Certified values in reference materials are considered the best estimates of true values, being used for validation and verification of analytical methods. A long-term series (1998 to 2019) of neutron activation analysis (NAA) of agricultural certified reference materials was investigated at the Radioisotopes Laboratory (CENA/USP). The average sodium mass fraction in SRM 1515 Apple Leaves and SRM 1547 Peach Leaves did not agree with the certified values and the discrepancy was confirmed by triple quadrupole inductively coupled plasma mass spectrometry (TQ-ICP-MS). Long-term data showed a clear increase of Na mass fraction over time. Such behavior, corroborated by results from other authors, seems to confirm the hypothesis of migration of Na from glass bottle walls into the certified reference materials. All rights reserved, Elsevier.

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