I am currently doing research on the adsorption of heavy metal using magnetic reduce graphene oxide. I use AAS to determine the concentration of the heavy metal.

I prepare the Pb solution with an initial concentration of 60 ppm then I run an adsorption test with 30 min interval for screening testing. the dosage of the adsorbent is 1g/L. I run the test one day prior to AAS testing.

The problem is, when I do the AAS testing, the initial concentration of the solution was 30 ppm followed by 28 ppm for the first 30 min, and a negative result at 90 min.

Then I try another testing to see whether the problem is my dilution, and it shows that my dilution is correct. I also try dilute several stock solution to see if the problem is my stock solution but the result suggest my stock solution is ok.

does heavy metal solution degrade over time? or is it become precipitate which makes the concentration lower?

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