2.4. Sample preparation
Liquid sample(1.000g)was transferred to a 10mL plastic centrifuge tube and acidified with 4 mL of acidifying solution (pHo2; containing mixtures of 0.3 mol/L oxalic acid and 2.1 mmol/L tert-pentanoic acid as internal standard).Then It was centrifuged at 14,000g for 12min at room temperature and the clear solution was filtered through a 0.45 m Acrodiscs syringe filters(Sigma-Aldrich, Denmark) and finally the supernatant solution was transferred into analytical vials. Aliquot(1.0 mL) of the solution was injected into the GC/MS system using auto sampler. The processed sample can be stored in the refrigerator(_4 °C) for a week and the freezer (__20 °C) for a month prior to the GC/MS analysis (studied as a part of this work ; data not shown).