Improve the sensitivity of haloacetic acids and phenols by increasing ion transmittance of an ion guide at higher pressure vacuum

Significance of the Topic

Trace-level detection of haloacetic acids and phenols is essential for monitoring water quality and ensuring regulatory compliance. However, their low molecular weights lead to increased ion losses in the higher-pressure stages of LC-MS. Optimizing ion transmission at these stages can significantly enhance analytical sensitivity.

Objectives and Study Overview

The primary goal was to enhance sensitivity for analytes with m/z ≤150 by adjusting the ion guide’s RF voltage under elevated vacuum pressure. The study combined computational simulations of gas dynamics and electric fields with practical LC-MS analyses of monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, phenol, and chlorophenols.

Methodology and Simulation Studies

Gas dynamics in the ion guide region of the LCMS-8060NX were modeled using ANSYS Fluent, simulating pressure around 100 Pa. The simulated flow fields were imported into SIMION to trace ion trajectories under various RF amplitudes. This dual simulation approach identified the RF voltages required to counteract collisional diffusion of low-mass ions.

Applied Instrumentation

• UHPLC: Shimadzu Nexera X2 with Shim-pack GIST-HP C18 column
• Mass Spectrometer: Shimadzu LCMS-8060NX in negative APCI mode
• Simulation Software: ANSYS Fluent and SIMION

Main Results and Discussion

Simulations revealed that low-mass ions (m/z <150) require higher RF amplitudes than theoretical estimates to maintain focused trajectories in the high-pressure region. Experimental optimization confirmed an optimal RF voltage of 60–70 V for haloacetic acids and phenols, yielding up to 2.6-fold increase in signal intensity compared to default settings. Higher-mass analytes showed minimal change in optimal voltage.

Benefits and Practical Applications

By tuning the RF voltage based on simulation insights, the method doubles the sensitivity for low-mass analytes without hardware modifications. This improvement facilitates lower detection limits and enhances throughput in environmental, food safety, and quality-control laboratories.

Future Trends and Potential Applications

Future work may integrate dynamic RF control, extend simulation-guided optimization to other ion guide designs, and apply machine learning to predict optimal settings. The approach can be transferred to trace analysis of other low-mass polar compounds in various fields.

Conclusion

A combined simulation and experimental strategy effectively enhances ion transmission for low-mass analytes in LC-MS. Adjusting RF voltage in the higher-pressure ion guide region is a straightforward and impactful way to improve sensitivity.

References

• Ueda M., Hattori T., Fukui W., Ibushi T., Mukaibatake K. Shimadzu Application MP-284: Enhance sensitivity by ion guide optimization.
• ANSYS Inc. ANSYS Fluent (2023).
• Scientific Instrument Services. SIMION (2023).