High-Sensitivity Measurement of Cr Isotopes by Double Spike MC-ICP-MS at the 10 ng Level

High-sensitivity and high-precision (2 SD ≤ 0.06‰) measurement of chromium (Cr) isotopes at the 10 ng level was successfully carried out using double spike multiple-collector inductively couple plasma mass spectrometry (MC-ICP-MS). To enhance the signal sensitivity and stability, the Aridus II desolvating nebulizer system was improved by placing its waste gas trap bottle in an ice chamber (5 °C cold trap). This setup, beyond Cr isotope analysis, can be applied to most heavy metal isotope measurements. The sensitivity of the ⁵²Cr signal is ≥300 V mg^-1 L (with a 10¹¹Ω amplifier and a 110 μL min^-1 uptake rate), an enhancement of ≥1.5 times compared to the Aridus II without the cold trap. In addition, the relative standard deviation of the ⁵²Cr signal varied ≤4% over 8 h, demonstrating high stability. The δ⁵³Cr values of common geological reference materials determined using 10 ng of Cr are in excellent agreement with results measured at 25 ng and 50 ng and are consistent with previous determinations, validating the accurate and precise Cr isotope ratio measurements. An empirical method is proposed to correct for the residual (after subtraction) effect of Fe interference on δ⁵³Cr determination. This method relies on a linear relationship between the [Fe]/[Cr] and δ⁵³Cr shift within one analytical session. Finally, we report the δ⁵³Cr values of 19 new reference materials, ranging from -0.44‰ to 0.49‰. Among them, GSS-7 (-0.44 ± 0.02‰, 2 SD, n = 5), GSS-4 (0.48 ± 0.02‰, 2 SD, n = 5), and GSD-10 (0.49 ± 0.05‰, 2 SD, n = 5) can be used as candidate reference materials for interlaboratory comparisons to complement existing ones that are mostly isotopically unfractionated from the bulk silicate earth.