1. Soil samples (5.0 g approx.) were oven-dried at 105-110 C until getting constant weight.
2. Weigh 1.0 g dried soil sample with a proper electronic balance into a 250-ml beaker.
3. Add 0.5 ml of DI water into a sample beaker and slurry.
4. Add 10 ml of concentrated HNO3 and a few boiling chips into the slurry samples and cover with a watch glass.
5. Heat gently the sample at 100 C on a hot plate for 2 hours a fume hood, and then allow the solution cool down for 15 min.
6. Add (drop-wise) 3 ml of 30% H2O2 into a boiled sample and continusly heat for another hour with intermittent stirring by gentle swirling of the beaker flux.
7. Filter the digested sample directly into a 25-mL volumetric flask. Wash the beaker and filter several times with small portions of water to complete a quantitative transfer. Allow the solution to be cool. Then dilute to the mark and mix well.
8. Keep the prepared sample solution for directly analysis.
9. Prepare a blank solution in the same manner.