How are metal samples prepared for spectrometric analysis?

Preparing metal samples for spectrometric analysis involves several physical and chemical processing steps to ensure the test portion is representative and suitable for instrumentation.1

Physical Preparation

For monolithic metal solids, the surface must often be cleaned by grinding off oxides or dissolving them with acid. Depending on the specific spectrometric technique, the sample may be cut to a required size, or processed into millings or drillings. In some forms of emission spectroscopy, a portion of the metal itself may serve directly as an electrode for the electric discharge.12

Chemical Dissolution

When the analysis requires the metal to be in a liquid state, it must be converted into an acidic aqueous solution. This is typically achieved using mineral acids:3
* Nonoxidizing acids (e.g., hydrochloric acid) are used to avoid the formation of protective oxide films.3
* Oxidizing acids (e.g., nitric acid) are effective for dissolving many metals, though they may create passive films on others.3
* Acid mixtures, such as aqua regia, are employed for resistant metals like platinum, palladium, and gold.3

If a sample is highly acid-resistant, molten salt fusion may be used, where the ground metal is heated with a flux in a crucible until molten, then dissolved.3