Make sure your DI water is at room temperature and acidified before the extraction. The stearic acid can fall out and it won't go back into solution if it hits the cold water.

Hi Felix. Also check the temperature at which you are evaporating your hexane. Too high of a temperature can result in the volatilization of the hexadecane in your standard. Somewhere around 65 C is recommended.

I'm getting low recoveries on my 7 mg/l matrix spike. I notice the orange gaskets are wet although the sample isn't leaking. Could this be the reason? Also are there any matrix interferences you are aware of?

Thanks

I would certainly double check your clamp on the sample reservoir and insure that it is creating a tight seal. Don't push the clamp all the way against the glassware--this will cause a slight tilt to the reservoir that can lead to leaks.
Also remember that the 7 mg/L spike is intended for your MDL studies. When you run these are you getting consistent results or are they all over the board? You should be getting consistent results to demonstrate proper technique. If you can't get consistent results for your MDL calculation with the 7 mg/L standard you might try the 10 mg/L standard G3026.
If you are looking to spike a sample or to run an LCS with your batch use a higher concentration standard such as the 40 mg/L G3025.