OECD TG 121: Estimation of the Soil Adsorption Coefficient (Koc) using HPLC

The adsorption coefficient is defined as the ratio between the concentration of the test substance in the soil/sludge and the concentration of the substance in the aqueous phase at adsorption equilibrium.
The adsorption coefficient normalised to the organic carbon content of the soil (Koc) is a useful indicator of the binding capacity of a chemical on organic matter of soil and sewage sludge. HPLC is performed on analytical columns packed with a commercially available cyanopropyl solid phase containing lipophilic and polar moieties, a moderately polar stationary phase based on a silica matrix is used. As a result of partitioning between mobile and stationary phases the test substance is delayed. The dual composition of the stationary phase having polar and non-polar sites allows for interaction of polar and non-polar groups of a molecule in a similar way as is the case for organic matter in soil or sewage sludge matrices. It allows to determine the relationship between the retention time on the column and the adsorption coefficient on organic matter. pH affects the sorption behaviour in particular for polar substances. For agricultural soils or tanks of sewage treatment plants, pH normally varies between pH 5.5 and 7.5. For ionisable substances, where test substance in its dissociated form (within pH 5.5 to 7.5) is higher than 10%, 2 tests should be carry out with both ionised and non-ionised forms in appropriate buffer solutions. The relationship between the retention on the HPLC column and the adsorption coefficient is just used for the evaluation, no quantitative analytical method is required and only the determination of the retention time is necessary.
A suitable set of reference substances and standard experimental conditions can be used, the method provides a fast and efficient way to estimate the adsorption coefficient Koc.The capacity factor k is described by the retention time tR and the dead time t0 as (tR-t0)/t0. The equation of the graph: log K versus log KOC obtained from  data of the reference substances can be calculated and the KOC of the test substance derived from the equation.



STUDY DESIGN
For the setting of the regression equation, at least 7 reference substances with known values of Koc are used including 1 substance for the determination of dead time (t0). The reference substances soultions are individually injected before and after sample solutions injections. The determination of the retention times and the plotting of log Koc as a function of log k allow to set the regression equation. After the determination of the test substance/s log k values (2 injections/sample solution), the corresponding Koc is calculated by the regression equation.

CRITERIA
Accuracy
Normally, the adsorption coefficient of a test substance can be estimated to within +/- 0.5 log unit of the value determined by the batch equilibrium method (OECD TG 106).  Higher accuracy may be achieved if the reference substances used are structurally related to the test substance.
Repeatability
log Koc values from duplicate determinations should be within a range of 0.25 log unit
Reproducibility
Typically, the correlation coefficient R for the relationship between log k and log Koc for a set of test substances is  0.95.
Study includes GLP managment and reporting.

REFERENCES AND GUIDELINES
OECD Guideline for Testing of Chemicals, No. 121 (22nd January 2001) - Estimation of the soil adsorption coefficient (Koc) using High Performance Liquid Chromatography (HPLC).

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Alberto Sala
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Valerio Frittella
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Guidelines

OECD TG 121