We have an ICP at work and I've had the guy that runs it check my tank water. We only looked at Mg, Ca, P, Na and Cu. Mg, Ca, Na & P were in line with what my Red Sea tests and refractometer came back with, but Cu was a no go because of the Na interference.
He said he could give me ppm, but ppb was out of the question. We just purchased an ICP-MS, but I haven't asked him to run it yet. He's still working on method development for the lab so I don't want to throw something personal on his plate.
I'm not sure what else companies test for when you send it out, but that's just what I asked for when I gave him the sample. I also typically grab his expired standards, dilute them and then run them with my test kits at home. We can't use them expired in the lab, but for my use they are fine.
You might be better off buying a standard and running it against your tests at home. I think that would provide much more value than sending it out unless you're looking for some off the wall element. We pay $56 for a single element, 250mL, 1000ppm standard. But you'd also need a volumetric flask and pipet so you could dilute it to a usable concentration. Our supplier can do custom standards, but I'm not sure what they cost.
Maybe I'm on to something here and should start selling reference standards for aquarium test kit verification.
