ABSTRACT
             Background and Objectives: In this study, nanosilica modified with HS-SiO₂ thiol groups was utilized as adsorbent for solid phase extraction, as a fast and reliable method of preconcentration and separation of very small quantities of selenium ions from water and blood samples.
             Methods: The samples included four natural water samples and one biological sample (blood serum) prepared in volumes of 25, 100, 200, 300, 400 and 500 ml. The samples were analyzed by solid phase microextraction, using thiolated-nanosilica (as adsorbent), ultraviolet-visible spectrophotometry and atomic absorption spectroscopy.
             Results: Optimized conditions for preconcentration of a 25 ml 0.2 mg/l selenium solution were pH 5, 40 mg of adsorbent, sample-adsorbent mixing time of 15 minutes and 5 ml of 2N sulfuric acid as detergent. The volume limit and concentration factor were 400 and 80, respectively. Limit of detection and relative standard deviation of the method were 0.46 μg/l and 0.9%, respectively.
             Conclusion: This study is the first to successfully utilize thiolated nanosilica for measuring low selenium levels. Thiolation of the absorbent increases selenium adsorption by thiolated-silica compared to SiO₂.
             Keywords: Solid phase extraction, Selenium, Preconcentration, Nano, UV-visible spectrophotometry.