Optimized conditions for liquid-phase microextraction based on solidification of floating organic droplet for extraction of nitrotoluene compounds by using response surface methodology

Solidification of floating organic droplet microextraction (SFODME) method followed by gas chromatography-flame ionization detector (GC-FID) was applied for ultra-preconcentration and determination of nitrotoluene compounds in water samples. The effects of several factors such as volume of organic solvent, sample solution temperature, extraction time, stirring rate and ionic strength were simultaneously investigated on the extraction efficiency using an experimental design. For the first time, quarter fraction factorial design was applied to screening in order to determine the significant factors in the extraction efficiency. Then, central composite design (CCD) was used for the optimization of important factors and the response surface equations were obtained. The optimized conditions were established to be 600 rpm for stirring rate, 30 min for extraction time, 60 °C for extraction temperature, 5 μL for organic solvent volume, and 3% (w/v) of NaCl for ionic strength. Limit of detections (LODs) for the extraction method were in the range of 0.3–0.5 μg L−1. Linear dynamic ranges were in the range of 0.5–200 μg L−1 for three nitrotoluene compounds and the obtained preconcentration factor was in the range of 535–640. The relative standard deviations of the proposed method were 4–10%. Finally, performance of the proposed method was tested for the extraction and determination of the nitrotoluene compounds at microgram per litre levels in samples and satisfactory results were achieved.