Anyone have any experience or knowledge in analyzing benzethonium chloride in finished consumer products at levels less than 0.5% with any degree of precision and accuracy? We have HPLCs with varied detectors (including HPLC-MSD) as well as GCs and GCMS.

Although not the same, I have analyzed pancuronium bromide (another quarternary amine) in biological samples at levels around 0.10 mg/100 mL using 5 mL sample--but it was by TLC. You can probably adapt this to HPLC pretty easily:
Silica TLC plate
Developed in 90% ACN/10% 1M Magnesium perchlorate
(the perchlorate is for ion pairing and I don't think that you specifically need magnesium perchlorate, it was just what I had laying around.)

I used Iodoplatinate to develop the spots, as pancuronium bromide has no chromophores. If your compound has chromophores, you can go with UV detection; if not, then MS would probably be good.

This method was adapted from "Analysis of Steroids. XLI. Ion pair HPLC separation of quarternary ammonium steroids on silica" j. chromatogr. 1991 jul 26; 550 (1-2): 639-44
You don't have a steroid there, but this might be somewhere to start method development from.
Best of luck!
Lisa

Check this out!!!

Journal of Chromatography, 469 (1989)317-328
Determination of Preservatives in Cosmetic Products II High-Performance Liquid Chromatography

Elsevier Science

This method worked well for me.

I've used reversed phase LC with an ion pairing agent to yield a reasonably sensitive assay for benzalkonium chloride with PDA detection. For details, contact me at juddman@peconic,net