Nifty! I find this very intriguing. I'm going to just spit out a brainstorm here. (and random questions) I'm not a chemist however so I'm working on ignorance here (it's never stopped me before
I would send a sample of the water column itself to provide a comparison between the water and the DSB
I would say the metals test as well as... ack... brain fart... what's the gas bubbles that build up, sulfar dioxide? see if thats remaining anywhere in substrate itself.
Can you do a general toxins test?
How are you planing on taking the samples?
I would think you would want to be able to take a sample without introducing much of the water column into the DSB
How deep is your DSB
? Is it possible, would it even make a difference, to take a sample that maintained the structure of the sample? In other words the sand on top remains on the top of the sample and the sand on the bottom remains on the bottom of the sample? It could provide additional info.
I would take multiple samples from the DSB
in different locations in the tank. Water flow over the substrate is commonly believed to be important to a healthy substrate. If you have various flow rates over your substrate your tests may help prove or disprove this.
I'm looking forward to seeing what results we get. And if it would help would be willing to take and ship a few samples myself (whats this going to cost?). What kind of lab will you be sending them to?
sorry for the rambling... like I said, brainstorming.