The most common cause of high ...

The most common cause of high results in Arsenic field kits is interference from Hydrogen Sulfide.  Most field kits will have a method for removing this interference.  One method is to pass the gas stream through lead acetate which reacts with the hydrogen sulfide to produce lead sulfide but doesn't react with the arsenic. The lead acetate is commonly used to soak some cotton which is placed in the gas stream.  You need to make sure that all of the gas passes through the cotton ball.   Another common method is to oxidize the hydrogen sulfide to sulfate before starting the test.  Some methods field utilize this approach.  The strong oxidant then needs to be removed usually by the addition of an organic to prevent its interference in the reduction of the arsenic to arsine gas.  It is important to follow instructions carefully.  Many firms make these test kits.  Hach test kit 28228-00 uses the lead acetate method and Hach test kit 28000-00 utilizes the oxidation method