How is mercury in drinking water detected? [closed]








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A document I've been reading states that:



> 
> Both the Maximum Contaminant Level Goal and the Maximum Contaminant Level were set at 2 parts per billion because current technology allows public water suppliers to detect and remove mercury levels that low.
> 
> 
> 


But I know of drinking water test strips that detect 0, 50, 100, ... 1000 ppb and are 1,5-Diphenylcarbazide based.


On a chem supplier catalog I've found that 1,5-Diphenylcarbazide can detect up to 0,000002 g/ml, which is 2000 ppb. But I know from a question on this website that the above reagent can detect up to 250 ppb.


And as per urine and blood concentrations we are looking at 10 ng/ml they detect.


So what is the threshold for 1,5-Diphenylcarbazide test and how they detect those minute amounts anyway? 


Is there a DIY method of detecting something like 2 ppb at home?


P.S. I am talking about Hg2+ ions, if it is not clear


