Ametryn

Phototransformation in water

Description of key information

Studies have shown ametryn to be susceptible to photolysis in solution under artificial sunlight throughout the environmentally relevant range of pH 5 to pH 9, with a half-life in the region of 20 to 29 hours (1.7 to 2.4 12-hour days), and also under natural sunlight at pH 7 with a half-life in the region of  2544 hours (212 12-hour days).

Additional information

Three studies are available that describe the phototransformation of the substance.

The key study is a Klimisch-1, GLP-compliant study, sponsored by Sipcam Agro S.A. and carried out in 2002 by Bioagri (Prata Cione AP) in accordance with a method equivalent or similar to OECD 316 (Phototransformation of Chemicals in Water - Direct Photolysis), 2008; no EC method. The objective of the study was to determine the half-life and photolysis rate of Ametryn in aqueous solution by artificial sunlight and to quantify and identify the degradation products. Solutions of Ametryn (ca. 22 mg/L) at pH 5, 7 and 9 were exposed to artificial sunlight from a Xenon lamp (250 W/m2) for 23.8 hours at ca. 25°C. Periodic samples were taken, in which the Ametryn and degradation products were quantified using HPLC, and identified using LC-MS. The results showed Ametryn to be unstable under artificial sunlight, with a half-life estimated to be 23.4 hours at pH 5, 20 hours at pH 7 and 23 hours at pH 9. Only a single degradation product was seen, which mass balance consistently showed to account for ca. 100% of lost test substance, and which was identified as EITD, the des(methylthio) analogue of Ametryn. Ametryn was stable in the dark controls.

The first supporting phototransformation study is a Klimisch-1, GLP-compliant, sponsored by Ciba-Geigy and carried out in 1989 by Agrisearch Incorporated (Spare WC) in accordance with EPA Guidelines, Subdivision N, 163-2 (Photodegradation studies in water). A solution of Ametryn (ca. 10 mg/L) at pH 7 was exposed to natural sunlight at ambient temperatures (5°C to 29°C) for 30 consecutive days. Periodic samples were taken and analysed for 14C-Ametryn and degradation products by TLC. The results showed Ametryn to photolyse slowly under the conditions of the test, with a half-life estimated to be 212 days (12 hours light each, i.e. 2544 hours) and a rate constant of 3.3E-3 per day. Four degradation products were identified, namely the des(methylthio) analogue (GS-32083), the desethyl analogue (GS-11354), the desisopropyl analogue (GS-11355) and the desethyl-des(methylthio) analogue (GS-28304), although each individual degradation product represented less than 5% of the total radiocarbon.

The second supporting phototransformation study is also a Klimisch-1, GLP-compliant, sponsored by Ciba-Geigy and carried out in 1989 by Agrisearch Incorporated (Spare WC) in accordance with EPA Guidlines, Subdivision N, 163-2 (Photodegradation studies in water). A solution of Ametryn (ca. 10 mg/L) at pH 7 was exposed to artificial sunlight from a mercury arc lamp (3.6 to 4.7E-5 W/cm2 – ca. twice the intensity of bright natural sunlight) for 48 hours at ca. 25°C. The results showed Ametryn to photolyse under the conditions of the test, with a half-life estimated to be ca. 29 hours (2.4 12-hour days) and a rate constant of 2.4E-2 per hour. Several degradation products were isolated and characterized by TLC, with the primary degradation product being the des(methylthio) analogue (GS-32083), which represented ca. 35% of the initial radiocarbon dose at 48 hours. Additional products were the desethyl-des(methylthio) analogue (GS-28304, 7% of dose), the desethyl analogue (GS-11354, 5%), GS-17794 (no structure provided, 5%) and the 6-hydroxy analogue (GS-34048, 15%). This author concluded that solution photolysis may be an environmentally relevant transformation process for Ametryn.