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In 1999 , chlorpyrifos residues were found in 24 % of apple samples tested , at a mean value of 0.022 ppm . As in our earlier study [ 11 ] , the iodine contents in more than 60 % of salt samples were below the mandated range ; compared with the national sample , however , the median concentration in this specific district was three times higher ( 24 vs 7 ppm ) but still below the minimum mandated level of 30 ppm . The concentrations varied from 1 to 117 ppm , with a global median of 24 ppm . To check the reproducibility of the TXRF unit a reflector with a high concentrated sample of 100 ppm is prepared and an alignment with a maximum countrate adjusted . Using a new development that incorporates improvements with state-of-the-art technologies in its Electrical Substitution Radiometers ( ESRs ) and taking full advantages of new materials and modern electronics , TIM measures TSI to an absolute accuracy of 100 ppm .