Portable arsenic test kit for drinking water

Does anyone have experience with getting false positive results from arsenic test kits? I tested the water with a portable kit and the results showed 0.025 - 0.05 mg/l of arsenic. I sent the sample to the lab and their test showed 0 mg/l. What could be the reason for the difference in the results?   Which arsenic test kit do you use?  Can you recommend one which gives the most reliable results?