Classification of the geographic origin of cigarettes according to Pb isotope ratios by inductively coupled plasma dynamic reaction cell mass spectrometry

Abstract

Trace Pb is accumulated in the same isotopic ratio as it occurs in the source soil, and the isotopic composition of Pb could be used to reflect these sources and provide powerful indicators of the geographic origin of agricultural products. In this study, we developed a simple and valid method based on inductively coupled plasma dynamic reaction cell mass spectrometry (ICP-DRC-MS) for the determination of Pb isotope ratios to distinguish between the geographic origins of cigarettes. The cigarette digestion solutions were directly analysed by ICP-DRC-MS with pressuring a non-reactive gas Ne. In the DRC, Ne molecules collide with Pb ions and result in a 2.4-fold improvement in the average internal precision of Pb isotope ratios, which may be due to the improvement of the ion transmission or sensitivity (via collisional focusing) and the reduction of the plasma noise (via collisional energy damping). Under an optimum DRC rejection parameter q (RPq = 0.45), the main matrix components (K, Na, Ca, Mg, Al, Fe, etc.) originating from cigarettes were filtered out. Other important parameters such as detector dead time, dwell time per data acquisition and total integrated time per isotope were also optimized. Mass discrimination of the ²⁰⁸Pb/²⁰⁶Pb ratio in Ne DRC mode increased to 0.3% compared to the vented mode; this mass bias could be accurately corrected by using the NIST 981 Pb isotope standard solution. The accuracy and precision of this method were evaluated by using two cigarette reference materials (Oriental tobacco leaves CTA-OTL-1 and Virginia tobacco leaves CTA-VTL-2). The results of ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb were 2.0842 ± 0.0028 (2δ) and 0.8452 ± 0.0011 (2δ) for CTA-VTL-2, which were in agreement with the literature values (²⁰⁸Pb/²⁰⁶Pb = 2.0884 ± 0.0090 and ²⁰⁷Pb/²⁰⁶Pb = 0.8442 ± 0.0032), respectively. The Pb isotopic composition (²⁰⁸Pb/²⁰⁶Pb = 2.0812 ± 0.0028 and ²⁰⁷Pb/²⁰⁶Pb = 0.8460 ± 0.0018) of CTA-VTL-1 was reported for the first time in our study. The precision of Pb isotope ratios (²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb) for the cigarette samples ranged from 0.05 to 0.12 % (N = 6). The proposed method has sufficient precision to distinguish between 91 cigarette brands that originated from four different geographic regions.