I have tried some acid mixtures to digest, in a microwave digestion system, the graphite powder, but I couldn't completely digest it without the use of nitric acid and since I try to measure the nitrogen content I can not use this acid in the mixture.
See Watanabe and Narukawa, 2000, Analyst, 125:1189-1191.
0.25 g or less graphite, 5 mL each nitric and sulfuric acids.heated for 20 min at 60% microwave power, followed by a period of standing (for 1 min) and the addition of 4 ml of nitric acid. The sample was re-heated for 10 min at 65% power, then left for 1 min, after which 4 ml of nitric acid were added. Five more cycles were carried out with 10 min heating at 65% power, 1 min standing and the addition of 4 ml of nitric acid.
There are simpler ways with higher temperature, pressure.
See Inorganic Ventures:
http://www.inorganicventures.com/advice/dissolving-graphite-measure-icp
A sulfuric acid/perchloric acid mix has also been used, but one must be aware of explosion hazards when concentrating perchloric acid; and a perchloric acid hood should be used.
Camelia
Since you want to measure nitrogen impurity in your graphite powder, you cannot use aqua-regia to digest your sample. If you have access to XPS or ToF-SIMS, that could tell you a lot about your sample. Stick some powder on kapton or carbon tape and try analyzing your sample.
Also, ICP could be an interesting option if you can suspend the powder in a suitable solvent. please make sure that the size of your graphite particles is less than the ICP orifice. I think a 0.5 um particle size would be OK, but its better to check the instrument specification. Also, keep your suspension shaking as you feed the ICP so that the powder doesn't aggregate.
For detemination of N content in a graphite, elemental analysis indeed works quite well, and therefore the process is based on combustion, not on digestion. As graphite is not easily burnt, a catalyst must be added, usually WO3 in equal amount as graphite. It is very important to add it, otherwise graphite combustion will be incomplete, and therefore the measured amount of N2 will be wrong.
Alain
Reporting to Huang et al. (*) graphite can be oxidized to graphite oxide prior to elemental analysis with a mixture of concentrated H2SO4 and H3PO4 (4:1 v/v) to which KMnO4 (4.5 g / 100 ml) is added. It can take about 3 days at room temperature to complete the oxidation process.
(*) N. M. Huang, H. N. Lim, C. H. Chia, M. A. Yarmo, and M. R. Muhamad, "Simple room-temperature preparation of high-yield large-area graphene oxide", Int. J. Nanomedicine, Vol. 6, 2011, 3443–3448.
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