1The application of colourimetric cavity ringdown spectroscopy to the detection of trace 2 compounds in water has been investigated using nitrite and iron (II) as test analytes. Sam-3 ples were contained within one of three commercially-available flow cells ranging in optical 4 pathlength from 0.1 mm to 2.0 mm, and positioned within a two-mirror ringdown cavity. A 5 measurement of the decay rate of the intensity of an optical pulse introduced into the cavity 6 allows an ultrasensitive determination of optical absorption by the sample. A calibration us-7 ing the known absorption coefficient of potassium permanganate at 532 nm was first carried 8 out in order to determine the detection sensitivity in terms of minimum detectable absorption 9 per unit pathlength when using each flow cell. The detection of nitrite and iron was then car-10 ried out by using well-known colour reactions, namely the Griess reaction for nitrite and the bathophenanthraline method for iron, to convert the analytes into strongly absorbing deriva-12 tives, which were quantified by a cavity ringdown measurement. In this first application of 13 colourimetric cavity ringdown spectroscopy to the liquid phase, detection limits of 1.9 nM for 14 nitrite and 3.8 nM for Fe(II) were demonstrated in a flow cell of path length 1.0 mm. The 15 volume of sample analysed is only 196 nL, so that detection limits of this order correspond to 16 the detection of less than 1 billion molecules. The detection method is therefore suitable for 17 integration into a microfluidic sensing platform. 18

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Broadband cavity enhanced absorption spectroscopy (BBCEAS) as a detection technique for a microfluidic flow cell made of Zeonor ®