Optimization of supercritical fluid extraction combined with dispersive liquid–liquid microextraction as an efficient sample preparation method for determination of 4-nitrotoluene and 3-nitrotoluene in a complex matrix

Supercritical fluid extraction (SFE) combined with dispersive liquid–liquid microextraction (DLLME) followed by gas chromatography with flame ionization detector (GC-FID) was developed for the extraction and determination of 4-nitrotoluene and 3-nitrotoluene in soil sample. The effects of different SFE experimental parameters on the extraction recovery were studied simultaneously using a central composite design (CCD) after a 2n−1 fractional factorial experimental design. The variables of interest in SFE were pressure, temperature, modifier volume, and dynamic extraction time. From this statistical evolution the pressure, dynamic extraction time, and modifier volume were found to have significant effects on the results achieved from SFE-DLLME-GC-FID, while temperature was not statistically significant at a 95% confidence level. The optimal SFE conditions for 4-nitrotoluene and 3-nitrotoluene were a pressure of 350 atm, temperature of 35 °C, dynamic extraction time of 30 min and modifier volume of 150 μL. Under the optimal conditions, the extraction calibration plots were linear in the range of 0.25–25 mg kg−1 and the limits of detection (LODs) were 0.12 mg kg−1 for both of the analytes. Performance of the present method was evaluated for extraction and determination of 4-nitrotoluene and 3-nitrotoluene in soil samples, and satisfactory results were obtained (RSDs < 6.5%).