A highly sensitive and large concentration range colorimetric continuous flow analysis for ammonium concentration

A continuous flow method for the determination of ammonium concentration in seawater from a nanomolar to a micromolar level is described. To prevent spurious peaks derived from salinity difference, a gas-permeable hydrophobic membrane filter was used to separate the manifold into an outgassing section and an indophenol blue reaction section. The indophenol blue reaction section was adopted for colorimetric analysis and is equipped with a 1-m path length liquid capillary cell and a fiber-optic spectrometer, which is able to record the absorbance at multiple wavelengths. The minimum detection limit at wavelength 630 nm is 5.5 ± 1.8 nM, and the calibration curves are linear to at least 2,000 nM. In addition, the minimum detection limit at wavelength 530 nm was 13 ± 5.3 nM, and linear calibration curves were observed until at least 10,000 nM. The slopes of the calibration curves were similar for standards prepared using filtered seawater and ultrapure water. The ammonium concentration of the ultrapure water was similar to those of ion-exchanged water and unfiltered low-nutrient seawater, but was significantly lower than those of filtered seawater and solutions that contained sodium hydroxide. Therefore, ultrapure water is optimal for both blank and standard preparations because of its stable quality and availability. Given its large concentration range and the use of readily available blanks, this method is suitable for the determination of ammonium concentration and helps our understanding of ammonium dynamics in the ocean.