Cucurbit[8]uril-Based Potentiometric Sensor Coupled to HPLC for Determination of Tetracycline Residues in Milk Samples

The determination of chlortetracycline, doxycycline, oxytetracycline, and tetracycline in milk samples by HPLC coupled to a cucurbit[8]uril-based potentiometric sensor is herein presented. The new tetracycline-selective electrode is based on a polymeric membrane incorporating cucurbit[8]uril as a macrocyclic host, potassium tetrakis(p-chlorophenyl) borate as an ionic additive, 2-fluorophenyl 2-nitrophenyl ether as a plasticizer, and multi-walled carbon nanotubes as nanostructured materials. A microfluidic wall-jet flow-cell is implemented as a potentiometric detector after chromatographic separation by a C8 column using a gradient mobile phase of sulphuric acid and acetonitrile. The proposed methodology was validated following International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) and European Union (EU) guidelines. Linear regression models provided R2 in the range from 0.9973 ± 0.0026 to 0.9987 ± 0.0012 for all tetracycline antibiotics. The limits of detection and quantification ranged from 13.3 to 46.0 μg L−1 and 44.4 to 92.1 μg L−1, respectively. Precision intra-day, inter-day, and inter-electrode showed relative standard deviation values lower than 12.5%, 13.5%, and 12.9%, respectively. Accuracy was assessed by analysis of spiked milk samples around the maximum residue limit, yielding recovery values in the range from 81.3 to 108.5%. The simple, sensitive, cost-effective, and reliable HPLC-ion-selective electrode method justifies its use as a competitive alternative for the analysis of tetracycline residues in the food quality control sector.