Proficiency test in Japan for the elements in tea-leaf powder

► A proficiency test (PT) of Al, Ca, Fe, K, P, and Zn in tea leaf powder. ► The certified values of NMIJ CRM 7505-a were used as reference values. ► The validity of the reference values was confirmed by inter-laboratory comparison. ► The z-scores and the δ-scores were calculated to assess the reported results. ► The possible reasons for questionable or unsatisfactory results were summarized.