When using hexane on a water sample to extract oil and grease-what techniques have the best recovery rate?

Question

why hexane? the epa requires it.

Answer 1

The best way is the one required in the EPA method, because that is the ONLY way that the numbers can be accepted.  NAy alternatives must be listed in the method or you have to go through hoops to get them accepted.  Hexane is used because anything that works better has been banned due to environmental concerns (freons, methylene chloride, etc).

Answer 2

EPA requires it because hexane is pretty clean and its relatively non-toxic.  

Recover rate depends on many factors.  In general, multiple extractions usually work better than single large volume extractions.  Meaning, rather than extracting with 60mL of hexane over your sample, using 20mL of hexane and extract samples 3 times usually works better.

Also, the volume of hexane compared to the volume of sample/water is important as well.  If you have a 100mL sample, extracting with 20mL of hexane 3 times probably won't give you a good recovery.  I would use the same volume as the sample to extract or more.

Next, what are other processes involved to determine the recovery rate? Is there a drying step involved?  Sometimes the drying step can lead to sample loss instead of the hexane extraction step.  Rinsing the wall of the glassware during drying step prevents compound from sticking to the glassware.  A few drops of decanol can prevent volatile compounds from evaporating etc.

Answer 3

The old method used a seperatory funnel and the sample was shaken with several aliquots of hexane,   Each successive aliquot had less to remove.  

Hexane is used because of its low affinity and solubility in water,  oils and grease are very soluble in the hexane, and because it is volatile and easy to remove after the oil and grease is extracted