New Design 1013 Ω Amplifiers for Measurement of Small Ion Beam Currents

Significance of the Topic

Accurate measurement of extremely small ion beam currents (down to femtoampere range) is critical for high‐precision isotope ratio analyses in thermal ionization mass spectrometry (TIMS) and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Such measurements underpin studies on scarce or trace materials, including extraterrestrial samples, geological microdomains, and ultra‐low concentration isotopes, where signal-to-noise ratio and detection limits directly affect data quality.

Aims and Study Overview

This study introduces a novel amplifier design incorporating 1013 Ω feedback resistors for Thermo Scientific Triton Plus TIMS, Neptune Plus MC-ICP-MS, and Noble Gas lines. The objectives are to enhance signal-to-noise performance by a theoretical factor of ten over standard 1011 Ω amplifiers, reduce Johnson-Nyquist noise, and maintain fast settling times via a proprietary relay matrix that allows remote switching of Faraday cups.

Methodology and Instrumentation

Key methodological features include:

• Amplifier Design: Integration of ultra-high-ohmic (1013 Ω) resistors in the feedback loop with optimized current amplifier architecture to minimize settling time and electronic noise.
• Relay Matrix: Enables seamless switching between Faraday cups without opening the amplifier housing, enhancing operational flexibility.
• Instrumentation Platforms: Tested on Thermo Scientific Triton Plus TIMS and Neptune Plus MC-ICP-MS systems, measuring ion currents across 1010, 1011, 1012, and 1013 Ω feedback resistances.

Main Results and Discussion

Noise Performance and Baseline Stability:

• Baseline reproducibility over 11 minutes yields 1 SD noise of 0.2 µV (normalized to 1011 Ω) for the 1013 Ω amplifier, compared to 0.4 µV and 2 µV for 1012 Ω and 1011 Ω respectively.
• Johnson-Nyquist noise scales with the square root of resistor value, offering a theoretical tenfold signal-to-noise improvement when moving from 1011 Ω to 1013 Ω.

Settling Time:

• Signal decay curves for an ~800 fA input show recovery to within 100 ppm of original signal in under 3 s for the 1013 Ω amplifier, demonstrating rapid response despite high gain.

Precision and Reproducibility:

• Internal Precision: Measured 2 SE on 143Nd/144Nd follows counting‐statistics limits closely, with improved precision at low beam intensities when using 1013 Ω versus 1011 Ω resistance.
• External Reproducibility: At 50 fA Nd beam intensity, 143Nd/144Nd ratios show 2 SD of ±0.000102 for 1013 Ω compared to ±0.000387 for 1011 Ω.
• 100 pg Nd Loads: Individual 100 pg loads yield 143Nd/144Nd reproducibility of ±0.000075 (2 SD) with intensities ~200 fA on 1013 Ω amplifiers.

Faraday Cup vs. SEM Comparison:

• The 1013 Ω amplifiers outperform secondary electron multipliers (SEMs) for beam intensities above ~20 kcps, offering better internal precision and resolving power across a dynamic range extending from sub-fA to pA levels.

Dynamic Range:

• Upper voltage limits for 1011, 1012, and 1013 Ω amplifiers are 50 V, 5 V, and 0.5 V respectively, covering ion currents from tens of fA up to nA scale. Noise‐limited lower detection thresholds reach ~30 cps equivalent.

Benefits and Practical Applications

• Enhanced Signal-to-Noise: Up to 10 × improvement over conventional 1011 Ω amplifiers allows measurement of extremely weak ion beams with greater confidence.
• Improved Throughput: Fast settling times and remote Faraday cup switching facilitate high‐volume, high‐precision isotope ratio analyses.
• Broader Applicability: Enables new research in geochronology, cosmochemistry, forensics, and environmental tracing where sample size or concentration is limiting.

Future Trends and Opportunities

• Integration with Next-Generation MC-ICP-MS Platforms: Further miniaturization and digital signal processing could push detection limits into the attoampere range.
• Expanded Isotope Systems: Application to elements with ultra-low natural abundances (e.g., 234U, heavy metal isotopes) and in situ microanalysis of mineral zonations.
• Automated Workflows: Coupling high-ohmic amplifiers with robotics and machine learning for real-time quality control and data interpretation.

Conclusion

The introduction of 1013 Ω feedback amplifiers represents a significant advance in faraday cup detection technology for TIMS and MC-ICP-MS. By substantially reducing electronic noise while maintaining rapid settling behavior, these amplifiers enable robust and precise analysis of femtoampere‐level ion beams, broadening the analytical capabilities in isotope geochemistry and related fields.

References

• Koornneef J.M., Bouman C., Schwieters J.B., Davies G.R. (2014) Measurement of small ion beams by Thermal Ionisation Mass Spectrometry using new 1013 Ohm resistors. Analytica Chimica Acta, 819:49–55.