Determination of volatile halocarbons in seawater

Seawater contains considerable amounts of dissolved halogenated organic substances. The following chapter deals with the low relative molecular mass fraction, i.e., substances with one to four carbon atoms. This group of compounds is often called halocarbons or volatile halogenated organic compounds (VHOC). 

Most of the halocarbons produced by industry are intermediates for the manufacture of various products. Halocarbons are also used as solvents for dry cleaning and degreasing in the mechanical industry. Much of this production ends up in waste waters. Others, such as chlorofluorocarbons (CFCs) have been used as aerosol propellants which were discharged to the atmosphere on a short time scale. Some halocarbons are slowly leaking to the environment for other reasons, as for example CFC-12 used as heat exchanger in refrigerators. CFC-12 concentrations in the atmosphere therefore are still increasing, while concentrations of many other CFCs decrease as a result of restrictions on production and use. 

Halogenated organic substances are a potential risk to the stratospheric ozone, provided their residence times in the atmosphere are long enough for them to reach the stratosphere. The impact on the ozone chemistry increases with atomic number, i.e., bromine is more aggressive than chlorine. The atmospheric residence times of the most stable compounds are of the order of a hundred years, while others break down within a few days. Residence times are longer in seawater, except in anoxic waters Ballister and Lee, 1995 Tanhua et al., 1996). 

Samplers should be as large as possible, because the increasing ratio with size of volume over potentially contaminated area reduces the risk of sample contamination. The volume should be at least 5 L, preferably 10 L, even if less than 1L is used for the halocarbon measurements. 

To ensure that the samplers are clean, halocarbon free water should, if possible, be analysed on a regular basis. Such water is not easy to obtain. Distilled water as well as water deionized by other means is usually heavily contaminated with one or another halocarbon, and therefore is unsuitable. Seawater sparged with purified nitrogen is virtually non-con-taminated, but enough water to fill a set of up to 24 samplers is hard to purify this way. Another option is to monitor seawater of the lowest possible concentration regularly, and make sure that the scatter of results lies within a reasonable range as compared with the analytical uncertainty. 

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