Hyphenation of Sorbent Extraction and Solid-Matrix Time-Resolved Luminescence Using Tetracycline in Milk as a Model Analyte

Hyphenation of sorbent extraction and solid-matrix time-resolved luminescence (TRL) was demonstrated using tetracycline (TC) in milk as a model analyte. The performance of a C18-impregnated silica layer was evaluated as both an extraction sorbent and a TRL substrate. To extract TC, a 10 × 6 mm glass-backed C18 layer was dipped into a 10 mL milk sample for 10 min followed by a 3-min water immersion for cleanup. The sorbent was then spotted with a TRL reagent solution at pH 9 that contained 5 mM europium nitrate and 5 mM EDTA. After a brief desiccation period, TRL was measured directly on the sorbent surface with a commercial fluorescence spectrophotometer. By eliminating the need to elute the analyte from the sorbent, organic solvent was not needed and sample preparation was greatly simplified. The integrated signal showed a linear dependence (R² = 0.9938) on TC concentration in the 0−3000 μg/L range. The same protocol was applicable to screening TC in fat-free, 2% low-fat, and whole milk at 300 μg/L, the US. regulatory tolerance level set by the Food and Drug Administration (FDA). This easy, fast, and low-cost screening method is friendly to the environment and particularly suitable for liquid samples.

Keywords: Sorbent; extraction; tetracycline; time-resolved luminescence; milk; screening