Acid Digestion for Metals Testing and RoHS/WEEE Compliance
The choice of acid digestions for metals is dictated by the sample type. For non-metallic samples, total dissolution is generally not achieved. This is the result of many samples consisting of materials relatively inert to nitric and hydrochloric acids, or aqua regia (combination of the two). These materials include most plastics, silicon, and carbon-based samples. However, by achieving a reasonably fine grind on the samples, a significant amount of surface area is exposed. Hot, concentrated acids then provide an aggressive leach of the material so the results generated meet the purpose of the regulation. Of particular importance is that the inert materials generally are not the components that would be expected to potentially contain lead, cadmium, mercury, and hexavalent chromium.
Samples that contain metallic components or are themselves a metallic material can usually be completely dissolved. Silver and gold components are sometimes present, which requires special treatment to achieve dissolution (e.g. dilute nitric for silver; aqua regia for gold). Other metallic components (e.g. solder, micro circuitry, etc.) are readily attacked using aqua regia digestions. For most samples, an aqua regia digestion performed under elevated temperature and pressure in a sealed Teflon bomb is appropriate.
If the sample is digested using the closed vessel approach, Mercury is included through that point. A portion of the digestate is then split and sent for separate handling and further digestion prior to instrumental analysis for Mercury. The other portion of the digestate is ready for Lead, Cadmium, and Total Chromium analysis using the appropriate instrumental approach.
Note: The standard approach at ALS Environmental is to analyze for Total Chromium first. If the Total Chromium concentration is below the action limit, then no further determination is necessary (i.e. Hexavalent Chromium cannot be greater than Total Chromium). If Total Chromium is detected near the action limit or above it, then a re-analysis of the sample is performed using an alkaline digestion followed by a colorimetric determination for Hexavalent Chromium.