When ammonium in samples are injected into a basic (pH 12) NAOH carrier/reagent1, NH3  is formed. The NH3 passes across a gas-permeable membrane into a collector stream (reagent 2) containing bromothymol blue at pH 6.3. Collection of NH3 into the bromothymol blue raises the pH, creating a bluish color change proportional to the initial ammonium.

Comments: The FIAlab-2500 is configured so that the M1 port (output of injected sample/R1/carrier) tubing passes through the first mixing coil, through one side of the membrane cell, and then to waste.  The other side of the membrane cell is connect to the R2 port.   This flows through the membrane cell, through the second mixing coil, then through the absorbance flowcell. Pump speed should be set to 30.  Sample loop for Mid to High concentrations should be 6" of 0.03" ID tubing, and the 1 cm flowcell should be used..  Sample loop for Low concentrations should be 12" of 0.03" ID tubing, and the 10 cm flowcell should be used. Interferences: Any component which will interfere with the ability of NH3 to pass across the membrane.