Non-Target and Post-Target Analysis of Organic Environmental Contaminants in Suspended River Sediments

Importance of the topic

Industrial and urban activities release persistent organic pollutants (POPs) into river systems, where they accumulate in suspended sediments. Monitoring these contaminants in the Niagara River—Lake Ontario’s main inflow—is vital to assess ecological risk and inform remediation efforts. Comprehensive profiling of known and unknown organic pollutants supports water quality management and regulatory compliance.

Objectives and study overview

The study aimed to extend routine monitoring of 21 polycyclic aromatic hydrocarbons (PAHs) and selected chlorinated compounds by (1) screening for additional POP classes not covered in standard protocols and (2) eliminating laborious wet chemistry clean-up steps through direct thermal desorption coupled to advanced chromatographic analysis. Samples were collected upstream at Fort Erie and downstream at Niagara-on-the-Lake in 2009.

Methodology and instrumentation

• Sample preparation: Approximately 20 mg of homogenized suspended sediment was thermally desorbed from 50 °C to 300 °C (100 °C/min) using a CDS Analytical Pyroprobe 5200; analytes were held at 35 °C for 5 min before ramping to 320 °C (5 °C/min).
• GC×GC-TOFMS: A dual‐column setup (Restek Rxi-5MS and Rxi-17SilMS) with a 3 s modulation period; Pegasus 4D acquired mass spectra at 200 spectra/s over 45–1000 m/z.
• High-Resolution GC-TOFMS (GC-HRT): Single Restek Rxi-5MS column; Pegasus GC-HRT delivered 25 K mass resolution at FWHM (33–650 m/z, 5 spectra/s).

Main results and discussion

• GC×GC provided superior chromatographic resolution, separating coeluting linear and branched alkanes, alkenes, PAHs and other heteroatom‐containing compounds.
• Over 120 PAHs and alkyl homologs were detected, exceeding the routinely monitored suite by more than 100 additional compounds.
• Mass defect plots based on accurate mass measurements (<1 ppm) enabled rapid classification of homologous series and tentative structure proposals for unknowns.

Benefits and practical applications

This integrated thermal desorption–GC×GC/GC-HRT workflow streamlines analysis by removing the need for solvent-based extraction and cleanup. It dramatically increases the number of compounds identifiable in sediment matrices, supports more comprehensive environmental surveillance, and enhances the ability to discover emerging contaminants.

Future trends and opportunities

• Expansion of high-throughput thermal desorption techniques to other solid and semi-solid environmental samples.
• Integration of machine learning algorithms with mass defect and retention modeling to improve unknown identification confidence.
• Development of real-time, in-field screening tools based on high-resolution ion mobility or ambient MS coupled with GC×GC for rapid water quality assessment.

Conclusion

Thermal desorption coupled to GC×GC-TOFMS and GC-HRT offers a powerful platform for non-target and post-target analysis of organic contaminants in suspended sediments. The approach enhances chromatographic separation, expands analyte coverage far beyond routine target lists, and harnesses accurate mass data for confident unknown identification, all while simplifying sample preparation.