Headspace gas chromatography uses headspace gas—from the top or "head" of a sealed container containing a liquid or solid brought to equilibrium—injected directly onto a gas chromatographic column for separation and analysis. In this process, only the most volatile (most readily existing as a vapor) substances make it to the column. The technique is commonly applied to the analysis of polymers, food and beverages, blood alcohol levels, environmental variables, cosmetics, and pharmaceutical ingredients.
Introduction
Chemists often use the phrase "standard temperature and pressure" or "STP" to convey that they are working at a temperature of 0 °C and one atmosphere of pressure (International Union of Pure and Applied Chemistry). There are three states of matter under these conditions: solids, liquids, and gases. Although all three are distinct states, both solids and gases can dissolve (or disperse) in liquids. The most commonly occurring liquid in the biosphere is water. All components of the atmosphere are capable of dissolving in water to some degree. The bulk of the stable natural components of the atmosphere are nitrogen, oxygen, carbon dioxide, gaseous water, argon, and other trace gases.
Materials that exist primarily in the gas phase at STP (i.e., "evaporates more than 95% by weight within six months under ambient evaporation testing conditions") are referred to as "volatile." Many natural and man-made (anthropogenic) materials are stable in two states at STP, earning them the title "semivolatile." A naturally occurring volatile that is sometimes found in aqueous solution is methane; water itself is semivolatile. Man-made or anthropogenic chemicals also occur in these classes. Examples of volatile anthropogenic chemicals include the refrigerants chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs). Semivolatile anthropogenics can exist as mixtures, such as petroleum distillates or as pure chemicals like trichloroethylene (TCE).
There is a need to analyze the dissolved gas content of aqueous solutions. Dissolved gases can directly interact with aquatic organisms or can volatilize from solution (the latter described by Henry's law). These processes can result in exposure that, depending on the nature of the dissolved material, can have negative health effects. There is natural occurrence of various dissolved gases in groundwater and can be a measure of health for lakes, streams, and rivers. Dissolved gases also occur as a result of human contamination from fuel and chlorinated spill sites. As such, headspace gas chromatography offers a method for determining if there is natural biodegradation processes occurring in contaminated aquifers. For example, fuel hydrocarbons will break down into methane. Chlorinated solvents such as trichloroethylene, break down into ethene and chloride. Detecting these compounds can determine if biodegradation processes are occurring and possibly at what rate. Natural gas extracted from the earth also contains many low molecular weight hydrocarbon compounds such as methane, ethane, propane, and butane. For example, methane has been found in many water wells in West Virginia.
RSKSOP-175 analysis method
One of the most widely used methods for headspace analysis is described by the United States Environmental Protection Agency (USEPA). Originally developed by the R.S. Kerr USEPA Laboratory in Ada, Oklahoma as a "high quality, defendable, and documented way to measure" methane, ethane, and ethene, RSKSOP-175 is a standard operating procedure (SOP) and an unofficial method employed by the USEPA to detect and quantify dissolved gases in water. This method has been used to quantify dissolved hydrogen, methane, ethylene, ethane, propane, butane, acetylene, nitrogen, nitrous oxide, and oxygen. The method uses headspace gas injected into a gas chromatographic column (GC) to determine the original concentration in a water sample.