gradeand purchased from Labscan (Dublin, Ireland). Membrane filters(PVDF, 0.45 m) were obtained from Millipore (Ireland).
2.2. Samples and sample preparation
Initially, method optimization and validation experiments wereperformed with a biological semi-skimmed milk sample (fat con-tent <1.5% w/w), purchased from local market and was found to besatisfactory as a blank for the target analytes. Milk samples werekept at 4◦C, until use.
2.3. Apparatus
Chromatographic analysis of OCPs and PCBs was performedusing a Trace GC Ultra instrument (Thermo Scientific, Waltham,MA, USA) coupled to an ISQ mass spectrometer controlled bya computer running X-Calibur software. Aliquots of 1 L wereinjected using an AI/AS 3000 auto sampler (Thermo Scientific).The separation was performed using a SLB fused silica column30 mm × 0.25 mm i.d., with film thickness of 0.25 mm (Supelco,Bellefonte, PA, USA). Helium was the carrier gas at a constant inletflow rate of 1.5 mL/min (constant flow). The GC oven temperatureprogram was as follows: initial temperature 60◦C held for 5 min,ramped at 8◦C/min to 300◦C (held for 2 min) (total acquisition timeprogram: 45 min). The ion source and transfer line were kept at225 and 250◦C, respectively. In the full-scan mode, electron ioniza-tion mass spectra at m/z of 50–500 were recorded at 70 eV. In theselected ion monitoring (SIM) mode acquisition, target ions weremonitored at different time windows defined by the correspondingretention times. Three diagnostic ions were chosen for each analyte,according to the mass spectra characteristics obtained in the full-scan mode as well as by comparison with NIST library. The qualitycriteria used were the following: the relative retention time of theanalytes, corresponded to that of the matrix-matched calibrationsolution with a tolerance of ±0.5% and the relative intensities ofthe selected ions did not differ more than 10% from the relativeintensities of the same ions acquired from a spiked sample.The SEM images, after gold coating were taken by a JOEL micro-scope (JSM-5600, JEOL, Tokyo, Japan) to assess the morphology ofMNPs.To characterize the functionalized MNPs and ensure their properfabrication, FT-IR spectroscopy (FTIR, Perkin Elmer, spectrum 100,Waltham, MA, USA) was employed. In addition, elemental analy-sis of Fe3O4@SiO2@C18 NPs was performed on a Vario Macro CNS(Elemental Analyzensysteme GmbH, Hanau, Germany).The surface area of MNPs was calculated based on N2adsorption–desorption porosimetry according to the BET methodon an Autosorb-1 porosimeter (QUANDACHROM, Bounton Beach,FL, USA). Before measurement, the sample was degassed at 80◦C,for 5 h.Finally, the crystal phases of Fe3O4, Fe3O4@SiO2andFe3O4@SiO2@C18 MNPs were investigated by X-ray diffrac-tion, using a D8 Advance Brüker diffractometer operating withCu K ( = 1.5406˚A) radiation and a secondary beam graphitemonochromator. Powder samples were scanned over an angular2 range from 10 to 90◦, in steps of 0.02◦(2), at a rate of 2 s perstep.
2.4. Preparation of magnetite octadecylsilane nanoparticles
2.4.1. Synthesis of magnetite (Fe3O4) nanoparticles
The nanoparticles of Fe3O4were synthesized by co-precipitation of Fe2+and Fe3+ions, at alkaline conditions andunder hydrothermal treatment [29,30]. Briefly, 0.7 g of FeCl3and0.42 g of FeCl2·4H2O were dissolved in 100 mL deionized water,degassed continuously with N2to agitate the mixture and prevent
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