Don't know about your sample but I'd suggest alkaline fusion followed by dissolution in nitric acid. You'll however have to find the right flux and very probably check if you need to pre-oxidize your metals first. You'll also probably want to avoid Pt-Au crucibles in your case.

I suggest you contact your local representative of Claisse-Panalytical or Katanax to discuss this with them (fluxers suppliers).

Check this :https://www.azom.com/article.aspx?ArticleID=11593

Dear Pierre Caulet,

Thank You for your suggestion. Alkaline fusion seems the right choice for my sample preparation (PGM on ceramic monolithic substrate).

What about try acid digestion?. Normally people to determine the metals by ICP-MS using HCl:H2SO4: HClO4.