Isotopic analysis of ultra-trace Cu using TIMS with the continuous heating method: optimization of the data integration range and method

A procedure for the isotopic analysis of ultra-trace Cu using thermal ionization mass spectrometry (TIMS) coupled with the continuous heating method was optimized. A chemical activator was applied to overcome the low ionization efficiency of Cu, which significantly enhanced the ion signal intensity by approximately 20 times while maintaining a negligible background contribution. Data integration methods and ranges were experimentally evaluated using 20 ng, 10 ng, and 1 ng of SRM 976 reference material. The analytical results showed that Method I (summed intensities) was more effective than Method II (simple arithmetic mean) for reducing the contributions of noise-dominated signals. The integration range of over 25% was chosen as the optimized range to effectively exclude noise while compensating for thermal mass fractionation. The experimental results showed that a minimum of 10 ng of Cu was required for reliable isotopic analysis. Good analytical performance, including accuracy, precision, and uncertainty, was obtained for 20 ng and 10 ng samples, but the 1 ng sample was not accurate due to noise and fractionation instability. The optimized TIMS procedure was verified by analyzing a certified reference material of Cu. The statistical agreement with the certified value confirmed that the optimized continuous heating TIMS method is a reliable technique for Cu isotopic analysis at ultra-trace levels.