Hey kad,
The best would be to do it on impure platinum sponge (after precipitating Pt with NH4Cl and reducing it to sponge),although it can also be performed on whole pgm blacks but that would be inefficient.Regarding the weight of individual metals in the specimen,its not mentioned in the published article of Gilchrist and Wichers.
And removing Pd from mixed bag through Nitric is also severely inefficient,there are ample other options to do the task with minimal hassle.
Hint
rop Pt and Pd through orthodox methods and then go after specialized tools to get high purity.
By the way nice to hear from u after so long,whats going on lately?