I am working on an SPE method for organochlorine pesticides for EPA method 608.3. All of my compounds have great recoveries except for aldrin and heptachlor (about 45% recovery).

For reference I am using a Promochrom SPE-03 unit and Biotage High Capacity C18 SPE disks.

My extraction procedure is as follows:
Condition Disks
• Hexane>DCM>MeOH>Reagent water (pH 7)
• Load Sample at 25 mL/min
• Rinse 3x with 10 mL of reagent water
• Dry disks for 15 minutes at 5 L/min
Elute Disks
• Elute 3x with 15 mL of 50:50 DCM:EtOAc
• Elute 3x with 15 mL of 10:90 DCM:Hexane
Then I dry and concentrate the extracts as I would with a LLE.

I increased the nonpolar solvents in the extraction to try and pull the compounds of the disk, went from just DCM and EtOAc, to adding the 10:90 DCM:Hexane. I also increased the soak time of the disks from 1 minute to 3 minutes without any improvement.

Any ideas on how I can boost the recoveries of aldrin and heptachlor?