This part of our ICP Operations guide provides some suggestions when attempting to work with mercury, gold, silicon, osmium, or sodium.
A common problem with Hg is stability. For an overview of Hg stability please our article entitled Mercury Chemical Stability. In March of 2003, the EPA published a bulletin describing the use of Au to stabilize Hg solutions: Mercury Preservation Techniques. When working at the ppb level we have found that using HCl rather than nitric acid will maintain the stability of Hg+2 solutions in plastic (LDPE) containers.
Another problem with Hg is loss during sample preparation. When performing acid digestions, the use of closed vessel digestion or the use of condensers should be considered. Ashing should be avoided. Only use validated sample preparation procedures.
Here are some additional suggestions when working with mercury:
- The presence of reducing agents in the solution may reduce Hg to the metal causing false high results due to the volatility of the element where the introduction system delivers more Hg to the plasma as a result.
- The use of plastic introduction systems will cause unusually long washout times. Glass is preferred and the use of HCl rather than nitric acid will reduce the washout time.
- The use of nitric acid matrices for ppb Hg determinations by ICP-MS should only be attempted using Au as a stabilizing agent (see above link).
The chemical stability of Au is very similar to that of Hg. The following suggestions may be helpful:
- Nitric acid solutions of Au at the low ppm and ppb levels are not stable. Use HCl matrices.
- Do no use Pt crucibles when ashing samples containing Au. Au will alloy with the Pt.
- When measuring Au in the presence of significantly greater amounts of Pt using ICP-MS, be aware of the resolving capability of your instrument.
The following suggestions are advised when working with silicon:
- Si is a common contaminant. In addition to the obvious use of laboratory glassware, common sources of contamination include silicon oil/grease, plastics containing catalyst residue, and air particulates.
- Sio is easily dissolved using an equal mixture of HF:HNO3:H2O. SiO2 is readily soluble in either HF or NaOH. Regardless of the mode of dissolution, solutions should be stored in plastics known to contain no catalyst residues or that have been leached with dilute HF for 48 hours.
- Exercise caution when heating solutions containing Si and HF. Si may be lost as the volatile H2SiF6 when heated. When water is present H2SiF6 will not form. If you wish to remove Si from the sample then add sulfuric acid and heat in a Pt crucible.
- Silicon dioxide is soluble in caustic media. When acidified it is stable at low ppm levels but will slowly polymerize and precipitate out of solution. Common preparations involve sodium carbonate fusions in Pt crucibles and dissolution of the fuseate with HCl - make sure the ppm level of Si upon dilution is low ppm and the solution is not allowed to sit for extended periods.
- HF (even low ppm levels of HF) containing samples should not be put through glass or quartz introduction systems when Si, B, Na, or Al are analytes of interest.
Keep the following in mind when working with osmium:
- Os should not be exposed to any oxidizing agents to avoid the formation of OsO4. The tetroxide is very volatile and toxic.
- A common mistake is to dilute Os containing solutions with solutions containing nitric acid. Tetroxide formation is slow but will cause false high readings due to the increased amount of the gaseous tetroxide reaching the plasma.
- Only work with Os in HCl containing solutions and use a separate waste container. Check with your safety coordinator or manager before using and attempting to dispose of Os.
- Use glass introduction systems if at all possible when measuring Os. The washout times from plastic introduction systems are slower.
The single most common problem with Na is contamination. Sodium is literally everywhere. Thousands of tons of salt are transferred from the ocean to the air in the form of sub-micron particulates and can travel for hundreds of miles inland. For more information on contamination, please refer to the following sections of our Trace Analysis series: