Frequent Questions

Issues with surrogate and target compounds during the soil extraction for herbicides.

We have had continuous issues with surrogate and target compounds during the soil extraction for herbicides. Could you give us any insight into where in the process we are most likely to incur these losses?

 

  • Method 8151A, Section 7.2.2.5 details the solvent exchange from acetone/ether to ether. Ensure that the target pHs are met or exceeded (i.e., more basic or more acidic as applicable), and be prepared to leave a little (<5%) of the aqueous phase behind in order to avoid acetone crossover to the hydrolysis phase.
  • Residual acetone may cause low recovery of some compounds during hydrolysis due to aldol condensation. Aldol condensation is a reversible reaction between two constituents (or sometimes two of the same constituent) that contain aldehyde, ketone, and even acidic alcohol functional groups.  It may occur in acidic or alkaline media, although it proceeds by two different reaction mechanisms.
  • The drying step is very critical to ensuring complete esterification.  Any moisture remaining in the ether will result in low herbicide recoveries.
  • Monitor the pH for at least 15 minutes with stirring, as indicated in the method.  This will help to ensure that the herbicide is in its acid form.
  • Ensure that after addition of acetone, mixing continues for at least 20 minutes.  Acetone needs sufficient interaction time with the matrix to penetrate the pore space of the solid material and solvate both herbicides and water (field moisture).
  • Ensure that at least 75 percent of the extraction solvent is recovered after the third extraction cycle.  There likely will be some herbicide partitioned into retained solvent; with consecutive extractions portioning will be favorable for removal of most of the constituent mass.  Three successive extractions have shown to be good in general.
  • Ensure that during separatory‐funnel extraction the pH of the aqueous phase is less than 2; the herbicide must be in its acid form to partition into the organic extraction solvent efficiently.
  • Following separatory‐funnel extraction, there should be no acetone remaining since the acetone is more soluble in the aqueous phase (polar) than in the organic phase (diethyl ether, non‐polar).
  • Ensure that the final extract is completely dry using anhydrous sodium sulfate prior to blow‐down.  It should be dried as described in Section 7.3.6.  Water will interfere with the derivatization efficiency.
  • Hydrolysis samples should be processed in a water bath at 60‐65°C.  Lower temperatures can cause problems.
  • To check methylation, obtain a reference standard in the acid form and methylate this without extraction.
  • If LCS spikes are in the acid form, check to see if you are losing herbicides in the alkaline hydrolysis step by comparing results with and without the hydrolysis step.
  • Make sure that all glassware, sodium sulfate, and filter media are acid-rinsed.
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