If you don't want to go down ICP rabbit hole, the plasma can ONLY count elements, not compounds. If the plasma sees a spike in the nitrogen range, it has no idea where that nitrogen came from - was it n2, no2, no3, nitrogen in organics, ammonia, ammonium, etc.?
Something like salinity is likely done with a hydrometer or conductivity meter. Compounds like no3, po4, ammonia, etc. are done with color changing indicators and light. Alk done with titration. Calcium can also be done with a conductivity meter, I think, but most likely use titration. Most companies will not even tell you how that they test this - Oceanmo does. My guess is that they use salifert or red sea test kits, or at least the same reagents in titration.
Elements that are in compound form cannot be separated from useful vs harmful forms. The iodine group is easiest to understand where there are many different forms in every tank yet only iodate is the only one that matters to us. There is no way to know if i2, iodate, iodide, organic iodine, etc. dominates or is deficient. Iodine (the element) was coming back in ICP test kits and companies were suggesting supplementing. In the early days of ICP people were harming their tanks by dosing too much since the tests cannot give an actionable result.
Lastly, things can change in the vials when they are shipped. Something like organic phosphorous can turn into po4. There are many other chemical interactions that can change in transit.
In the end, I don't ever see a need to use ICP ever again. I do change water, so I can use a good salt like IO and not worry about adding too many elements. My Calcium Reactor gets a lot of the major traces if I use natural media. This has worked for a few decades.
If you read through any of these threads, Cristoph will openly discuss how often the machines need calibrated, how certain peaks in the plasma readings can give errors, etc. He is great to pay attention to.
As for traces, don't forget that your fish food probably has all that you need. If you feed your fish well, then you are probably set. If you are fish less, then this is a whole different discussion for another time.
IMO, I think that ICP is a waste of money and does more harm than good for the hobby. However, the buy a lot of booths at shows and advertising on web sites.
