Arsenic bound proteins confirmatory test
You may wish to give this suggestion a try, courtesy of the literature on constructing nano-particles. 

First, dissolve the protein samples in say ethanol or other flammable solvents.

Next, place it into an alcohol lamp with a wick composed preferably from glass fiber, but one may be able to employ woven cotton or a cotton string.

Lastly, light the lamp and place a bowl over the lamp to capture/condense escaping particles.

My recollection is that the target metal of interest, is slowly draw through the wick and thermally broken down. Still looking for the source, but here is a related article.

This is clearly relates to the final step in the Marsh Test, where the arsenic presence is distinctive.

Note, this proposed procedure, if successful, avoids the conversion of a sample of a compound into deadly AsH3 gas per a nascent hydrogen pathway, which is perhaps not required here, as you already have constructed (via an alcohol lamp), a path to 'burn' the As(III).