I have been running EPA method 548.1 for over 5 years, and I am trying to find two more labs interested in performing an ATP study to get EPA approval to change the extraction cartridge. I have tested a couple different cartridges that are cheaper (1/2 the price of the biorex5 or the Anion exchange disk), easier to use (only two conditioning steps using a total of 12mL), and they remove the interferences caused by Ca, Mg, and sulfate. I can spike 600 ppm of Ca and Mg and achieve 100% recovery without any dilution or adding any EDTA. I have also spiked 750 ppm SO4 and can achieve 100% recovery. Unfortunaltely, I can not use these cartridges unless they are apporved by the EPA, which requires a study done by three different labs. Anyone interested please place a post.