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The adsorption coefficient KOC of charcoal (Probe 2: C-Fix = 80.5%) was determined using the HPLC method in accordance with the OECD guideline 121 and in compliance with GLP standards.

The study was performed using HPLC with a cyanopropyl chemical bound resin on a silica based column as stationary phase (pH = 5.5; 25 °C). As mobile phase methanol/water (55/45% v/v) was used. Seven reference items with different retention times (tR) were chosen for the calibration on behalf of the retention time of the test item charcoal (Probe 2: C-Fix =80.5%) on the column.

First, a solution of the reference items was measured three times with HPLC, then a solution of the test item (three measurements), and finally the solution of the reference items was again measured three times.

For each reference item, the capacity factor k’ was calculated from the retention time of an unretained compound (sodium nitrate) and the retention time of the respective reference item. A calibration function was set up using the literature values for KOC of the reference items and the mean capacity factor of the six determinations. Based on the data set log k’ versus log KOC, the equation for the regression line was calculated.

The chromatogram of the test item showed one peak (mean TR = 2.254 min).The log KOC was calculated as 0.661±0.026 (mean ± standard deviation) with a range of 0.633–0.686. As this log KOC-value lies outside the calibration range obtained with the used reference substances (1.32-5.63) and also outside the determinable log KOC-range of the HPLC screening method (according to OECD guideline 121: 1.5-5.0), the log KOC of the test item charcoal (Probe 2: C-Fix = 80.5%) should be stated as < 1.5.

Due to the limited solubility of the test item charcoal (Probe 2: C-Fix = 80.5%) in the solvent, an extract instead of a solution was used for the measurements. Therefore, the results are necessarily related to the used charcoal extract with a nominal concentration of 208,910 mg/L in methanol/water (75/25% v/v) under the testing conditions used (25±1°C; pH 5.5).

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