High precision δ41K using MC-ICP-MS in high resolution and MS/MS mode

Importance of the Topic

Potassium plays a critical role in geological and biological systems, with isotope ratio variations (δ41K) offering insights into processes ranging from crustal weathering to cellular function. However, the close mass overlap of 41K and the 40Ar1H interference has historically limited precision in isotope ratio measurements. High-precision δ41K analysis thus demands advanced mass spectrometric techniques to resolve or remove isobaric species and achieve reliable data.

Objectives and Study Overview

This application note evaluates the performance of the Thermo Scientific Neoma MC-ICP-MS in two configurations: the extra-high resolution XHR mode and the MS/MS mode with a collision/reaction cell CRC. The goal is to compare sensitivity, precision, and suitability for different potassium isotope applications using standard reference materials SRM3141a and SRM9996b.

Methodology

The study employed two parallel experimental protocols:

• XHR mode The instrument was equipped with a high-sensitivity Jet Interface and adjustable slits, and operated under hot plasma conditions (approximately 1300 W RF power) to minimize 40Ar1H formation
• CRC mode A pre-cell mass filter and collision/reaction cell filled with H2 and He gases neutralized 40Ar1H, allowing low-resolution measurement with enhanced sensitivity at ambient plasma power

Each measurement spanned 180 seconds (45 cycles of 4 seconds), with bracketing of sample and standard solutions at defined concentrations and blank corrections at sequence start and end.

Instrumentation

• Thermo Scientific Neoma MC-ICP-MS with XHR option extra-high resolution slits and electrostatic analyzer modifications
• Thermo Scientific Neoma MS/MS MC-ICP-MS with collision/reaction cell using H2 and He gases
• ESI Apex Omega Q desolvating nebulizer
• ESI microFAST MC autosampler with Dual Loop Syringe Loading and controlled injection rates of 5 to 1000 microliters per minute
• Qtegra ISDS software for instrument control and mass resolving power calculations

Main Results and Discussion

Sensitivity differed markedly between methods: approximately 22 volts per ppm in XHR mode versus more than 1250 volts per ppm in CRC mode, reflecting reduced ion transmission at high resolution. Signal intensity matching via the autosampler was critical; a 1 percent mismatch induced approximately 0.01 per mille bias in δ41K for XHR. Internal precision from single-session bracketing yielded one standard deviation of 0.020 per mille at 1 ppm potassium in XHR and 0.057 per mille at 100 ppb potassium in CRC. Despite lower sensitivity, XHR mode demonstrated better external precision due to greater system stability.

Benefits and Practical Applications

• XHR mode offers superior precision when sample quantity is ample, making it ideal for high-accuracy δ41K studies in geology and environmental science
• CRC mode provides substantially higher sensitivity, enabling potassium isotope analysis on trace samples below 100 ppb, suitable for micro-samples in biology or limited geological specimens

Future Trends and Opportunities

Advances in reaction cell chemistry, slit design, and plasma optimization may further suppress interferences and improve precision. Integration of automated sample-standard matching, higher repetition rates, and novel data processing algorithms could expand applications to in-situ microanalysis, real-time monitoring of biological processes, and high-throughput environmental screening.

Conclusion

The Neoma MC-ICP-MS platform demonstrates flexible operation for high-precision potassium isotope analysis. The XHR option excels in accuracy when sample volume allows, while the CRC mode extends capabilities to trace-level measurements. Selection between modes should be guided by sample availability and precision requirements.

References

• Hobin K et al Anal Chem 93 8881-8888 2021
• Chen H et al J Anal At Spectrom 34 160-171 2019
• Morgan LE et al J Anal At Spectrom 33 175-186 2018
• Chen H et al Earth Planet Sci Lett 539 116192 2020
• Telouk P et al J Anal At Spectrom 37 1259-1264 2022
• Moynier F et al Chem Geol 571 120144 2021