2.4. Solution Preparation

A total of 30 g of NaHCO₃ was quantitatively transferred to a 1000 mL volumetric flask (hereinafter referred to as ‘flask’) and dissolved in a small amount of water. After dissolving the batch, the flask was made up with water.

PBS consists of two solutions: solution A (0.02 mol L⁻¹ Na₂HPO₄ at pH 7.4) and solution B (0.29 mol L⁻¹ NaCl). Solution A: A total of 1.42 g of NaHPO₄ was quantitatively transferred to a 500 mL flask and dissolved in a small amount of water. After dissolving the batch, the flask was made up with water. The pH at 7.4 was adjusted with 85% H₃PO₄. Solution B: A total of 8.47 g of NaCl was quantitatively transferred to a 500 mL flask and dissolved in a small amount of water. After dissolving the batch, the flask was made up with water. PBS was obtained by mixing both prepared solutions A and B in a ratio of 1:1. PBS is stable for one year. PBS-15% methanol was obtained by mixing 850 mL of PBS and 150 mL of methanol.

A total of 116.9 g of NaCl was quantitatively transferred to a 1000 mL flask and dissolved in a small amount of water. After dissolving the batch, a total of 33.7 mL of H₃PO₄ was pipetted into the flask. The flask was made up with water. The pH at 1.6 was adjusted with NaOH. The solution is stable for six months.

A total of 2 mL of CH₃COOH was pipetted into a 100 mL flask. The flask was made up with CH₃OH.

All of these solutions were kept at 5 ± 0.5 °C. Before direct use, they were tempered at a laboratory temperature of 21 ± 0.5 °C.

A total of 100 µL of OTA stock solution at concentration of 1000 µg L⁻¹ was pipetted into a 25 mL flask. The flask was made up with CH₃OH.

Stock and working standard solutions were kept at −20 ± 0.5 °C.

A total of six calibration OTA standards (0.10, 0.25, 0.50, 1.00, 2.00, and 4.0 ng mL⁻¹) were prepared with a linear response on each day of the measurement from the working solution (40 µg L⁻¹) by its dilution in the mobile phase (MP) in a ratio reaching the target concentration. The determination coefficient was 0.9999. A blank sample consisting of the mobile phase was also prepared fresh daily.

The MP consisted of two solutions: solution A (acetonitrile:acetic acid, 99:1) and solvent B (water:acetic acid, 99:1). Solvents were used in ratio 40:60; A:B.