SPE cleanup of soil extract for PAH analysis

[kg5000]

Hello everyone,
I am new to the site and am hoping someone can help me as i desperately need it. My ultimate goal is analysis of soil extracts via epa method 8270 (GC-MS for semivolatiles). Specifically, i am concerned with the epa 16 priority PAHs. Here is an overview of my method.

1. microwave extraction using 25ml acetone.

2. exchange 25ml acetone to 2ml hexane via nitrogen evap.

3. SPE cleanup w/ non bonded silica cartridge (500mg si, 3mL). Precondition with 5ml hexane, load extract, elute with 10mL DCM:Hexane (2:3).

4. Concentrate for injection

My question is concerning the SPE step. Will enough of my interference compounds adhere to the silica? My hope is that my PAHs will pass through the Si leaving aliphatics and other interference compounds behind. I also have C-18 cartidges (1g, 6mL) but from what i understand these are only good for an aqueous matrix. However i believe there would be a way to load my extract (expecting my PAHs to adhere to the C-18 bedding) then elute interference compounds as waste and then elute my analyte.

Additionally, I was given an SPE method (for Si cartridge) which stated that i should precondition with hexane, load hexane extract, elute interference compounds with hexane and then elute PAHs with DCM:Hexane. I tried this and all the compounds came out with the first fraction of elution. That would be expected due to the hydrophilic nature of the Si bedding, right?

I am looking for any and all feedback (especially concerning the selection of my solvents for elution).

Thanks,
Kyle Gray

[Uwe Neue]

Whether your interferences stick to the silica or not depends on the quality of the silica cartridges (especially dryness) and the polarity of these interferences. It is difficult to predict up front how well you will be doing. The procedure that you are planning to use looks fine.

Interferences are often more polar, and a removal of interferences via an prewash with hexane does not sound like a smart idea to me.

It is possible to do a combined cleanup using C18 in conjunction with your standard cleanup. However, this is more work and creates additional complications. I would go with the standard simple procedure first, and use a C18 procedure only if the standard procedure does not work. If you need to go with a C18 procedure, write a note and I can help you with the development.

[kg5000]

Thank you for your reply!

The Prewash w/ hexane is not intended to remove the polar interferences. It is just to condition the bedding prior to loading my extract, correct?

[Uwe Neue]

The prewash is intended to prepare the cartridge for the subsequent loading of your sample. However, all polar interferences will be retained more strongly on the cartridge than your PAHs, and a good equilibration with hexane is important.