Identification of Unknowns in Ground and Surface Water by LC/Q-TOF

Importance of the topic

Environmental release of pharmaceuticals from wastewater into surface water and groundwater raises ecological and public health concerns. Reliable detection of both known and unknown drug residues and their transformation products is essential to assess transport, removal efficiency in natural and engineered systems, and potential exposure through drinking water sources.

Study objectives and overview

This work employs high-resolution liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (LC/Q-TOF-MS) and a suite of accurate-mass tools to identify target, suspect, and unknown pharmaceuticals and their degradants in the South Platte River and adjacent alluvial groundwater near Denver, Colorado. The goal was to elucidate removal processes and transport behavior of these contaminants from urban wastewater effluent into riverbank infiltration zones used as drinking water sources.

Methodology and used instrumentation

Water samples were collected from ten alluvial wells and the river over multiple campaigns. A 100 mL aliquot underwent offline solid-phase extraction for 200× concentration. Chromatography was performed on an Agilent 1290 Infinity II LC with ZORBAX Eclipse XDB-C8 column (150 × 4.6 mm, 3.5 µm) at 25 °C using a 30-min gradient from 10 % to 100 % acetonitrile (0.1 % formic acid in water as the aqueous phase). Detection was achieved with an Agilent 6545 LC/Q-TOF with Jet Stream ESI source in positive mode. Data acquisition covered m/z 50–1,000 with Auto MS/MS fragmentation at 15 and 30 eV. Processing and unknown screening were conducted in Agilent MassHunter with Personal Compound Database/Library (PCD/PCDL), molecular structure correlator, diagnostic ion filters, Kendrick mass defect, and biochemical pathway tools.

Main results and discussion

• Twenty-two pharmaceuticals across beta-blockers, antibiotics, antidepressants, caffeine, and sucralose were confirmed in surface water; eight persisted in alluvial groundwater 100–500 m from the river.
• Neutral tracers gabapentin and sucralose exhibited minimal attenuation during aquifer transport, indicating limited removal for these compounds.
• Charged drugs such as atenolol and diphenhydramine achieved 90–99 % removal, likely via cation exchange and hydrophobic sorption to aquifer sediments.
• Mixed-mode removal (≈60 %) was observed for venlafaxine and its desmethyl metabolite, reflecting partial sorption under groundwater pH conditions and raising concerns about N-nitrosodimethylamine formation during chloramination.
• Persistent compounds carbamazepine, lamotrigine, and sulfamethoxazole showed variable groundwater concentrations tied to seasonal wastewater inputs, highlighting their resilience and transport potential.

The integrated workflow of suspect screening, iterative unknown identification, diagnostic fragment ions, and structure correlation proved effective for comprehensive pharmaceutical profiling in environmental waters.

Benefits and practical applications

• Low-ng/L detection capabilities support risk assessment for riverbank filtration as a drinking water source.
• Flexible workflows enable rapid inclusion of emerging contaminants by updating accurate-mass libraries and fragmentation databases.
• Insights into natural attenuation and removal mechanisms can guide water treatment design, monitoring strategies, and regulatory decision-making.

Future trends and opportunities

Continued enhancements in mass spectrometry resolving power, sensitivity, and data-processing algorithms will drive more comprehensive non-targeted screening. Expansion of suspect databases through cheminformatics and improved MS/MS spectral libraries will facilitate discovery of novel transformation products. Integration of advanced analytical data with environmental fate modeling and bioassays offers a holistic framework for evaluating contaminant impact and developing targeted remediation solutions in aquifer systems.

Conclusion

Applying LC/Q-TOF-MS alongside accurate-mass data tools provides a robust platform for detecting and identifying both known and unknown pharmaceuticals in surface and groundwater. The study demonstrates diverse transport behaviors linked to compound chemistry and aquifer interactions. Ongoing refinement of analytical methods and expansion of reference resources will enhance monitoring and management of emerging water contaminants.

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