I have been running into a very strange analytical problem with a batch of sediment samples:
- I run a perfect calibration, beautiful OPRs, run some samples without any issues.
- When I run the 1st "problem" sample I get an Hg0 and HgII peak but no MeHg
- All the subsequent samples/OPRs run after the "problem" sample come out as flat lines.
- The next day the instrument runs fine.
PS: the sediment was distilled prior to ethylation and analysis, I am using EPA 1630 with H2SO4 digestion.
Has anyone ever encountered a similar problem?
Hi Amina, does it look like the Hg(II) and Hg0 get bigger (i.e., demethylation), or is the MMHg just missing?
The 1st problem sample has no MeHg but large HgII, then flat lines for everything (including OPRs). Methanol/KOH extraction (instead of distillation) solves that problem, but I still would like to know what is going on.
Interesting...thanks for the info on dist vs methanol. were these sulfidic-y?
Dear Emina, may you check the behavior of MeHg in sediment? Because sing EPA 1630 with H2SO4 digestion it can't work for those three species in the same time. Try to dissolved 250 mg of sediment in the mixture of 2 mL of concentrated HNO3 and 1 mL of 30% H2O2 prior microwave digestion. Because using this procedure i got perfect results
I think I have my explanation. There are some reddish granules in some of the sediment. I have noticed them while cutting the cores, tube worms seemed to use them to build their casts. I now suspect that these are pyrite granules (with oxidized outer layer). Pyrite granules + sulfuric acid during distillation = many headaches. Also explains why Methanol/KOH worked.
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