Preparation Method: Coal Sample Digestions

Debby was trying to find the best way to digest coal samples for the analysis of mercury, arsenic, and selenium. Her initial attempted methods led to poor recoveries. Debby asked Paul for his recommendations as she prepared for her next attempt using a microwave digestion approach.

Dear Debby,

The use of closed vessel microwave digestion offers several advantages including the retention of the 'volatile' elements, but the difficulty with coal is that the matrix cannot be decomposed using the 'micro-waveable' acids. Nitric acid will not decompose coal and elements like Hg will be adsorbed to the unattacked aromatic ring system and the teflon vessel causing low recovery.

The method I am familiar with uses 1 gram of sample + 18 mL sulfuric + 15 mL conc. perchloric acid in a glass digestion flask (do not use perchloric acid in a microwave furnace under any circumstances) with condenser. The condensor is needed to contain the volatiles you mentioned. In addition, 10 mg of V is added as either the pentoxide or in solution form as V+5to act as a catalyst (I use 1 mL of the Inorganic Ventures' 10,000 ug/mL V Custom Grade standard which is very pure and the V is in the +5 oxidation state). I have been able to decompose even charcoal and coke using the V+5 catalyst. In addition, Hg does chemiadsorb onto plastic - even in strong acid media - and many losses thought to be due to volatilization are in fact due to loss through adsorption on the container. This is why the digestate must be diluted in HCl. I have found that adsorption of ppb Hg solution is not a problem in either plastic or glass when in an HCl matrix.

Please consult the Acid Digestions of Organic Samples portion of our Trace Analysis Guide. The text goes into further detail on the digestion of organic matrices.

Thank you for your question and best of luck.

Serving you in chemistry,

Paul R. Gaines, Ph.D.
CEO of Inorganic Ventures & Fellow Chemist

DISCLAIMER: Advice offered by the chemists at Inorganic Ventures is intended for the individual posing the question. Feel free to contact us to verify whether these suggestions apply to your unique circumstances.