Factors affecting chromium isotope measurements using the double-spike method.

Area of Science:

• Geochemistry
• Isotope Geochemistry
• Analytical Chemistry

Background:

• The double-spike technique is crucial for correcting instrumental fractionation in stable metal isotope measurements.
• Chromium (Cr) isotope measurements have gained prominence, relying on the double-spike method.
• The necessity of precise double-spike calibration and other accuracy-affecting factors in Cr isotope analysis remain underevaluated.

Purpose of the Study:

• To theoretically assess the necessity of precise double-spike calibration for Cr isotope measurements.
• To develop and validate a robust double-spike method for Cr stable isotopic composition analysis.
• To identify and correct for factors influencing the accuracy of Cr isotope measurements.

Main Methods:

• Theoretical calculations to evaluate the impact of instrumental mass fractionation of the double spike.
• Development of a novel double-spike method using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS).
• Testing of key factors affecting Cr isotope measurement accuracy, including matrix elements like Fe and Ti.

Main Results:

• Theoretical analysis confirmed that uncorrected instrumental mass fractionation of the double spike does not introduce systematic bias in δ⁵³Cr measurements under specific conditions.
• The developed method effectively corrects for the presence of Fe and Ti (up to 5% wt) in samples.
• Achieved long-term internal reproducibility of ±0.04‰ (2SD) and external reproducibility of -0.07 ± 0.05‰ for δ⁵³Cr.
• δ⁵³Cr values for various USGS geological reference materials were measured, showing consistency with existing data for BHVO-2 and BIR-1.

Conclusions:

• Precise calibration of the double spike's mass fractionation is unnecessary, simplifying the calibration procedure for Cr isotope analysis.
• The new Cr isotope method is applicable to diverse geological rock samples with varying chemical compositions.
• This research streamlines Cr isotope measurements, enhancing their applicability in geological studies.