Blood tests - Serum, Plasma or Whole

Trying to piece together the Copper / Zinc levels and ratio to establish which is either to supplement. However the picture is somewhat confusing when it comes to which part of blood to test.

Some labs test Serum Copper and Plasma Zinc, others do both Plasma Copper and Zinc, while yet others do both Whole blood Copper and Zinc! Then there is Magnesium which is normally done as RBC Magnesium. Why are Copper and Zinc not measured from RBC?

Can anyone help me to understand which is likely to return the best results RBC, Plasma, Serium or Whole blood.

Thanks,