Chemical Stability and Compatibility - Stability of Ta and Nb upon dilution

Depending upon the final diluted concentrations it is possible that 2% HNO3 only is not capable of stabilizing the Nb and Ta. We have observed low ppb levels of Ta are not stable long term (e.g., weeks) when diluted solely in 2% (v/v) HNO3, and we would recommend increasing the HNO3 content to 5% as it could help with this issue. If that does not solve the problem, then adding trace HF (0.1% v/v or less) would likely solve the issue. It is also possible that aggressively cleaning the preparation containers could help stabilize the diluted standards, as we have observed that <100 ppb mixed element solutions often benefit from storage in pre-cleaned containers. In that regard, our recommendation would be to leach the labware in 5% (v/v) HNO3 at 60 degrees Celcius for at least 60 hours followed by extensive rinsing. It may be that a combination of higher HNO3 content (5% vs. 2%) in conjunction with acid-leached preparation containers would solve the issue, at least to the extent of allowing daily measurements to proceed.