Does anyone one have a simple chromatographic method (HPLC or GC) for analysing quaternary ammonium chlorides?

Here is one method. There is also a method in USP for benzalkonium chloride. In addition, the titration for these can be fairly simple. These can also be fairly easily titrated using methods found in USP for Benzethonium chloride and also for benzalkonium chloride. There is also a titration using standardized sodium lauryl sulfate in a two-phase titration. It is called the epton titration. The end point is when the water phase turns the LIGHTEST shade of purple.

Benzalkonium Chloride
Mobile Phase Preparation
Hexane Sulfonic Acid solution - 1g of 1-hexane sulfonic acid dissolved in 800 mL of deionized water and 2 mL of phosphoric acid . The volume is brought to 1000 mL with deionized water.

The mobile phase is a 50:50 mixture of hexane sulfonic acid solution and acetonitrile.

Standard Preparation
The standard was prepared by diluting benzalkonium chloride in 100 mL acetonitrile. Dilute as appropriate in mobile phase.

Sample Preparation
Prepared as necessary in mobile phase.

Chromatographic Conditions
Column - Zorbax Rx - C8 - 25 cm x 4.6 mm
Temperature - Ambient
Flow Rate - 1.0 mL/minute
Detector - UV at 255 nm
Injection Volume - 20 microliters

Good Luck

What quaternary ammonium ions are you looking for? Quats may also be analyzed by cation exchange chromatography, depending on the application.