Non-Target Analysis of Runoff Waters from Urban Fires Using LC/QTOF-MS“Finding Nylon Degradation Products in Drinking-Water Sources”

Significance of the topic

Runoff from urban fires can introduce a complex mixture of combustion products and degraded materials into surface waters that serve as drinking-water sources. Analyses that extend beyond targeted screens are essential to detect unexpected contaminants and to identify chemical tracers of fire-related pollution. This study demonstrates how high‑resolution LC/QTOF-MS non‑target screening in both negative and positive electrospray modes can reveal both previously reported wildfire markers (benzene polycarboxylic acids and benzene sulfonic acids) and novel polymer degradation products derived from commonly used materials such as Nylon 6. Identifying such markers supports source attribution, retrospective surveillance, and informs water‑quality risk assessment and remediation strategies.

Objectives and study overview

• Perform non‑target LC/QTOF-MS analysis of runoff waters from two urban wildland‑urban interface (WUI) fires (Marshall Fire 2021 and Nederland Fire 2025).
• Compare negative and positive ESI data to detect both known wildfire tracers and previously unreported fire‑derived compounds.
• Characterize and identify unknowns, with emphasis on compounds that differentiate downstream (impacted) from upstream (reference) sites and that may reach drinking‑water reservoirs.

Experimental methodology

• Sampling: Water samples (100 mL) were collected from multiple locations during the Nederland Fire event (contaminated stream, impacted Barker Reservoir used for drinking water, and an upstream uncontaminated site). Marshall Fire samples were collected three days after extinguishment. The Nederland event also involved firefighting with ~4 million liters of tap water that drained into a reservoir supplying Boulder.
• Sample preparation: Concentration by solid phase extraction (SPE) prior to instrumental analysis.
• Chromatography and MS: Reversed‑phase LC coupled to a high‑resolution quadrupole time‑of‑flight mass spectrometer (Agilent 6546 Q‑TOF) with a 1290 Infinity UHPLC. Column: Zorbax Eclipse XDB‑C8. Mobile phases described as 0.1% formic acid in water and acetonitrile; chromatographic program held initial composition (10% A) for 5 min followed by a linear gradient to 100% A over 30 min as reported. Data were acquired in both auto MS/MS (data dependent) and data independent acquisition modes.
• Data processing: MassHunter Explorer 2.0 and MassHunter Qual/Quant workflows were used for automated suspect screening, non‑target feature discovery, and MS/MS interpretation. Mass accuracy better than 2 ppm and resolving power >50,000 enabled confident formula assignment and retrospective data mining.

Used instrumentation

• LC: Agilent 1290 Infinity UHPLC.
• Column: Zorbax Eclipse XDB‑C8.
• Mass spectrometer: Agilent 6546 LC/QTOF‑MS (high resolution, <2 ppm mass error, >50,000 resolving power).
• Software: Agilent MassHunter Explorer 2.0, MassHunter Qual, MassHunter Quant; ChemVista libraries and suspect lists from prior wildfire studies.

Main results and discussion

• Negative ESI results: The study confirmed a suite of benzene polycarboxylic acids (BPCAs), including chlorinated and sulfonated analogues (benzene sulfonic acids, BSAs), previously associated with biomass burning and ash leachate. Automated suspect screening and library matching yielded ~30 BPCA-related compounds and several emerging contaminants (pesticides, pharmaceuticals) detected at low ng/L concentrations.
• Positive ESI results and discovery of Nylon degradation products: Positive‑ion non‑target analysis uncovered a novel homologous series not present in standard databases. A recurring diagnostic ion at m/z 114.0913 (putatively a caproic amide formula) appeared across multiple chromatographic peaks. Major observed ions at nominal m/z 114, 227, 340, 453, and 566 were spaced by ~113 Da, indicating a repeating unit consistent with caprolactam (C6H11NO) minus hydrogen (caprolactam-derived unit ~113.084 Da), the monomer used in Nylon 6 polymer formation.
• MS/MS confirmation: Fragmentation of a prominent ion (m/z 453.3454) produced key fragment ions at m/z 114.0916 and 227.1763, corresponding to single and double caprolactam units, supporting the interpretation that these features are low‑molecular‑weight degradation oligomers of Nylon 6 (1–5 repeat units observed chromatographically). The homologous pattern and even‑electron mass behavior (N rule) plus mass defect trends reinforced the polymeric assignment.
• Spatial distribution: Non‑target feature analysis (e.g., volcano plots and Venn comparisons) showed these nylon‑derived oligomers and other unique features predominantly in downstream samples impacted by fire runoff, with minimal presence upstream, indicating a fire and urban source linkage.
• Concentration context: BPCAs and BSAs were observed in the ng/L range (log10 presentation in the original data), consistent with trace‑level contamination but detectable by HRMS methods used here.

Contributions and practical applications

• Source tracing: The identification of Nylon 6 degradation products provides a novel class of chemical tracers for urban fire impact, complementing known wildfire markers (BPCAs/BSAs) and enabling more specific attribution to urban structural fires where synthetic materials are abundant.
• Drinking‑water relevance: Detection of these products in water bodies directly connected to drinking‑water reservoirs highlights the potential for urban fire runoff to introduce synthetic polymer degradation products into potable water sources, warranting monitoring and assessment.
• Analytical practice: The work demonstrates the value of acquiring both negative and positive ESI data in comprehensive environmental screening and underscores the importance of high mass accuracy (<2 ppm) and high resolving power for robust unknown identification and retrospective analysis.

Future trends and possibilities for application

• Expanded monitoring: Integrate targeted assays for nylon degradation oligomers and BPCA/BSA suites into routine post‑fire water monitoring programs, particularly for reservoirs and supply streams downstream of urban fires.
• Degradation pathway studies: Investigate formation mechanisms of nylon oligomers during combustion, thermal degradation, and post‑fire leaching to understand persistence, solubility, and transformation products under environmental conditions.
• Toxicology and risk assessment: Evaluate the toxicity and human‑health relevance of detected nylon oligomers at environmentally realistic concentrations to inform water treatment needs and advisory guidance.
• Method development: Refine SPE workflows, chromatographic separation, and targeted MS/MS transitions for greater sensitivity and quantitation of polymer degradation products; leverage retrospective HRMS data mining to screen historical samples.
• Broader polymer screening: Extend non‑target approaches to detect degradation products of other common synthetic polymers (e.g., PET, polyamides other than Nylon 6, polyurethane) released in urban fires.

Conclusions

• Combining negative and positive ESI LC/QTOF‑MS non‑target workflows revealed both expected wildfire markers (BPCAs and BSAs) and previously unreported nylon degradation oligomers in urban fire runoff waters.
• Homologous ions differing by ~113 Da and diagnostic fragments in MS/MS support the identification of Nylon 6 degradation products (caprolactam‑derived oligomers) that were primarily observed downstream of fire‑impacted sites, including a drinking‑water reservoir.
• High mass accuracy and advanced software tools are critical for confident unknown identification and retrospective data mining; follow‑up work should address quantitation, environmental fate, and toxicological implications.

References

1. Ferrer I, et al. Wildfires: Identification of a New Suite of Aromatic Polycarboxylic Acids in Ash and Surface Water. Sci Total Environ. 2021;770.
2. Ferrer I, Thurman EM. Chemical Tracers for Wildfires–Analysis of Runoff Surface Water by LC/Q‑TOF‑MS. Chemosphere. 2023;339.
3. Thurman EM, et al. Occurrence of Benzene Polycarboxylic Acids in Ash and Streamwater after the Cameron Peak Fire. ACS EST Water. 2023;3(12).