What's the Point of Introducing C18E in D-SPE?

Some samples (such as avocado, peanuts, olive oil, etc.) contain more fat. Due to the limited solubility of fat in acetonitrile, the recovery rate of some fat-soluble pesticides (such as HCB, DDT, etc.) is reduced.
How to overcome the low experimental results caused by fat?
You can remove the fat following below 2 methods.
1. Freeze the fat, and put the extractor purified sample in the freezer for more than 1h (or freeze overnight)
2. Remove fat by reversed-phase adsorbent adsorption, for example, add C18E or C8 adsorbent to the extract.
If the target analyte contains pesticides with very non-polar pesticides (such as HCB, DDT, etc.), the presence of fat will lead to differences in the extraction efficiency of the internal standard TPP and such pesticides which have low result due to high loss. For this phenomenon, it can be avoided by delaying the internal standard to the pre-analytical step. At the same time, the quality control internal standard (PCB138@Kow 6.83 or PCB153@Kow 7.75) can be added to verify the loss of such pesticides during the whole process. If the quality control internal standard recovery rate is above 70%, it means that there is no obvious loss of such pesticides