A sensor for monitoring a species which is a component of a fluid. The sensor includes an internal reflection window which, in use, is in direct contact with the fluid. The sensor further includes a mid-infrared light source which directs a beam of mid-infrared radiation into said window for attenuated internal reflection at an interface between the window and the fluid. The sensor further includes a first narrow bandpass filter which preferentially transmits mid-infrared radiation over a band of wavelengths corresponding to an absorbance peak of the species. The sensor further includes an infrared detector which detects filtered mid-infrared radiation transmitted through the first filter and a processor for measuring the intensity of the detected mid-infrared radiation transmitted through the first filter, and determines therefrom an amount of the species in the fluid.