In situ carbon stable isotope measurement for graphite using LA-MC-ICP-MS

Abstract

Graphite, a nonmetallic mineral composed of carbon, relies on carbon stable isotopes for tracing the material source, growth enrichment and mineralization processes. In this study, we leverage laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) to establish an in situ analysis method with medium resolution (∼5000) for carbon stable isotopes. Two potential in-house graphite reference materials from the Lutang deposit in Hunan Province and the Changshouyuan deposit in Jiangxi Province, China, were prepared using the tableting method. Twenty measurements on each of the two samples indicate relatively homogeneous carbon stable isotope composition with a RSD of uncorrected ¹³C/¹²C ratios at approximately 0.030% (LTSM: 0.032%, CSY: 0.027%). Additionally, the laser repetition rate was gradient changed (2–20 Hz) to investigate the influence of concentration effect caused by changes in the signal-to-noise ratio (SNR) on the measurement results. When the SNR > 1.23, ¹³C/¹²C-2SE no longer drops significantly. Constructing working curves of the SNR and isotope ratio can correct the concentration effect. No matrix effect was observed between the two graphite samples. A slight matrix effect was observed between graphite and dolomite and a strong matrix effect exists between graphite and calcite. The δ¹³C analysis of graphite from six different mining areas was conducted, and the LA-MC-ICP-MS results, when corrected using the standard-sample-bracketing (SSB) method and LTSM samples, exhibit reasonable agreement with the δ¹³C values reported in previous and current studies using IRMS. This study demonstrates the feasibility of in situ measurement of carbon stable isotopes in graphite using LA-MC-ICP-MS, characterized by high efficiency, cost-effectiveness and high spatial resolution. The two types of graphite are suitable to be used as reference materials for in situ measurement of carbon stable isotopes in graphite using LA-MC-ICP-MS.