OECD Draft: Phototransformation of Chemicals on Soil Surfaces

Photolysis involves the transformation of a chemical resulting from direct or indirect absorption of a solar photon. Photolysis can be an important dissipation pathway for a chemical that exhibits significant light absorption above 290 nm cut-off of the solar irradiation at the earth´s surface. The purpose of this test is to investigate the potential effects of solar irradiation on the test substance on soil surface. Photolysis rate and half-life will be determined for the test item. The transformation product pattern and mass balance can be investigated with the usage of 14C-labelled test item.

Study Design

Test setup

Typically, the test will be carried out with 14C-labelled test item. As light source a combination of xenon lamp and optical filters to simulate natural sunlight in the region of 290 to 800 nm will be used. Incubation on wet and dry soil will be accomplished at a temperature of 20°C. The soil will be prepared as a thin-layer on glass plates with a thickness of about 2 mm. The test will be conducted using multiple flask design where each custom-made flask is connected to a flow-through system. A moderate stream of air will be used as carrier gas to collect CO2 and other volatiles in distinct traps consisting of different solutions.

Course of the test

One test substance concentration will be used for the test. To distinguish between photochemical and other reactions (e.g. hydrolysis) additional samples will be incubated in the dark. The maximum study duration will not exceed the equivalent of 30 days of midsummer sunlight at an appropriate latitude (e.g. 10 days of artificial irradiation). Apart from samples taken directly after application, at least 6 sampling time points will be included. Time intervals will be chosen in such a way that the pattern of decline of the test substance and possible transformation products can be established.

Analysis of the test substance and transformation products

In case of the 14C-labelled test substance the analytical methods will base on LSC and HPLC coupled with UV and radio detection. LC-MS/MS can be carried out as additional analytical method. The analytical method and sample preparation will be established to allow a limit of quantification of ≤ 5% of the applied radioactivity.


  • Calculations of half-lives or DT50 and DT90 values
  • Transformation product pattern including characterisation/ identification of major products.

Guidelines and Literature

  • OECD Guideline for Testing of Chemicals, Draft Document, "Phototransformation of Chemicals on Soil Surfaces"; adopted January, 2002.