To correct this problem, try one or both of the following procedures:
 
  1. During the de-fat procedure, do two 10 minute soaks in petroleum ether.  This is particularly helpful with high fat (> than 20%) samples.
  2. It is possible you have a sample which has been highly processed and has bound the fat to the extent the fat is not available to the solvent before the TDF process.  In this case, after running the TDF instrument, remove the bags, place them on the rinse stand and perform the 2 standard acetone rinses.  Immediately after the rinses (no air drying) remove the bags from the stand and soak the filters in fresh pet ether for 10 minutes. Place the bags back on the rinse stand and immediately perform 2 pet ether rinses (just like the acetone rinses).  When the solvent has evaporated, seal the bags and dry them normally.