Tips and Tricks for ICP-MS General Applications

This Information Applies To: Agilent ICP-MS Systems

Issue:

This is a small collection of general application tips and tricks for ICP-MS

Recommendations:

HCl and Polyatomic Interferences

Traditionally, HNO3 alone was preferred to avoid polyatomic interferences in ICP-MS without CRC. However, the lack of Cl- ion caused numerous problems with several elements. Therefore, we recommend using HCl in CRC system when you have:

• Poor extraction efficiency during preparation (e.g., Sn in soil extracts).
• Poor stability for many elements (Hg, Sn, Mo, W, Ag, As, Se, PGMs, REEs).
• Poor stability of many elements in standard solutions due to lack of co-existing ions/ligands.
• Poor washout characteristics.

Where possible, adding some HCl to the samples (0.5% minimum) may help with these cases.

Sample Container:

• Where possible, use PP or PE (low density) for containers, avoid glass.
• Dispose after use.
• Polypropylene is better for long-term storage as it is less permeable.
• Fluorinated polymers are an excellent choice as they are very clean.

HF use:

• Please use the PFA kit (PFA spray chamber, PFA nebulizer, platinum cones).

Washout solutions:

• Rinse port (HNO3)
• Basic rinse (H2O, H2O2, NH4OH, EDTA, Triton X) stock solution
• Acid mix (10% HCl, HNO3, 0.01% HF)
• Acid matrix in sample

Solving problems with Ag and Pb analyses:

• Though solubility rules say that some metals (Ag, Pb, Hg) precipitate as chlorides, due to presence of chloride ions in water. In water, both Ag and Pb would indeed precipitate as chlorides. The situation is different in acid. AgNO3 is water soluble, but Ag+ is easily reduced to Ag(metal). The reduction is acid catalyzed or photo catalyzed, so nitric acid alone is not good for Ag.
• Though adding a little HCl may precipitate Ag+ as chloride, adding more brings it back into solution as [AgCl2]- or higher Cl- complexes, depending on the Cl- concentration. By having Ag complexed with HCl as a source of Cl-, you shift the equilibrium away from free Ag+ ion to silver chloro-complexes. This slows the rate of silver loss by reduction.
• Pb is less of a problem. It is fine in diluted nitric acid. Since you will have Pb in acid, the water solubility rules again do not apply unless Pb and Ag concentrations are way above solubility levels anyway, typically not the case for analytical work. At any HCl concentration above 0.005 M, Pb2+ forms a soluble anionic complex.
• Something that might work even better than HCl in a final dilution is getting your nitric acid down to 1% or lower with 0.1% cysteine. This acid-soluble sulfur ligand will grab those soft acid cation species (Pb and Ag) and hold them in solution. The problem is that higher nitric acid concentrations will oxidize the cysteine at a rate dependent on nitric acid concentrations. Immediately after dissolution, cysteine will be oxidized to cystine (the disulfide bonded dimer). Prolonged solution in strong oxidizer like nitric acid will take the oxidation further, diminishing its chelating power. That is why less than 1% nitric acid is good when using cysteine. So, it is not recommended to add cysteine before final dilution, or to the undiluted digest.

Keywords: ICPMS, SQ, QQQ, 7500, 7700, 7800, 7900, 8800, 8900