Samples Containing Magnesium

Organic Matrices

These encompass a wide variety of materials including oil additives, petroleum matrices, coal, organic plant material, biological material, synthetic organics, etc. Samples can be digested with sulfuric acid and hydrogen peroxide, or sulfuric/nitric/perchloric. For more detailed information about acid digestions of organics, please see the following article: Acid Digestions of Organic Samples.

The sulfuric peroxide procedure is as follows:

1. Add a 0.1 gram sample to a 125 mL Erlenmeyer flask.
2. Add 5 milliliters of 98% sulfuric acid.
3. Heat until dense white fumes form. Sample will be dark in color.
4. Add 30% hydrogen peroxide drop-wise until solution clears.
5. Continue heating until solution remains clear with dense white fumes and does not revert back to a dark color.
6. Remove from heat. Cool and dilute to desired volume with 18 megohm water.

It is also very acceptable to dry ash organic samples for Mg analysis in a Pt crucible and then bring the resulting MgO into solution using a sodium carbonate fusion. For more information, see the portion of our Trace Analysis Guide that discusses Ashing.

The following is a general guide for carbonate fusions:

1. Make certain that the sample is well mixed with the sodium carbonate.
2. A 5-9's pure sodium carbonate is recommended and available from EM Science.
3. Mix the sample with the flux at no more than a 1:20 ratio. Typical sample to flux ratios are in the 1:10 area.
4. If organic matter is present, either the sample is mixed with the flux initially and heated slowly to 500 °C for ~ 2 hours before bring up to full temperature, or the sample can be pre-ashed at 500 °C and then the ash mixed with the flux.
5. Use Pt as the crucible container material.
6. Perform the fusion at 1000 °C in a muffle furnace. Avoid flames since this fusion is difficult to perform in a flame due to the high melting point of the sodium carbonate.
7. Most fusions are complete in 15 minutes and some require up to 45 minutes.
8. Dissolve the fuseate in dilute HCl (1:1).

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