Quantification of phthalate and DINCH metabolites in human urine and maternal breast milk: Assessing maternal body burden and infant exposure

Based on toxicological evidence, human exposure to phthalates (PHs) and diisononylcyclohexane-1,2-dicarboxylate (DINCH) may contribute to adverse health effects, especially during vulnerable developmental stages. To support the exposure assessment for this group of endocrine disruptors, we developed and validated a method for the analysis of 14 PH and 3 DINCH metabolites in human urine and maternal milk, applied the method in a pilot study, and identified crucial obstacles in the path of establishing maternal milk as a routine matrix in human biomonitoring. Urine and milk samples were extracted with solid-phase extraction (SPE) and QuEChERS salts, respectively, and analysed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). The method accuracy was confirmed for urine samples via a certified standard reference material and the G-EQUAS intercomparison programme. We identified a need for sampling protocols, reference materials, and external method verification schemes in order to establish maternal milk as a routine matrix. Finally, the method was tested for its applicability in a pilot biomonitoring study on 30 paired urine and milk samples from lactating mothers, with medians ranging from <LLOQ - 15 µg/L in urine and <LLOQ - 16 µg/L in maternal milk and generally higher detection rates in urine. Furthermore, the results indicate extensive monoester formation under cooled storage conditions, resulting in potentially high infantile exposure to phthalate monoesters for which, to date, no guidance values exist despite their demonstrated toxicity.