Generation of Standard Gas Mixtures

As seen throughout this discussion of the measurement of gases in the atmosphere, a critical component is the accurate calibration of the technique for the gas(es) of interest. This clearly requires sources of such calibration gases, which however, vaiy depending on the particular gas. 

In the simplest case, the gas of interest can be purchased in a gas cylinder with known concentration provided by the supplier. In the United States, NIST 

Caution must be exercised in using cylinder gases in some cases, however. For example, N02 in air from cylinders commonly contains a few percent HN03 as an impurity, and nickel carbonyls are present in CO stored in cylinders. 

In other cases, the species cannot be preprepared as a mixture in air and hence flow systems must be used to generate them. For example, HN03 strongly adsorbs to surfaces and hence it is not possible to make a stable calibration mixture that can be stored. Some larger organics also do not have long-term stability when stored in gas cylinders. 

A similar approach is the use of diffusion cells. In this case, the liquid is held in a container that has a capillary of fixed length and diameter through which the vapor over the liquid diffuses. The vapor exiting the capillary is swept into a flow of gas to provide the gas mixture this approach has been used to prepare mixtures of terpenes in air, for example (Larsen et al., 1997). The concentration of the gas can be varied by using capillaries of varying internal diameter and length. 

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Xiong, G. and Pawliszyn, J., Microwave-assisted generation of standard gas mixtures, Anal. Chem., 74, 2446-2449, 2002. 

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