1. Make standards using sodium phosphate at the following uM concentrations: 0, 2, 5, 7, 10, 20, 40, 60, and 80. Use the screw top glass tubes. 2. Dry the samples and standards under nitrogen evaporator (Rm 209), and add 150 ul 70% perchloric acid. Oxidize the samples and standards in a heating block (180C, Rm 218) overnight or for 1 hour if the heating block has been pre-heateed. 3. Rinse the sides of the tubes with 830ul xenopure water and vortex. 4. Add 170 ul 2.5% ammonium molybdate (by weight) and vortex. 5. Add 170 ul 10% ascorbic acid (by weight) and vortex. Make the ascorbic acid fresh every time. 6. Incubate for 15 min at 50°C. Allow tubes to cool. 7. Use spectrophotometer to read absorbance at 820nm.
