Method development and validation for low-level propineb and propylenethiourea analysis in baby food, infant formula and related matrices using liquid chromatography-tandem mass spectrometry

Two simple, selective and rugged liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods were developed and validated for determination of propineb and propylenethiourea (PTU) in infant formula, fruit-based and cereal-based baby food and raw materials used in production of infant formula, including carbohydrates, protein isolates, vegetable oils and emulsifiers. The sample preparation procedure for propineb analysis was based on streamlined derivatisation to form and stabilise the target analyte (propylenebisdithiocarbamate-dimethyl), followed by extraction using a modified QuEChERS procedure with a dispersive solid phase extraction (d-SPE). The PTU determination employed an aqueous extraction with optimised protein precipitation and single-step SPE clean-up. To achieve maximum sensitivity, electrospray ionisation and atmospheric-pressure chemical ionisation were employed for LC-MS/MS analysis of propineb and PTU, respectively. Validation of the developed methods was performed in accordance with Document SANTE/11813/2017. Mean recoveries were in the range of 86–120% for propineb and PTU, respectively, with interday and intraday repeatabilities below 13%. A limit of quantification (LOQ) of 0.003 mg kg^–1 was validated for most of the evaluated analyte/sample matrix combinations with the exception of PTU in soy protein isolate and soybean oil, for which an LOQ of 0.01 mg kg^–1 was obtained. This is the first report that provides validated methods for monitoring propineb and PTU in infant formula and baby foods at concentrations compliant with the maximum residue levels established in the EU legislation.