Kinetic Method for Manganese

Trace levels of manganese can be determined by measuring its catalytic effect on the oxidation of diethylamine by sodium periodate.  The rate of formation of the oxidation product is followed spectrophotometrically.  A series of external standards are prepared by adding 1.00-mL aliquots of standards containing 0.010, 0.020, 0.030, and 0.050 mg/mL of manganese to separate tubes containing 5.00 mL of diethylamine.  A 5.00-mL aliquot of a sodium periodate solution is added to the first tube and the absorbance is measured 10.0 min after mixing, yielding an absorbance of 0.050.  The remaining standards are treated in the same fashion, yielding absorbance values of 0.081, 0.109, and 0.169, respectively.  An analysis of a reagent blank gives an absorbance of 0.020.  A 5.00-mL sample of blood was dry-ashed and the manganese extracted using ion-exchange chromatography.  After diluting the sample to 3.00 mL, a 1.00-mL aliquot was treated in the same manner as the standards, yielding an absorbance of 0.098.  What is the concentration of Mn in the blood sample in ppm?