The EPA Method 3051 is used to partially digest 0.2500 +/- 0.0002 g of crushed soil samples in 10 mL of concentrated nitric acid heated to 175°C for (insert time) in a CEM MARSXpress microwave unit. The resulting digestion solution is filtered and diluted to 50 mL, resulting in a 20% HNO3 solution. Finally, the amounts of 14 trace elements (Al, Ba, Be, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, P, Pb, and Zn) in these solutions are measured by ICP-OES.
Materials and Methods
Safety Warning: Nitric acid poses a hazard to skin, eyes, and internal organs.
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Digestion Solution Preparation: Materials Sixteen soil samples are digested in each batch. The samples are digested in clean and dry fluorocarbon (PFA Teflon®(?)) (look up volume) vessels with Fisher Scientific Trace Metal Grade concentrated Nitric Acid. A Fisherbrand 2-10 mL Finnipipette with a Finntip 10 mL tip is used to measure the 10 mL of nitric acid used in the digestion. A CEM MARSXpress heats the nitric acid and sample mixture. Rinsed Whatman #4 Qualitative 125mm diameter circle filter papers are used to filter the material through Pyrex 60º glass funnels into 50mL Pyrex glass volumetric flasks. Approximately 6 MegaOhm resistance distilled water is used throughout the procedure. Three standards are used: NIST Standard Reference Material 2711 Montana Soil (Moderately Elevated Trace Element Concentrations), Environmental Resource Associates D050-542 (further crushed to <80 mesh size in alumina shatterbox), and an internal standard, BS-1 #13a 60-65cm.
Digestion Solution Preparation: Methods Initial preparations minimize the potential for contamination of the digested sample solutions. These include:
1. Thorough soaking of the digestion vessels in a 1:1 nitric acid (Pharmco ACS Reagent Grade) solution followed by rinsing in distilled water.
2. Drying the digestion vessels prior to use
3. cleaning the 50 mL volumetric flasks and glass funnels in multiple rinses of 1:1 nitric acid (Pharmco ACS Reagent Grade) and distilled water
4. Rinsing the filter papers in distilled water and drying them in the oven (sometimes)
5. Cleaning the inside of the microwave with distilled water and Kimwipes after each use.
An ANALYTICAL Plusbalance is used to weigh out 0.2500 +/- 0.0002 g of soil
sample onto Fisherbrand 4”x 4” weighing paper. This sample is then poured into a numbered digestion vessel. The Fisherbrand 2-10mL Finnipipette is used to add 10 mL of concentrated nitric acid to the digestion vessels containing the soil samples. The vessels are then weighed on a Sartorius balance before placing them in the microwave carousel which is in turn placed in the CEM MARSXpress microwave unit. The samples are rotated to ensure homogenous distribution of heat in the microwave using a pre-programmed heating and cooling cycle. After removal from the microwave, the samples are reweighed on the Sartorius balance to check that no significant weight change has occurred during heating, which would indicate the loss of material by venting of vapors from the digestion vessels.
The next step in the procedure is the filtering of the nitric acid and soil sample solution. Because this procedure only partially digests the soil samples, there still remains much solid material after heating which must be separated from the nitric acid containing digested ions in solution. The glass funnels containing the conically folded filter papers are placed on a stand with their outlets running into the 50 mL glass volumetric flasks. The inner caps of the digestion vessels are covered in droplets of condensed vapor from the heating and so are rinsed into the funnel with distilled water. The contents of the digestion vessel are then poured into the filter with distilled water (used to keep the filter paper from tearing). The liquid portion of the vessel’s contents filters through the paper and drips into the volumetric flask and the solid portion remains in the filter. The digestion vessel is rinsed twice with distilled water, which is then poured into the filter.
Once all of the digestion solution has been filtered, the filter paper is removed from the funnel and placed aside to dry. The glass funnel is rinsed into the volumetric flask with distilled water. Subsequently, the digestion solution is diluted to 50 mL with distilled water. Occasionally the samples must be diluted to 100 mL due to tearing of the filter paper during initial pouring or to overfilling of the 50 mL volumetric flasks.