OECD 316: Phototransformation of Chemicals in Water – Direct Photolysis

Direct photolysis involves the transformation of a chemical resulting from direct absorption of a solar photon. Direct 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 in water. Direct photolysis rate, half-life and quantum yield 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 in sterilised (buffered) aqueous solution will be accomplished at a temperature of 25°C. The test will be conducted using multiple flask design. Based on the phase of the study either hermetically closed quartz glass cuvettes or custom-made incubation flasks connected to a flow-through system will be used. In the latter case 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 sunlight exposure during an appropriate season and latitude (e.g. 14 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.

Tier 1 (Theoretical screen):

The maximum possible direct photolysis rate constant and the corresponding half-life for the test substance will be estimated based on the absorption spectrum of the test substance.

Tier 2 (Degradation rate):

The decline in the concentration of the test substance and the corresponding direct photolysis rate constant will be determined (Tier 2-1). According to these results, if further tests are necessary, an actinometer will be developed to calculate the quantum yield of the test substance(Tier 2-2).

Tier 3 (Mass balance and transformation products):

Bass balancing and transformation product pattern will be determined for the test substance. Degradation kinetics will also be followed for any major transformation product, if possible.

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.

Special test setup

  • Non-labelled test substance can also be used for the test to investigate the degradation kinetics (e.g. DT50) of the test substance.


  • 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, No. 316, "Phototransformation of Chemicals in Water – Direct Photolysis"; adopted October 3, 2008.
  • Leifer A. The Kinetics of Environmental Aquatic Photochemistry. Theory and Practice. ACS Professional Reference Book, 1998