Dynamic time correction for high precisionisotope ratio measurements

Significance of the Topic

This note addresses a key challenge in high-precision isotope ratio analysis by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS): the differing response times of Faraday cup detectors equipped with high-resistance (1013 Ω) amplifiers. Improved signal-to-noise offered by these amplifiers can enhance precision on small ion beams, but slow response and decay can bias transient measurements such as laser ablation. A dynamic, time-based correction (tau correction) is therefore crucial to realize the full analytical benefits.

Objectives and Study Overview

The primary aim was to demonstrate and validate a dynamic tau correction for laser ablation MC-ICP-MS using Thermo Scientific Neptune XT with 1013 Ω amplifier technology. Specific goals included calibration of amplifier gain factors, determination of tau decay constants, application of tau correction in transient Pb isotope measurements on MPI-DING reference glasses, and evaluation of precision and accuracy improvements relative to standard 1011 Ω amplifiers.

Methodology and Used Instrumentation

The study employed a Teledyne Photon Machines Analyte G2 excimer laser (193 nm, 40 µm spots, 3 J cm⁻², 8 Hz, 30 s) coupled to a Thermo Scientific Neptune XT MC-ICP-MS. A desolvating nebulizer (Teledyne Cetac Aridus II) aspirated a 2.5 ppb Nd solution for gain calibration and tau constant determination. The Neptune XT featured both 1011 Ω and 1013 Ω amplifier channels, configured to measure Nd isotopes for calibration and Pb isotopes (206, 207, 208) on glass samples.

Main Results and Discussion

• Gain calibration factors for three 1013 Ω amplifiers were established (~0.01015) with 50 ppm uncertainty.
• Tau decay constants (τ) were 0.6524±0.0006 s, 0.6497±0.0008 s, and 0.6406±0.0010 s (n=10), and remained stable over time.
• Dynamic tau correction applied on an integration-by-integration basis corrected the lag and smoothing of signals acquired on 1013 Ω channels, aligning them with 1011 Ω traces.
• For Pb isotope ratios (208Pb/206Pb and 207Pb/206Pb) on MPI-DING glasses, tau correction reduced external relative standard deviations by up to fourfold and eliminated systematic biases exceeding reference uncertainties.

Benefits and Practical Applications

Implementing a dynamic tau correction enables accurate and precise transient isotope ratio analyses (e.g., laser ablation, transient signals) using high-resistance amplifiers. Laboratories can exploit improved signal-to-noise of 1013 Ω technology without compromising data quality, extending the range of samples and low-intensity signals amenable to Faraday detection.

Future Trends and Opportunities

• Integration of automated tau calibration routines within instrument software for real-time corrections.
• Extension of dynamic time corrections to broader amplified detector arrays and other transient techniques (e.g., single-particle analysis).
• Application to isotope systems beyond Pb and Nd, facilitating high-precision studies in geochemistry, environmental science, and materials research.

Conclusion

The demonstrated dynamic tau correction effectively compensates for slow response and decay of 1013 Ω amplifiers, unlocking up to fourfold improvements in precision for transient isotope ratio measurements. This advance allows routine use of high-gain Faraday detection in laser ablation MC-ICP-MS, enhancing analytical capabilities for low-intensity signals.

Reference

Koornneef J. M. et al., Anal. Chim. Acta 819 (2014) 49–55
Kimura J.-I. et al., J. Anal. At. Spectrom. 00 (2016) 1–11
von Quadt A. et al., J. Anal. At. Spectrom. (2016)
Klaver M. et al., J. Anal. At. Spectrom. 31 (2016) 171–178
Pettke T. et al., J. Anal. At. Spectrom. 26 (2011) 475–492
Hu Z. et al., Spectrochim. Acta B 78 (2012) 50–57
Hirata T. et al., J. Anal. At. Spectrom. 18 (2003) 1283
Iizuka T. et al., Chem. Geol. 220 (2005) 121–137
Iizuka T. et al., Chem. Geol. 282 (2011) 45–57
Jochum K. P. et al., Geostand. Geoanal. Res. 29 (2005) 333–338