with occasional shaking. A second 5 mL aliquot of hydrogen peroxide was introduced and digested at 85 ± 2 °C (oil bath) for 1 h. The contents were evaporated to a small volume (1.2 mL). Twenty five milliliters of ammonium acetate (1.0 mol/L, adjusted to pH 2 with HNO3) was added to the cool and moist residue, shaken, centrifuged and the extract separated described in step one.