Iron Oxide Pigment Analysis by ICP

William writes...
I have an iron oxide pigment sample which has to be analyzed for Pb and As by ICP. The levels of those two are expected to be less than 10 ppm. How would I go about doing this?

Dear William,

This is a tough one. The pigment is most likely an ignited oxide and is very resistant to acid dissolution.

First, you'll want to "open out" the structure. I like the use of sodium carbonate for this purpose. You mix a 2:1 ratio of high purity (5-9s) sodium carbonate (1:1 may also work just as well) with the pigment-mix as completely as possible. Then heat the sample to 1000 °C in a muffle furnace for 15 minutes. In this case, the opening out procedure does not result in a genuine melt but rather a sintered lump which should subsequently dissolve very quickly. Please note that As loss has been reported in some carbonate fusions. These reports may not be valid due to the difficulty of making an As measurement, etc. Method validation through spike recovery (or analysis of a NIST SRM if available) would be necessary.

The measurement of the Pb and As by ICP-OES can be attempted, but I do not give it much of a chance — too much of a spectrum from the Fe. I would prefer you dissolve the melt in dilute nitric and measure the As and Pb using ICP-MS or GFAA. The MS detection limits should be < 1 ppb (in the ppt range) but be careful of ion quenching, i.e., dilute the fused sample (assuming 1 gram sample + 2 grams carbonate) up to 1000 gram with DI water in leached LDPE bottle (preleached with nitric). If the Pb is 1 ppm in the spl then it will be 1 ppb in the sample solution. Confirm absence of severe sensitivity loss due to ion quenching through analysis of synthetic or known blank pigment. The GFAA alternate may be easier in the final analysis, especially if the MS acts up.

Please feel free to call with any questions and the best of luck to you.

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.