Analysis of 6PPD-Q in Reagent and River Water using LCMS-8060RX

Significance of the topic

6PPD-quinone (6PPD-Q) is a transformation product of the ubiquitous tire antioxidant 6PPD and has been implicated in acute toxicity to aquatic organisms following urban runoff events. Robust, sensitive analytical methods for quantifying 6PPD-Q in aqueous matrices are therefore critical for environmental monitoring, regulatory compliance, and understanding exposure risks to freshwater ecosystems.

Objectives and overview of the study

This work describes development and validation of a rapid liquid chromatography–tandem mass spectrometry (LC–MS/MS) method using a Shimadzu LCMS-8060RX coupled with a Nexera X3 UHPLC for quantitation of 6PPD-Q in reagent and river water. The primary aims were to achieve detection limits, accuracy, and reproducibility compatible with or exceeding requirements in the U.S. EPA draft Method 1634, enabling direct-injection workflows with minimal sample concentration.

Methodology

Standards and internal standards were prepared in a 1:1 water:acetonitrile matrix following EPA 1634 guidance. Isotopically labeled standards (13C6-6PPD-Q as the electronic internal standard and D5-6PPD-Q as the non-isotopic internal standard) were used for quantitation and to correct for matrix effects. Surface water from an East Coast river and reagent water were used as matrices for method performance evaluation. Samples were spiked with the labeled standards, centrifuged, and transferred to autosampler vials prior to direct LC–MS/MS analysis.

Used instrumentation

• UHPLC: Nexera X3 (Shimadzu)
• Mass spectrometer: LCMS-8060RX triple-quadrupole (Shimadzu) with electrospray ionization (ESI)
• Analytical column: Restek Force C18, 1.8 µm, 50 × 2.1 mm
• Mobile phases: 0.1% formic acid in water (A) and 0.1% formic acid in acetonitrile (B)
• Typical operating conditions summarized: fast gradient to high organic for short cycle times, interface and gas temperatures and nebulizing/heating/drying gas flows optimized for sensitivity

Analytical details

The method employed multiple reaction monitoring (MRM) transitions selected for 6PPD-Q and the labeled standards. Instrument voltages and collision energies were optimized for each transition. A small injection volume and a rapid gradient allowed short analysis times and supported high-throughput direct-injection workflows.

Main results and discussion

• Linearity and calibration: The calibration for 6PPD-Q was linear across 0.002–10 ng/mL with excellent fit (R² > 0.99) and low relative standard error.
• Sensitivity: Method detection limit (MDL) in river water was 1.1 pg/mL, demonstrating sensitivity more than an order of magnitude better than the LOQ requirements in EPA draft Method 1634.
• Accuracy and precision: Percent accuracy across calibration levels ranged broadly (reported 83–118%), with %RSD generally <15% for calibrants.
• Recoveries and matrix effects: Reagent water recoveries ranged approximately 99–133% depending on concentration, while river water recoveries were higher and more variable (approximately 104–148% at certain levels), indicating matrix-driven ionization differences and potential positive bias in some spiked river-water measurements.
• Blanks and interferences: Unspiked reagent and river water samples showed no detectable 6PPD-Q above the method LOQ in the study, supporting method selectivity for the target analyte under the applied conditions.

Benefits and practical applications of the method

• High sensitivity allows detection of 6PPD-Q at sub-picogram per milliliter levels, enabling early detection in environmental monitoring programs.
• Direct-injection capability reduces sample processing time and the need for extensive concentration or cleanup, increasing throughput for routine monitoring.
• Use of isotopically labeled internal standards improves quantitation reliability in complex matrices by compensating for matrix effects and instrumental variability.
• Compatibility with EPA draft Method 1634 objectives supports regulatory and research applications focused on aquatic toxicity risk assessment.

Limitations and considerations

• Elevated and variable recoveries in river water suggest matrix effects that can bias results if not properly controlled; continued use of isotopic standards and matrix-matched QC is essential.
• Direct injection is efficient but may be more susceptible to matrix-related ion suppression/enhancement in heavily contaminated waters; selective sample cleanup or dilution may be required for some matrices.
• Method performance should be verified locally (MDLs, recoveries, precision) when implemented on other instruments or different water bodies.

Future trends and potential applications

• Wider adoption of sensitive direct-injection LC–MS/MS methods for routine monitoring of 6PPD-Q in urban and stormwater monitoring networks.
• Integration with automated sampling and online preconcentration systems to expand temporal resolution while retaining sensitivity.
• Application of high-resolution mass spectrometry and non-target screening to discover related tire-derived transformation products and improve understanding of environmental fate.
• Refinement of sample preparation approaches (e.g., optimized SPE, matrix-matched calibration) to reduce matrix bias and improve quantitative accuracy in diverse surface waters.
• Use of harmonized methods in regulatory frameworks to support risk assessment and mitigation strategies for aquatic ecosystems.

Conclusion

The described LC–MS/MS method on the Shimadzu LCMS-8060RX coupled with Nexera X3 UHPLC provides excellent sensitivity, linearity, and throughput for quantifying 6PPD-Q in aqueous matrices. Achieving an MDL of ~1.1 pg/mL in river water and reliable calibration down to 0.002 ng/mL, the approach supports direct-injection workflows that can accelerate environmental monitoring while meeting or exceeding draft regulatory sensitivity goals. Attention to matrix effects and rigorous QA/QC remain important for accurate environmental measurements.

References

1. U.S. Environmental Protection Agency. Draft Method 1634: Determination of 6PPD-quinone in aqueous matrices using liquid chromatography with tandem mass spectrometry (LC–MS/MS). December 2023.
2. Tian Z, Zhao H, Peter KT, et al. A ubiquitous tire rubber-derived chemical induces acute mortality in coho salmon. Science. 2021;371(6525):185–189. doi:10.1126/science.abd6951.