What should if I do if my sample is particularly dirty?

Stuff a little glass wool in your cartridge before you begin.

When I performed oil and grease liquid/liquid extraction (Method 1664A) I used a filter paper funnel to hold my sodium sulfate. Now I am told that I can’t do that when performing 1664 by SPE. Why is that?

When performing 1664 using large volumes of solvent, the solvent rinses the oil and grease standard through the filter paper. With SPE, much less solvent is used and the standard becomes trapped in the paper giving low recoveries. Use either glass wool or a sintered frit to hold your sodium sulfate.