Determination of selected gadolinium-based contrast agents in soil: method validation and application

The increasing use of reclaimed water for agricultural irrigation raises concerns about the environmental accumulation of persistent contaminants, including gadolinium-based contrast agents (GBCAs). These compounds, widely used in medical imaging, are excreted unmetabolized and poorly removed by conventional wastewater treatment plants, leading to their release into aquatic environments and potential accumulation in soils and crops. In this study, a method was developed, validated, and applied-for the first time-for the determination of three commonly used GBCAs-gadoteric acid, gadoteridol, and gadobutrol-in clay soil. Quantification was achieved by ion chromatography coupled to inductively coupled plasma mass spectrometry (IC-ICP-MS). Suitable separation was accurately optimized using a mobile phase containing 30 mM EDTA and 20 mM NH₄NO₃ at pH 9.8. Elution of all target species was achieved within 7.5 min. The method was evaluated using real clay soil samples from an agricultural field in Castelló (Spain) fortified with selected GBCAs at various concentrations. Mild extraction with NH₄NO₃ yielded recoveries below 50%, which allows assessment of the bioavailable fraction. To achieve quantitative recovery, a more severe alkaline extraction protocol using KOH was subsequently developed. The optimized procedure demonstrated excellent analytical performance, with LOD and LOQ below 2 and 5 ng g^-1, respectively. This methodology enabled the first quantitative determination of GBCAs in soil samples collected from municipal parks irrigated with reclaimed water. Furthermore, the KOH-based extraction was extended to the quantification of gadoteric acid in peat substrates from greenhouse-scale cultivation trials, yielding a mean recovery of 80% with a relative standard deviation (RSD) of 12%. The application of this method confirmed the accumulation of the contrast agent in peat irrigated with fortified tap water. These findings underscore the necessity for robust, matrix-specific analytical workflows to monitor the environmental fate of GBCAs and to accurately assess their potential transfer into the food chain.