A Total Solution for the Quantitative Analysis of Benzocaine Metabolites in Fish Tissue

A Total Solution for the Quantitative Analysis of Benzocaine Metabolites in Fish Tissue — Expert Summary

Importance of the topic

Monitoring residues of veterinary anesthetics and their metabolites in aquaculture products is critical for food safety, consumer protection, and regulatory compliance. Benzocaine metabolites can persist in edible fish even when the parent drug is absent, and lipid-rich fish matrices complicate trace-level analysis. Robust, sensitive, and high-throughput analytical workflows are therefore required to support routine surveillance and market control programs.

Objectives and overview of the study

This application note presents a complete, validated LC/MS/MS workflow for quantitative determination of three benzocaine metabolites—4-aminobenzoic acid (PABA), 4-acetamidobenzoic acid (AcPABA), and N-acetylbenzocaine (AcBZ)—in fish tissue. The method integrates a streamlined QuEChERS extraction, Captiva EMR–Lipid passthrough cleanup, reversed-phase LC separation, and sensitive triple-quadrupole MS detection. Validation is demonstrated across three fish species with varying fat content (tilapia, seabass, Norwegian salmon) to ensure broad applicability.

Methodology

Sample preparation and cleanup

• Homogenize 5 g fish tissue and perform a QuEChERS-style extraction using water and acetonitrile with EN extraction salts.
• Centrifuge and dilute extract prior to passthrough cleanup with Captiva EMR–Lipid HF cartridges to remove lipids and hydrophobic interferents.
• The workflow avoids solvent evaporation and reconstitution steps, reducing hands-on time to ~25–30 minutes per batch of 10 samples and decreasing potential analyte loss or variability.

Chromatography and detection

• Column: Altura ZORBAX Eclipse Plus C18, 2.1 × 100 mm, 1.8 µm.
• Mobile phases: 0.05% formic acid in water (A) and acetonitrile (B); gradient elution with 0.25 mL/min flow; 2 µL injection volume.
• Detection: Agilent 6495D triple quadrupole MS with Jet Stream heated ESI in positive-ion dynamic MRM mode. Data processing with MassHunter software.

Calibration and QC strategy

• Standards prepared in 80% ACN/water; nine-point calibration from 0.1 to 100 µg/L.
• Matrix-spiked QC levels at 1, 5, and 10 µg/kg (corresponding to ~0.4, 2, and 4 µg/L in extract). Two technical preparations per QC level with two injections each (n = 4) were used to assess repeatability and recovery.

Instrumentation used

• Agilent 1290 Infinity III LC system.
• Altura ZORBAX Eclipse Plus C18 column (2.1 × 100 mm, 1.8 µm).
• Agilent 6495D Triple Quadrupole LC/MS with Jet Stream ESI source (positive polarity, dynamic MRM).
• Captiva EMR–Lipid HF cleanup cartridges and Bond Elut QuEChERS extraction consumables.

Main results and discussion

Analytical performance

• Linearity: Excellent calibration linearity across three orders of magnitude (0.1–100 µg/L) with R2 > 0.999 for all analytes.
• Sensitivity: Instrument detection limits (IDLs) and method detection limits (MDLs) in the low parts-per-trillion range (IDLs down to 0.004 µg/L; MDLs 0.01–0.02 µg/L), translating to tissue-equivalent MDLs of ~0.02–0.05 µg/kg.
• Precision: Response RSDs ≤ 2.7% at the lowest calibration level; %RSD of recoveries generally < 3% across QC levels and matrices (one exception: PABA at the lowest spike in salmon showed 7% RSD, still within CODEX criteria).
• Accuracy: Matrix-spiked recoveries between 80 and 120% for all analytes across all three fish matrices and QC levels.
• Matrix effects: ME% values fell within 80–120% (reported 93–110%), indicating minimal ion suppression/enhancement attributable to effective EMR lipid cleanup.

Chromatography and peak shape

• The Altura Ultra Inert surface reduced secondary metal-ion interactions, providing symmetric peaks for polar analytes such as PABA and minimizing peak tailing. All targets were baseline separated with sharp, reproducible peaks within a 10‑minute run time.

Practical workflow advantages

• The combined QuEChERS + Captiva EMR approach effectively addresses lipid interferences in high-fat fish matrices, enabling direct LC/MS/MS analysis without derivatization (unlike some GC/MS strategies for polar metabolites).
• Faster sample preparation and elimination of evaporation/reconstitution reduce labor and sample handling variability, supporting higher throughput in routine testing laboratories.

Benefits and practical applications

• Enables routine surveillance and regulatory testing for benzocaine metabolites in aquaculture products with high sensitivity and robust quantitation.
• Adaptable across a range of fish fat contents, allowing a single validated workflow for diverse sample types.
• The production-ready combination of commercially available consumables and instrumentation facilitates transfer to QA/QC and contract testing laboratories.

Future trends and potential developments

• Broader analyte panels: Expanding the method to include other veterinary drugs and metabolites for multi-residue monitoring using the same extraction/cleanup approach.
• Isotope-labeled internal standards: Routine inclusion would further improve quantitation robustness across complex matrices.
• Automation and throughput: Integration with robotic sample preparation or 96-well formats to increase laboratory throughput.
• High-resolution MS complementarity: HRMS could be used for non-target screening and retrospective data mining while triple quadrupole remains the primary tool for routine quantitation.
• Regulatory harmonization: Adoption of validated, standardized protocols across jurisdictions to support consistent MRL enforcement.

Conclusion

The described workflow combining QuEChERS extraction, Captiva EMR–Lipid cleanup, Altura ZORBAX Eclipse Plus C18 LC separation, and Agilent 6495D triple-quadrupole detection provides a sensitive, accurate, and efficient solution for quantifying benzocaine metabolites in fish tissue. The method meets stringent performance criteria (ppt-level MDLs, R2 > 0.999, recoveries 80–120%, and low %RSD), is validated across low- to high-fat fish, and is well suited for routine regulatory and surveillance laboratories.

References

1. Priborsky J.; Velisek J. A Review of Three Commonly Used Fish Anesthetics. Reviews in Fisheries Science & Aquaculture 2018, 26(4), 417–442.
2. Lin S.; Qiu W.; Hua Y.; Yang Y. Rapid Determination of Caine-Based Anesthetics and Their Metabolite Residues in Fish Using a Modified QuEChERS Method Coupled with UPLC-MS/MS. Food Chemistry: X 2024, 24, 102032.