Development of a Paste-type Certified Reference Material of Tomato for Elemental Analysis: Certification and Long-term Stability Study

A paste-type certified reference material (CRM) of tomato was developed for accurate elemental analysis of canned foods. Accurate mass fractions of cadmium (Cd), lead (Pb), and tin (Sn) in the tomato paste CRM were determined by isotope dilution inductively coupled plasma mass spectrometry. The certified values and their expanded uncertainties (k=2 at approximately 95% level of confidence) for Cd, Pb, and Sn were (0.297 ± 0.015) mg/kg, (0.213 ± 0.010) mg/kg, and (221.2 ± 9.5) mg/kg, respectively. Homogeneity of the CRM was 0.3%, 1.8%, and 0.2% for Cd, Pb, and Sn, respectively, which were estimated using the relative standard deviation of the measurement results of 10 separate sample bottles from one batch of the CRM. Major contributions to the measurement uncertainties for Cd and Sn were the uncertainties of the isotope ratios of Cd and Sn in the sample and the elemental standards. The uncertainty for Pb was mainly affected by sample homogeneity. Stability tests were conducted for 6 and 12 months at −20, 4, and 25°C, and the storage temperature for the CRM was determined to be −20°C. To investigate long-term storage, the mass fractions of Cd, Pd, and Sn in the CRM were re-determined after storage for 7 years. The results were in good agreement with the certified results implying that this CRM is stable for 7 years if kept at an appropriate storage condition with vacuum packaging the amber glass bottles in aluminum-coated polymer bag.