Complex substances

For a complex substance (UVCB/multi-constituent), the ‘substance’ to be tested in the environmental fate studies must be identified. This may be the whole substance, specific constituent(s) or fraction(s) of the substance. Guidance on selection of the appropriate test substance is given in ECHA Guidance R11 (ECHA, 2017).

Each constituent in a multi-constituent substance or UVCB substance adds complexity to the analytical methodology and to the interpretation of the study results.

Where it is appropriate to test specific constituents of the substance, each additional constituent selected will also add to the cost of the study, particularly for radiolabelled studies, where the cost associated with synthesis of radiolabelled constituents is significant.

Throughout ECHA Guidance R11, it is stated that the feasibility and proportionality of efforts should play a part design of the test, e.g. in selecting the ‘substance’ to be tested. Several approaches to selection and assessment of the appropriate test ‘substance’ are described in R.11.4.2.2.2. These essentially focus on achieving an assessment of the persistence and bioaccumulation potential of the multi-constituent/UVCB based on a ‘worst-case’ assessment of the constituents or fractions.

If testing whole substance, constituents (and impurities in the case of multi-constituent substances) potentially have a wide range of physical chemical properties and subsequently very different degradation pathways:

• This would affect their extraction efficiency and partitioning between the water and the organic content of the soil/sediment/fish.
• There would be large chromatographic differences between polar and non-polar compounds.
• It should be noted that synthesised multi-constituent and UVCB substances would contain a proportion of the starting materials. These could be confused with metabolites from the constituents.
• The sensitivity of the analytical method required to identify and quantify multiple constituents, and potential metabolites of these constituents may be a significant challenge.
• Analytical reference standards are unlikely to be available for individual constituents, increasing the difficulty of quantification.

The partitioning of constituents of variable solubility in a multi-constituent substance can also be a challenge, where the soluble constituents are dissolved preferentially. This is even more complex for Unknown or Variable Composition, Complex Reaction Products and Biological Materials (UVCB) such as petroleum-derived products (Letinski et al., 2016), wood pulp extracts or essential oils.