What are Dioxins?

Dioxins and PCBs belong to the group of compounds known as Persistent Organic Pollutants (POPs). They are known to bio accumulate due to their lipophilic nature  and, therefore, have health implications. As a result their emission into the environment and food chain is strictly controlled. Samples that are analysed, amongst others, are foodstuffs like fish, fish feed, and stack emissions from waste incineration sites. Limits are published by the World Health Organisation (WHO) and local authorities.  As a consequence, low levels of contamination have to be detected, providing a challenge to sample preparation and detection  systems.
 

Compounds of Interest
The term ‘Dioxin’ covers a wide range of halogenated aromatic compounds, including polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs and PCDFs). These compounds are formed as a result of incomplete  combustion  of hydrocarbons in the presence  of chlorine e.g. metal processing, domestic waste incineration, etc. They have high melting points and are stable to acids and bases; these characteristics make them very persistent  in the environment. PCDD/ Fs can be found in many environmental matrices such as soils, air, and water.

The basic structure  of PCDD/ Fs comprises two benzene rings joined by either a single (furan) or a double oxygen bridge (dioxin), see (Figure 1).

There are 210 possible combinations  of chlorine atoms on the skeletal structure  of dioxins and furans. However, only a few congeners are considered to have significant risk to human health. The toxicity of these compounds is measured  in TEF (Toxic Equivalence Factor), which is an internationally recognised calculation that weighs the toxicity of each individual congener against the most toxic compound in that family, in the case of PCDD/PCDF, this is 2,3,7,8-TCDD. The closer the ratio is to unity, the greater  the toxicity of that congener. Calculation of the total toxicity of a sample is achieved by multiplying the concentrations of the individual target  compounds by their respective TEFs. These values are known as TEQs (Toxic Equivalents); and the total TEQ of a sample is obtained  by summing the individual TEQs.

In addition to PCDF and PCDD, some polychlorinated biphenyls (PCBs) (Figure 2) that are similar in structure  and lipophilic properties as the dioxins have been identified as having similar toxic health effects. These are often referred to as non-ortho,  coplanar, or dioxin like PCBs, and their TEF is also measured  against 2,3,7,8-TCDD. For example, a PCB congener with a TEF of 0.01 is considered to be one hundred times less toxic than 2,3,7,8-TCDD (see Table 1).

Non-ortho PCBs are those which are not chlorinated at the ortho position, and as such are free to rotate  around  the single carbon carbon bond, resulting in a co-planar (‘flat’) configuration;  PCBs that have a single ortho chlorine are also able to adopt a relatively planar arrangement;  the twelve possible congeners that obey these rules are known as the WHO-12 PCBs. These compounds are monitored  along with the dioxins (see Table 1).