How did you condition the cartridge? How did you run the solid phase extraction? How are you determining the amount of material recovered?

If the > than 100% recovery was measured gravimetrically, the extra mass could be solvent which hasn't evaporated or is trapped within the extracted material. Try drying under vacuum and see if the mass changes. There could be salts from any buffers used in the extraction- washing with plain water for longer and using no buffers (or use volatile buffers) will avoid this problem.  There is a possibility of C18 material that came off, simply because the SPE cartridges are dry-packed and the act of packing causes some fine powders; Wash the cartridge with methanol followed by diethyl ether, followed by methanol again, followed by the conditioning procedure you use will alleviate this problem. The methanol wash is simply to have an intermediate solvent miscible with water. Depending on where the vitamin E sample is coming from, there may be traces of oil, or other non-polar compounds  in the sample too check with HPLC.

Contrary to a comment above, diethyl ether is compatible with C18. The technique, going from a polar organic solvent to a less polar organic solvent on C18, is called non-aqueous reverse phase. It is very useful for purifying non-polar compounds. Here are some references to the technique (although all are some sort of column chromatography, it shows that C18 is compatible with wholly organic solvents- the C18 is chemically bonded to the silica and these solvents won't hydrolyze the silyl ether that holds the C18 and any end-capping in place) :

http://pubs.acs.org/doi/pdf/10.1021/ac00253a021

http://www.sciencedirect.com/science/article/pii/S002196730092332X

http://www.isco.com/WebProductFiles/Applications/101/Application_Notes/AN58_Non-aqueous_Reverse_Phase_with_RediSep_Rf_Gold_C18.pdf which uses flash columns, similar in some ways to SPE cartridges, including being dry-packed.

If the % recovery is being determined by HPLC, check your calibration curve against known concentrations. Also, try to extract a "spiked" sample and check your recovery. The sample should be in the same matrix as whatever you are attempting to recover the vitamin E from.

Hi, Do you use HPLC DAD  for determination of Vitamin E? If yes,  Have you checked  the identity and purity of the target Vit. E's peak in sample? Recovery >100 % so it means that your peak might be not pure (could be contaminated with other compound).

Please note that diethyl ether should not be when you have used C18 sorbent. C 18 will get soluble in diethyl ether. Hence your product will be >100%.

How did you condition the cartridge? How did you run the solid phase extraction? How are you determining the amount of material recovered?

If the > than 100% recovery was measured gravimetrically, the extra mass could be solvent which hasn't evaporated or is trapped within the extracted material. Try drying under vacuum and see if the mass changes. There could be salts from any buffers used in the extraction- washing with plain water for longer and using no buffers (or use volatile buffers) will avoid this problem.  There is a possibility of C18 material that came off, simply because the SPE cartridges are dry-packed and the act of packing causes some fine powders; Wash the cartridge with methanol followed by diethyl ether, followed by methanol again, followed by the conditioning procedure you use will alleviate this problem. The methanol wash is simply to have an intermediate solvent miscible with water. Depending on where the vitamin E sample is coming from, there may be traces of oil, or other non-polar compounds  in the sample too check with HPLC.

Contrary to a comment above, diethyl ether is compatible with C18. The technique, going from a polar organic solvent to a less polar organic solvent on C18, is called non-aqueous reverse phase. It is very useful for purifying non-polar compounds. Here are some references to the technique (although all are some sort of column chromatography, it shows that C18 is compatible with wholly organic solvents- the C18 is chemically bonded to the silica and these solvents won't hydrolyze the silyl ether that holds the C18 and any end-capping in place) :

http://pubs.acs.org/doi/pdf/10.1021/ac00253a021

http://www.sciencedirect.com/science/article/pii/S002196730092332X

http://www.isco.com/WebProductFiles/Applications/101/Application_Notes/AN58_Non-aqueous_Reverse_Phase_with_RediSep_Rf_Gold_C18.pdf which uses flash columns, similar in some ways to SPE cartridges, including being dry-packed.

If the % recovery is being determined by HPLC, check your calibration curve against known concentrations. Also, try to extract a "spiked" sample and check your recovery. The sample should be in the same matrix as whatever you are attempting to recover the vitamin E from.