Development and optimization of an ultra-fast microextraction followed by HPLC-UV of tetracycline residues in milk products.

The present study developed a new ultra-fast microextraction (within 8 min) with acetonitrile followed by gradient reversed-phase HPLC-UV method to determinate six tetracyclines simultaneously in various milk products with HPLC analysis time of 9 min. Chromatographic separations were achieved with a phenomenex™ Synergi 4 µm Fusion-RP (150 × 4.60 mm) column at 220 nm, using acetonitrile/KHPO buffer (4 mmol·L, pH 2.5) as mobile phase. The HPLC method showed excellent peak resolutions (3.3-8.8) and peak symmetry (0.99-1.11) inspite of short analysis time. The extraction rates from milk products showed consistently very good values over all tetracyclines (milk 80.58 ± 5.39 %, yoghurt 82.61 ± 5.63 %, cream cheese 80.13 ± 6.32 %, buttermilk 81.07 ± 6.49 %, kefir 79.69 ± 6.51 %, skyr 78.12 ± 5.22 %, quark 65.37 ± 4.72 %). The optimized method was found to be specific, reproducible, robust. This study combines for the first time a fast, cheap quantification of six tetracyclines via HPLC-UV with a reliable microextraction applicable to various milk products using only standard laboratory equipment.