Sensitive magnetic dispersive solid-phase extraction using hydrophobic magnetic nanoparticles and GC-MS analysis for the determination of sterol composition in milk samples for the detection of palm oil
In this study, a simple and reliable method based on magnetic dispersive solid phase extraction (MDSPE) with modified magnetic nanoparticles followed by gas chromatography-mass spectrometry (GC-MS) has been developed for the determination of sterol composition in liquid milk samples and the detection of palm oil in milk fat. Hydrophobic magnetic nanoparticles were synthesised by assembling octadecyltrimethoxysilane (ODTMS) onto iron magnetic nanoparticles and used as an adsorbent for MDSPE. The method is based on the saponification of milk fats followed by extraction of unsaponifiable sterols by the MDSPE procedure and subsequent desorption of sterols into acetonitrile for GC-MASS analysis. The mass fraction of each sterol, as a percentage of the total sterol content, was used for the quantitative determination of sterols in milk samples. The main factors affecting the extraction efficiency, including extraction and desorption time, desorption conditions, sample volume, amount of magnetic adsorbent and saponification conditions, were investigated. The results of this study showed that addition of palm oil to milk caused substantial changes in sterol contents and the fractions of phytosterols (β-sitosterol, stigmasterol and campesterol) increased with increasing the addition level of palm oil. Based on the evidence for the presence of vegetable sterols in pure milk fat, a limit of detection of 3% for palm oil was obtained in milk fat. Under the optimum conditions, the inter-day precision and intra-day precision for sterols in a sample containing 3% fat (10% of the fat containing palm oil) were in the range of 1.1–1.8% and 1.3–1.9%, respectively. Also, the good recovery of the total phytosterol (97–97.2%) showed that the proposed method can be successfully applied to the determination of phytosterols in milk samples for the detection of palm oil addition.