LC-MS/MS Analysis of Herbicides in Drinking Water at Femtogram Levels Using 20 mL EQuan Direct Injection Techniques

Importance of the Topic

Monitoring herbicide residues in drinking water at extremely low levels is essential due to increasing concerns about environmental toxicity and human health impacts. Traditional offline concentration methods can be costly and time‐consuming, making online preconcentration techniques an attractive alternative for routine environmental and food safety laboratories.

Objectives and Study Overview

The aim of this work was to evaluate the performance of large‐volume direct injections (1, 5 and 20 mL) using an online preconcentration column coupled to an analytical column in a Thermo Scientific EQuan LC-MS/MS system. Key goals included assessing sensitivity improvements, reproducibility and throughput without offline sample preparation.

Methodology and Instrumentation

Sample Preparation

• Drinking water acidified with 0.1% formic acid was spiked with 12 triazine herbicides at concentrations ranging from 0.1 to 10 pg/mL.
• Calibration standards prepared at 0.1, 0.5, 1.0, 5.0 and 10 pg/mL.
• Direct injection volumes of 1, 5 and 20 mL were loaded without offline cleanup.

Chromatography and Mass Spectrometry

• Online system: Thermo Scientific EQuan with dual C18 columns (20 × 2.1 mm ID, 12 µm loading column; 50 × 2.1 mm ID, 3 µm analytical column) and a 6‐port switching valve.
• HPLC pumps: Surveyor Plus for loading (1–5 mL/min depending on volume) and Accela U-HPLC for analytical separation.
• Autosampler: HTC PAL with multi‐syringe macro for volumes up to 20 mL.
• Detection: TSQ Quantum Access triple quadrupole with ESI in positive mode; spray voltage 4000 V; ion transfer tube 300 °C; sheath gas 30; auxiliary gas 5; collision gas 1.5 mTorr; Q1/Q3 resolution 0.7 Da; scan width 0.002 Da.
• Qualitative confirmation via Quantitation-Enhanced Data-Dependent MS/MS with Reverse Energy Ramp (RER collision energy ramp from 45 to 25 eV).

Main Results and Discussion

Signal Enhancement and Sensitivity

• Signal‐to‐noise ratios and peak areas increased 3–5 fold when moving from 1 to 5 mL and from 5 to 20 mL injections.
• Detectable peaks at femtogram per milliliter levels for all targeted herbicides using 20 mL injections.

Reproducibility

• Eight replicate injections of a 1 pg/mL standard at 20 mL volume yielded RSDs below 12% without internal standards.

Qualitative Confirmation

• RER MS/MS scans provided rich product ion spectra suitable for library searching and ion ratio checks to reduce false positives.

Benefits and Practical Applications of the Method

Using online preconcentration with direct large‐volume injections eliminates offline cleanup, reduces sample handling and shortens analysis times. The flexible injection volumes allow laboratories to tailor sensitivity to regulatory requirements, making the method ideal for routine monitoring of trace herbicides in drinking water.

Future Trends and Potential Applications

Further advancements may include:

• Automation of multi‐residue analyses covering broader pesticide classes.
• Integration with real‐time water quality monitoring platforms.
• Enhanced column chemistries and higher‐throughput valve configurations.
• Application to other environmental matrices such as wastewater and soil leachates.

Conclusion

The Thermo Scientific EQuan system coupled with dual‐column LC-MS/MS enables sensitive, reproducible quantitation of herbicides at femtogram levels via 20 mL direct injections. This approach streamlines workflow by removing offline preconcentration and delivers high throughput for trace analysis in drinking water.

References

• Application Note 437: LC-MS/MS Analysis of Herbicides in Drinking Water at Femtogram Levels Using 20 mL EQuan Direct Injection Techniques, Thermo Fisher Scientific, 2008