Abstract: Apparatus and method of monitoring fluid presence, wherein electomagnetic radiation sources (1, 2) provide respective reference and sample beams (12, 22), the wavelength of the radiation preferably being matched with that of the absorption spectrum of a fluid to be monitored. A first reference cell (13), containing a reference sample of the fluid to be monitored, has the reference beam (12) passed through it, whilst a second, sample cell (33) containing any working sample of the fluid to be monitored, has both the reference and sample beams (12, 22) passed through it. Subsequently, the reference and sample beams are detected independently of each other to provide respective signals representative thereof, which signals are then compared to determine the presence, and preferably also the concentration, of any sample fluid in the second cell.