According to the U.S. Agency for Toxic Substances and Disease Registry, benzene is a known human carcinogen. Consequently, high concentrations of benzene in drinking water or environmental samples can be cause for concern, and a variety of analytical methods have been developed to monitor benzene concentrations. The Environmental Protection Agency has set a maximum permissible level for drinking water of 5 parts per billion (ppb).
Purge and Trap
Purge and trap (or P&T) is a way to prepare water samples containing benzene for gas chromatography. In this technique, nitrogen is bubbled through the water solution; as benzene is more volatile than water, its vapor accumulates and is carried into a trap by the increasing pressure. The benzene and other volatiles are adsorbed or stick to the activated charcoal inside the trap and form a liquid. Subsequent heating will return the benzene and other volatiles to the gas phase.
In gas chromatography (or GC), an inert gas such as helium carries the benzene and other volatiles through a column, which may be either a capillary or packed type. In the capillary type, the center of the column is open but the column has a very narrow diameter, while in the packed type the column is wider but completely packed with a porous solid support. In both capillary and packed columns, the "stationary phase" is a liquid that interacts with the gases as they travel through the column. The rate at which a compound can traverse the column depends on the temperature, rate of flow and solubility of each compound in the liquid "stationary phase." Typically, compounds with lower boiling points will emerge faster than compounds with high boiling points. All in all, the GC method separates the volatiles in the sample from one another so that each can be identified.
The GC technique is often coupled with mass spectrometry; when used together, these two techniques are called GC/MS. In this approach, the benzene and other volatile compounds emerging from GC are ionized with a beam of high-energy electrons and propelled by an electric field. Their velocity depends on their mass, which in turn will determine the degree to which they are deflected by a magnetic field. MS makes it possible to determine the molecular weight of a compound and identify it. Purge and trap high-resolution GC/MS can often detect benzene at levels as tiny as parts per trillion.
GC is sometimes used instead in conjunction with flame ionization detectors (FIDs), in which the gases emerging from GC are ionized by burning them in a hydrogen/air flame. The ions released from this combustion process are fed into an electrometer, where they alter its electrical conductivity. Benzene can also be extracted from water using a solvent like dichloromethane; this older method, however, is less accurate, so GC/MS or GC/FID are now more common.