Solubility of Gold in low concentrations When preparing ICP-MS calibration standards for trace metal (~100 ppt) analysis, the values for Gold do not remain stable for more than a day. The standards are prepared using commercially available 10 ppm solutions in Nitric Acid with traces of HF. 

A known amount of the Gold standard is added to 2 mL of 70% ultra pure Nitric Acid, an Indium Internal Standard and diluted to a pre-determined volume with ultra pure deionized water. The samples being analyzed are in 49% hydrofluoric acid to which has been added 2 mL of 70% ultra pure Nitric Acid, an Indium Internal Standard and diluted with an equal volume of ultra pure deionized water. 

Would preparing matrix-matched calibration standards solve this problem?
Whenever you prepare very dilute working standards, it's good practice to do a stability study to see how long you can store them. There are many factors that can influence the stability of these solutions. The choice of container can make a big difference. Dilute metal solutions should be stored in polyethylene or polypropylene containers, not glass. Another well-known problem is photoreduction. Gold solutions should be stored in opaque containers and be exposed to light as little as possible. Some solutions may also be sensitive to atmospheric exposure. (though I don't think it's an issue for gold)

I don't have experience with working with gold in hydrofluoric acid, but such a large change in such a short period of time in the absence of the above factors may indicate some actual reactivity with your matrix, for which matrix-matching might help, though you may be better off reformulating your solutions if that's the case. It may also be an option to just make fresh working standards very frequently or increase storage life through refrigeration.