We currently are running the TCLP analysis of Herbicides. Acetic acid is added to soil samples and then the extract is analysed using the 8151 method. After the final extraction the sample is derivatized to the methyl esters with diazomethane. If enough diazo is added the sample should remain yellow. We form our diazomethane in ether and then add an aliquot to each sample. Our recoveries have been about 60-70%. Even if we add double the amount of diazo the recoveries are about the same and do not remain yellow. I know the acetic acid will also react but I am not sure why the recoveries do not increase. After the TCLP method we use 250ml of the water extract to do the regular 8151 method. If anyone has had some experience with this problem I would appreciate the information.
By H W Mueller on Tuesday, November 28, 2000 - 11:54 pm:
Could it be that the equilibrium
RCO2CH3 + H+ = RCO2H + CH3OH
is taking place after the methylation?
My suggestion: Modify your reaction medium to a more favorable one for diazomethane (for example: Ether with a small amount of methanol in it).
By Donna Ruszaj on Wednesday, November 29, 2000 - 04:38 pm:
The reaction medium that we currently use is ether, isooctane and methanol. The acetic acid is added to simulate acid rain in the TCLP extaction at a high level. I think we loose some during the Herbicide 8151 extractions but it is still present.
By H W Mueller on Thursday, November 30, 2000 - 12:06 am:
It really looks like the acetic acid is the culprit, the concentration of iso-octane is probably too low to bother (the reaction prefers a polar medium). Unless the herbicides are very water soluble it should be easy to get rid of acetic.
By Donna Ruszaj on Friday, December 1, 2000 - 04:02 am:
The first extraction for the Herbicides involves making the water basic, adding NaCl, and extracting with MeCl2. Then the water is re-acidified and extracted with ether. The ether is dried with Na2SO4 and concentrated to 1ml. Then the diazo is added along with Methanol and isooctane. Would you suggest a straight water extraction? Since the Herbicides have some water solubility I'm afraid we may loose a percentage of them also.
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