Overcoming Au Stability Issues

Peter had Au stability problems when diluting one of our multielement standards into calibration solutions with an HNO₃ matrix. Most of the elements showed good calibration curves with ICP-MS, but Au often had too low intensities. He also found that after measuring his calibration solutions different times, the Au concentration increased. Peter wanted to know how he could overcome these Au stability problems.

	RE: Overcoming Au Stability Issues Dear Peter, Your question is a very good one. The problem is with the stability of Au in nitric acid media. We have found that there are only two elements that are not compatible with a 1% nitric acid matrix in a PE container at ppb concentration levels -- Au and Hg. Both elements chemi-adsorb on the container walls and are not stable for a 24 hour time period (refer to Part 7 of Reliable Measurements Guide, Stability of Elements at ppb Concentration Levels. As the concentration of the Au increases, the relative stability improves to the point that a 100 ppb solution can be used the next day but will not be stable for 72 hours. The stability continues to improve as the Au concentration increases to the point that a 1000 ppm Au solution in nitric acid will be chemically stable for more than 12 months. It appears that the surface of a PE container can take up to ~ 100 micrograms of Au or Hg before adsorption ceases (this is not a number that we have validated, but rather have deduced from stability studies - in fact the loss through chemi-adsorption is an assumption on my part at this time). The only solution to this problem that we have found thus far is to use an HCl matrix for both Au and Hg. The use of 1% v/v HCl solution rather than nitric acid should eliminate this problem. We are currently using a 10% v/v HCl matrix for ppb Au and Hg but we believe this is far more than needed. We have not yet initiated studies at lower HCl levels. Please feel encouraged to contact me with any further discussion on this or any related topic(s).     Paul R. Gaines, Ph.D. Serving you in chemistry