Optimization of data integration for ultra-trace zinc isotope ratio analysis by continuous heating thermal ionization mass spectrometry with single-collector sequential detection

Abstract

The thermal ionization mass spectrometry (TIMS) coupled with the continuous heating method was applied to the isotopic analysis of Zn at an ultra-trace level (1 ng) using the certified reference material IRMM-3702. Because nanogram-level Zn produces weak ion signals that require single secondary electron multiplier (SEM) sequential detection, a detailed evaluation of data treatment procedures was carried out to secure high analytical performance. Three integration methods (summed intensities, simple averaging, and intensity-weighted averaging) were experimentally evaluated for four integration ranges defined relative to the maximum ⁶⁴Zn⁺ intensity (full, “over 1%”, “over 25%”, and “over 75%”). The results showed that the analytical performance (accuracy, precision, and uncertainty) is strongly affected by both the integration method and the integration range. Method I (summed intensities) and Method III (intensity-weighted averaging) produced isotope ratios with relatively high precision and acceptable accuracy, while Method III showed slightly better analytical performance compared to Method I. Method II (simple averaging) produced large deviations and poor precision when applied to the full range, although the analytical performance improved when reduced integration ranges were applied. These findings indicate that the optimized data treatment procedure for the continuous heating TIMS analysis of Zn isotopes is the use of Method I or III with the integration ranges of “over 25%”, as they yield comparable analytical performance in this range. The results establish a methodological procedure for accurate and precise Zn isotope ratio measurements at ultra-trace levels, extending the applicability of TIMS as a competitive tool for geochemical, environmental, and biomedical research.