Hi,
Conjugating the antibody to the polymer and then loading the metal ion in has some issues.
That was the original protocol for labeling the antibodies: for example, see here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2600572/However, many proteins, including most antibodies, have regions of their structure that can non-specifically bind metal ions. Since they are low-affinity, they often keep "bleeding"/streaking metal ions throughout all of your staining and washes, and running on the instrument. This gives higher background in your sample.
This is one of the main reasons why the procedure was changed to load the polymer first, wash away all excess metal, and only *then* conjugate to the antibody. This way, the antibody never "sees" free metal.
If you do choose to test this, it may be clone-by-clone whether you can do this with low enough background. I would strongly recommend doing the comparison of the "antibody+polymer then load metal" (new) with the current MAXPAR "load metal into polymer then conjugate to antibody" so you have a rigorous test of the success.
I would also strongly recommend including EDTA or citrate into your washes, to hopefully pull off some of the non-specifically bound metal from your antibody.
Mike