Analytical Tips for Tin

Ben was having trouble analyzing Sn in a sediment sample that had been digested in aqua-regia. He noted that results from ICP-MS did not tie in with results from his ICP-OES. His team had tried different lines on the ICP-OES all with different results (189.927, 235.485, 283.998, and 242.170). Ben was aware of the potential interferences on the ICP-MS for the mass he was using (U++ and MoO) and stated that both elements were not present in the digest. Paul was asked if he knew of any interferences on the aforementioned lines.

	RE: Analytical Tips for Tin Dear Ben,      The 189.926 nm line is the best with respect to sensitivity and freedom from spectral interference. I do not suggest using any of the other Sn lines because the spectral regions are 'dirty' with several elemental interference issues. Since you mentioned the MoO and U++ interference, you must be using the 116 or 118 amu Sn lines. Can you try other Sn lines since other interferences are possible? I'd suggest the 120 amu Sn line.      In addition, Sn is a problem element from two aspects: 1) The SnCl4 has a BP of 114 °C causing a vaporization interference (i.e., more Sn gets into the plasma due to the vapor pressure). This is of concern if there is a significant amount of HCl present and the standards and samples do not have any HF (to fix the Sn) and different ratios of the Sn +2 and +4. 2) The second issue is the very problematic tendency of Sn to hydrolyze "out of solution" forming a semi-collodial suspension. If the particulate is less than 8 microns (depending on nebulizer) you will get varying amounts of Sn in the plasma. The degree of hydrolysis is a function of Sn oxidation state, reagent order of addition, acid content, Sn concentration, and there is the complicating factor of the chemical form of the standard as well as the age of the sample digestate. Again, it is best to 'fix' the Sn by adding small amounts of HF.      May I suggest that you try some different Sn masses first to confirm that there is no interference. If you get two masses to agree and the values are different from the ICP-OES data, then the problem is the Sn chemistry. This chemistry is difficult when Sn is stabilized with HF and nearly impossible when it is not.     Paul R. Gaines, Ph.D. Serving you in chemistry