Accurate measurements of lead using isotope dilution calibration curve method with the accounting for natural...

Isotope dilution mass spectrometry (IDMS) is a key method for high-precision measurements. Most implementations, however, implicitly assume that the isotopic compositions of the sample and the primary standard are identical, thereby neglecting the natural isotopic variations. Although this is a reasonable approximation for many chemical elements, ignoring the natural isotopic variations for elements such as lead, boron, or lithium can result in significant systematic errors.

An IDMS calibration curve method capable of accounting for natural isotopic variations is developed and applied for the determination of lead in food samples. The fundamental equations describing the measurement model are derived from first principles and are supported by a suite of Excel-VBA functions that facilitate easy interpretation of the graphical calibration data and their uncertainty evaluation. Measurements of the NRC ROCA-1 cacao powder revealed that neglecting isotopic mismatch between lead in the cacao sample and the primary standard can introduce a 7% bias in the estimated mass fraction of lead. In general, biases of up to 20% are possible. The results obtained using the IDMS curve are compared with the traditional double isotope dilution (ID²MS). Both techniques yield highly accurate results, but the graphical method is easier to implement, more precise, and is better suited for high-throughput analyses.

In this study, a novel IDMS quantitation approach based on a multipoint calibration curve is proposed. The method effectively eliminates systematic errors resulting from significant differences in the isotopic compositions of lead between the sample and the primary standard. The graphical nature of this calibration approach marks a significant departure from current equation-based IDMS methods, aligning more closely with the practices commonly found in analytical laboratories. Although the method is demonstrated on lead, a toxic element routinely measured in food commodities, this method can be extended to the accurate determination of other elements with significant natural isotopic variability, including technology critical elements such as lithium and boron.