An exemplary apparatus for measuring nitric oxide in a fluid includes a sample injection port; a pump and reservoir; a valve; a measurement chamber; an electromagnetic radiation source; controls and user interface; and a cartridge. The cartridge includes a sample chamber a mix chamber; a sample degassing chamber; and a planar reaction chamber. The cartridge preferably is single-use and disposable. Furthermore, additional inventive aspects and features are disclosed related to measuring nitric oxide in a fluid.