In situ carbon isotope analysis of diamonds using LA-MC-ICP-MS inspired by the distribution of ions and isotope ratios in ICP†

Abstract

Carbon-stable isotopes of diamonds provide clues regarding their growth processes. Thus, an accurate, efficient, and affordable method to determine the carbon isotope ratio is extremely urgent. Accurate and precise determination of the carbon isotope ratio with LA-MC-ICP-MS is limited by the high background intensity of ¹²C⁺ and instrumental settings. Hence, laser parameters were gradient-changed to investigate their influences on carbon isotope analysis. Besides, high-resolution spatial distributions of ¹²C⁺ and ⁴⁰Ar³⁺ intensities coupled with the ¹³C/¹²C value in the ICP were investigated in detail to elucidate the ionization kinetics of ⁴⁰Ar³⁺ and to determine the most stable zone for carbon isotope analysis in the ICP. Finally, two types of diamonds (a natural diamond (D-N-1) and synthetic diamond (D-HTHP)) were measured to verify their homogeneity and flexibility for in situ analysis of the carbon isotope on diamonds with LA-MC-ICP-MS. The signal-to-noise ratio (SINR) greatly affects precision, which could be significantly improved by optimizing laser parameters. The internal precision of in situ C isotope analysis is better than 0.2‰ (2SE) when the SINR is more than 4. The ionization efficiency of ⁴⁰Ar³⁺ was found to be controlled by the catalysis of C ions and the thermodynamic parameters of the ICP. The most stable zone for carbon isotope analysis in the ICP was found to be located at ≈1.4 mm ahead of the ¹²C⁺ signal-maximum point. Hence, the precision of ¹³C/¹²C could be improved when the torch was retreated ≈1.4 mm from the maximum ¹²C⁺ intensity point axially. The natural and synthetic diamonds exhibited similar down-hole fractionation behaviors of the C isotope during laser ablation, indicating that they can be used to correct each other with LA-MC-ICP-MS. The accuracy of the carbon isotope was validated by comparing the data with those of nano-scale secondary ion mass spectrometry (NanoSIMS) and laser ablation-isotope ratio mass spectrometry (LA-IRMS). This study suggested that in situ LA-MC-ICP-MS is a rapid, precise and accurate way to measure the carbon isotope of diamonds. The interactive influence of the ionization process of nuclides leads to elemental and isotopic fractionations, which is one of the mechanisms of the matrix effect.