Quantification of 3,5-di-tert-butyl-4-hydroxystyrene migrating from organic materials in contact with drinking water: True occurrence or analytical artefact?

The occurrence of 3,5-di‑tert‑butyl‑4-hydroxystyrene (S.I) diffusing from cross-linked polyethylene (PE-X) pipes has been a topic of debate among industries, regulatory authorities, and scientists due to inconsistent findings. This study aimed to clarify the origin of S.I in migration waters and provide analytical recommendations for the certification process of pipe materials. First, it was demonstrated that analysing migration waters by liquid-liquid extraction (LLE) followed by gas chromatography-mass spectrometry (GC/MS), according to EN 15,768 with an injector temperature of 250 °C, significantly biases the measured S.I concentrations, which are due to thermal degradation of 7,9-di‑tert‑butyl‑1-oxaspiro[4,5]deca-6,9-diene-2,8‑dione (S.II). Under these conditions, artificially high concentrations of S.I are obtained. Reducing the injector temperature to 150 °C improves S.I. accuracy, however, a reliable solid-phase microextraction (SPME) method was developed to quantify S.I. independently of S.II. Second, four types of PE-X pipes were evaluated using LLE with injector temperatures of 250 °C and 150 °C, as well as SPME. LLE significantly increases the likelihood of detecting S.I concentrations in the migration waters well above the 0.1 µg/L limit stipulated by the European Commission Implementing Decision. Although the fraction of S.I formed from S.II is small (0.7-2% with LLE), the concentration of S.II itself can reach up to 100 µg/L (legal limit), leading to substantially biased S.I values that exceed 0.1 µg/L. Consequently, materials could fail European certification despite being compliant, which would have significant commercial implications. Therefore, the use of SPME is recommended as an alternative method for analysing S.I in this context.