In-Sample Chemical Tracers for Quality Control in Environmental Samples: A Systematic Evaluation of Deuterated Pharmaceuticals for Sampling Campaigns

Understanding the integrity of water samples between collection and analysis is essential for the delivery of high-quality data. In this study, we evaluated the spiking of isotopically labeled chemicals into environmental samples postsampling as indicators of sample integrity. Our approach compared the ratio of unstable tracers (those sensitive to poor storage conditions or the matrix) to stable tracers (those insensitive to these factors) in samples to track sample integrity. We systematically evaluated 18 deuterium-labeled pharmaceuticals for this purpose in wastewater samples, classifying them into varying stability levels from highly stable to very unstable. Stability was determined under varying temperatures (37 °C, 20 °C, 4 °C, and -20 °C) for up to 84 days and using common preservation techniques (hydrochloric acid: HCl, sodium metabisulfite: SMS, and untreated). Based on our proposed selection criteria, of the 18 investigated, one was found suitable as an unstable tracer and 6 as stable tracers in acidified wastewater, while 3 unstable and 2 stable tracers were suitable for SMS-preserved wastewater and 4 unstable and 2 stable tracers when no preservatives are required. This study provides the first assessment of chemical tracers to measure the sample integrity for environmental sampling campaigns.