Dear So'aib,

Your outcome is quite common in such situations.  What is the other 20% in your system, I assume water?  In any event I would guess that some water will come from the matrix during the extraction.    The oil / wax that you see would probably be oils from the sample matrix and some compounds that hold on to water.

Probelm is that to get a good stable dry mass of your extract you need a large surface area over which to dry the extract.  Problem, you extract will contain some oily and / or waxy elements.   The solution will really depend on what kind of equipment you have available, ultimately the sample is not well dissolved in 20% water and no methanol, therefore I would suggest adding 50% tertiary butanol or 1,4-dioxane to the mixture, and then vacuum evaporating only the methanol.  Freeze dry the remaining mixture of water and solvent.  T-BuOH and 1,4-Dioxane are typically polar enough to help keep things in solution but freeze dry beautifully.

Kind regards,

Rob

Thank you for the comprehensive answer. Yes, you were right 20% is water. I also attempted another method;  MeOH  extract concentrate (after vacuum evap) was resuspended in ultrapure water and acidified to pH 1.5. Later, the mixture was subjected to LLE using EtOAc twice & vacuum evaporated. Finally EtOAc was purged with N2 gas. This time I got some moist residue (but unlike the thick wax using 80% MeOH alone). I understand this procedure is meant for free phenolic acid analysis, but does the dry residue valid for quantification of total phenol?

Be careful - when removing anything from your 80% methanol extract you might change the mass to which you want to refer to and thereby get false results.

I would suggest to determine the dry weight of an aliquot gravimetrically: heat a portion of the extract to 120° C until the weight becomes constant (usually 2 hours) and see how much weight is lost. The extract has to be homogenous and youAdd your answer should weigh the sample for determination of total phenolics at the same time.