Ashing Technique for Metals

Jason was trying to determine the presence of seven metals (Cr, Cu, Fe, Mg, Mn, Mo, and Ni) in solid raw material samples, dextrose and polyvinyl alcohol (PVA), using ICP-OES. He was analyzing a 5% dextrose solution and a 1% PVA solution in 1% nitric acid. These solutions resulted in metal content below the method's LOD. Jason tried wet ashing the samples to increase LOD, but the spike/recovery yielded poor results. He wondered if Paul could recommend an ashing technique for the seven metals in these samples.

Dear Jason,

The analytical problem you have is not simple. Try ashing several grams of sample in a clear quartz crucible after sulfation of the char using a few drops of concentrated sulfuric acid. This is done by first charring the sample on a hot plate. If you have never done a char and/or ignition, you may want to contact me. It is very important to keep the ashing temperature low (~450 °C) and the time as short as possible to prevent formation of the oxides of Cr, Fe, and Ni. These oxides can be very difficult to dissolve. Sulfation of the char to form the metal sulfates is an important step. Use a 1:1:2 mixture of HCl:HNO3:H2O to dissolve the ash.

It is likely that the low recoveries were due to incomplete ash dissolution. Hopefully the formation of the metal sulfates along with the lower ashing temperature and time will give good recoveries.

Please feel free to contact me if you have additional questions.

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.