2018 Volume 67 Issue 1 Pages 19-25
The determination of trace-level Rb was achieved by cool plasma ICP-OES, for which the optimum operating conditions for RF power and carrier gas were 750 W and 0.95 mL min−1. Such a highly sensitive analysis could be attributed to the effective suppression of spectral interference from Ar emission lines while measuring the Rb emission line, having improved the signal-to-background ratio by 20-fold. The present method provided an instrumental detection limit of 0.005 mg kg−1 for Rb. Apparent matrix effects of K and Ca were observed for the measurement of Rb by cool plasma ICP-OES, suggesting that approaches are required to cancel the matrix effect for the determination of Rb in real samples. Gravimetric standard addition incorporating an internal standard correction was applied to the determination of Rb in three certified reference materials (CRMs) for food analysis, i.e. NMIJ CRMs 7502-a (white rice flour), 7505-a (tea leaf powder), and 7512-a (milk powder). The observed values for these CRMs were (1.77±0.12) mg kg−1, (7.30±0.24) mg kg−1, and (8.91±0.29) mg kg−1, respectively. These observed values agreed with the certified values: (1.77±0.07) mg kg−1, (7.3±0.3) mg kg−1, and (8.93±0.31) mg kg−1, respectively. Both the observed values and the certified values were given as (mean±expanded uncertainty), while the expanded uncertainty was calculated using a coverage factor (k) of 2, giving a level of confidence of approximately 95 %.