Pesticides determination in crops by solvent assisted stir bar dispersive solid phase microextraction based on magnetic nano-composites

In this study, a novel method combining the principles and benefits of solvent assisted dispersive solid phase microextraction (SA-DSPME) [1] and stir bar sorptive extraction (SBSE) is presented. This new approach, termed solvent assisted stir bar dispersive solid phase microextraction (SA-SBDSPME), involves the addition of a magnetic nano composite (MNC) and a neodymium-wire (1.5 cm) magnetic stir bar into the sample solution. The proposed method was based on the dispersion of the synthesized MNC (Nio@TiO2@GO-CTAB) into the aqueous sample to maximize the interaction surface. In this way, the dispersion of the sorbent at a very low milligram level was received by injecting a solution of the MNC and disperser solvent into the aqueous sample. The cloudy solution created from the dispersion of the MNC in the sample solution. In the high stirring rates, the rotational forces surpass the magnetic field and the MNC disperses into the sample solution in a similar manner to DSPME. But, As long as the stirring rate is maintained at low speed, the MNC resists rotational (centrifugal) forces and remains on the stir bar surface in a manner closely resembling SBSE. After extraction, the stirring is stopped and the MNC returns to the neodymium-wire without the requirement of an additional external magnetic field. The MNC-coated stir bar containing the preconcentrated analytes is thermally desorbed directly into a gas chromatographic system coupled to a mass spectrometric detector (TD-GC-MS) using a SPME syringe. Several parameters affecting the extraction efficiency were studied by Fractional Factorial Screening (FFS) and a multivariate optimization was done using Central Composite Design (CCD) [2]. To prove the utility of the proposed method, the extraction of nine organophosphorus pesticides from typical agricultural crops was done as model analytical application and excellent analytical features were achieved in terms of linearity (R2  0.991), enrichment factors (1925–2644), limits of detection (lower than 20 ng.L-1), intra- and inter-day repeatability and reproducibility (RSD<6%) and relative recoveries (94-102%, 91- 103% and 92-105% for rice, tomato and onion samples, respectively).