors are the flame ionization detector (FID) and the thermal conductivity detector (TCD). Both are sensitive to a wide range of components, and both work over a wide range of concentrations . While TCDs are essentially universal and can be used to detect any component other than the carrier gas (as long as their thermal conductivities are different from that of the carrier gas, at detector temperature), FIDs are sensitive primarily to hydrocarbons, and are more sensitive to them than TCD . However, a FID cannot detect water . Both detectors are also quite robust . Since TCD is non-destructive, it can be operated in - series before a FID (destructive), thus providing complementary detection of the same analytes . </P> <P> Other detectors are sensitive only to specific types of substances, or work well only in narrower ranges of concentrations . They include: </P>

How does a liquid stationary phase separate the organic compounds in a mixture