α hch

Salmon sperm DNA with an average molar mass of 1.3 × 10⁶ Da and %G-C content of 41.2%, α-HCH, β-HCH, and γ-HCH were purchased from Sigma Chemical Co. (St. Louis, MO, USA). DNase I and DNase I buffer (10 × ) were purchased from Takara Bio Company (Dalian, Liaoning, China). The physicochemical properties of α-HCH, β-HCH, and γ-HCH are given in Supplementary Table 4. All chemical reagents were of analytical grade and used without further purification. DNA stock solution of 1 g L⁻¹ was prepared by dissolving 0.1 g DNA in 100 mL Tris-HCl buffer (10 mmol L⁻¹, pH 7.0). Stock solutions of α-HCH, β-HCH, or γ-HCH (1 g L⁻¹) were prepared using methanol as solvent. HCH working solutions were prepared by further diluting their stock solution by methanol to 10 mg L⁻¹. All solutions were stored at 4 °C in a refrigerator before use.

Standard OCPs including pp-DDT, pp-DDD, pp-DDE, α HCH, γ HCH, heptachlor, heptachlor epoxide, aldrin, endrin, chlordane, methoxychlor, and HCB were obtained from Sigma–Aldrich (Germany). Petroleum ether, diethyl ether, n-hexane, acetonitrile, anhydrous sodium sulfate, and methylene chloride were bought from Merck (Darmstadt, Germany). Florisil (PR Grade, 60–100 mesh) was purchased from Silica (Silica Co., USA). All solvents were of pesticide residue grade and subjected to a solvent purity test for residue analysis suitability. Florisil was activated at 130°C for 24 h and cooled to room temperature.