SPE extraction of hand soap containing Benzalkonium Chloride; has anyone else had dealt with this before?

I'm developing a method to quantitate BAC in a hand soap product of ours (I work in cosmetics), however I'm finding it extraordinarily difficult as the formulation only has 0.15% BAC in it, which makes pulling it out of the sample very very difficult. It's to the point where I can barely distinguish between which peaks are the homologues of BAC and which are background. As for method parameters, I'm basically following the USP until I have something more refined. I can generate a pretty solid calibration curve (r^2 = 0.99970) and do raw material assays at 50%, but any sort of extraction from product is basically impossible. I've been thinking SPE might be the way to go, but I don't have much experience in this arena.

Any and all advice would be greatly appreciated. 

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