Stir bar sorptive extraction combined with high performance liquid chromatography-ultraviolet/inductively coupled plasma mass spectrometry for analysis of thyroxine in urine samples

• A new composite coating was prepared for SBSE of thyroxines with high extraction efficiency.
• A method of PEG/OH-PDMS/γ-MPTS SBSE-HPLC-UV/ICP-MS was developed for human urine analysis.
• The method provided low LODs, high extraction efficiency and wide linear range.

In this work, polyethyleneglycol (PEG)/hydroxyl polydimethylsiloxane (OH-PDMS)/γ-mercaptopropyl trimethoxysilane (γ-MPTS) coated stir bar was prepared by sol–gel process and its extraction performance for the extraction of amphoteric thyroxines (3,3′,5,5′-tetraiodothyronin, T4; 3,3′,5-triiodothyronine, T3; reversed-3,3′,5-triiodothyronine, rT3) and their metabolite (3,5-diiodothyronine, T2) was studied. The preparation reproducibility of PEG/OH-PDMS/γ-MPTS coated stir bar was investigated, and the relative standard deviations (RSDs) in the same batch and among different batches were 3.3–14.3% (n = 5) and 7.7–16.6% (n = 3), respectively. The prepared PEG/OH-PDMS/γ-MPTS coated stir bar could be reused for more than 20 times. Based on this fact, a novel method of stir bar sorptive extraction (SBSE) combined with high performance liquid chromatography (HPLC)-ultraviolet (UV) and HPLC-inductively coupled plasma mass spectrometry (ICP-MS) for the analysis of target thyroxines in human urine samples was developed. The influencing factors of SBSE, such as sample pH, extraction time, stirring rate, salt effect, desorption solution and desorption time, were studied in detail, and the analytical performance of the proposed method was evaluated under the optimized conditions. The enrichment factors (EFs) of the developed method for four target thyroxines were in the range of 14.9–70.4 (theoretical enrichment factor was 100). The RSDs were ranging from 4.0% to 13.8% for SBSE-HPLC-UV (c = 25 μg/L, n = 6) and from 3.7% to 6.1% for SBSE-HPLC-ICP-MS (c = 0.5 μg/L, n = 5). The linear range obtained by SBSE-HPLC-UV was 2–500 μg/L for T2 and 5–500 μg/L for rT3, T3 and T4, with correlation coefficients (r) ranging from 0.9957 to 0.9998, respectively, while the linear range obtained by SBSE-HPLC-ICP-MS was 0.05–500 μg/L for T2 and rT3, 0.10–200 μg/L for T3 and 0.05–200 μg/L for T4 with r ranging from 0.9979 to 0.9998, respectively. The limits of detection (LODs) for the target thyroxines were 0.60–2.20 μg/L for SBSE-HPLC-UV and 0.0071–0.0355 μg/L SBSE-HPLC-ICP-MS, respectively. The developed method was applied for the determination of target thyroxines in urine samples, and the recovery for the spiking samples obtained by SBSE-HPLC-UV was in the range of 81.6–137.6% for human urine, while the recovery for the spiking urine samples obtained by SBSE-HPLC-ICP-MS were in the range of 72.0–121.5%.