EDIT* How about you guys get over yourselves rather than putting down a guy who is already frustrated by RNG? I'd probably just quit the game, taking flak* from people who don't even have the right answer? Pffft, not for me.
No need to be patronizing lol, wave scanner isn't going to help him much if he's on a planet with a large material pool isn't he? That might not have occurred to you.
OP, the way material distribution on planets works is not based solely on the percentages on the surface map. That is secondary to the number of elements within a tier. (Common, Normal).
What you want to find is a Planet where Arsenic is one of very few in that tier.
Here's one:
TIMBALDERIS AB 1 D A
Here, Arsenic is the only Common Material. That means every dice roll, if it comes up 'common' will yield Arsenic. All you are fighting against, is the chance for other tiers to spawn, not other mats of the same value.
Armed with that, you should be able to find a planet to suit whatever your needs are at the time.
I hope this made sense to you.