The determination of phosphorus in a microfluidic manifold demonstrating long-term reagent lifetime and chemical stability utilising a colorimetric method

The chemical stability of the vanadomolybdophosphoric acid method for phosphate determination in a microfluidic manifold is described. The reagent lifetime has been shown to extend to more than 1 year. A stopped flow regime has been implemented, which enabled a very simple microfluidic manifold design to be employed, and has the added advantages of low reagent consumption coupled with less waste generation and access to the complete reaction profile in the optical cuvette on the microfluidic chip. Optical detection was achieved with an UV-LED, integrated into the microfluidic chip holder, coupled to a portable spectrometer via an optical fibre. The manifold includes integration of reagent and sample introduction inlets, a mixing channel and an optical cuvette of 400 μm path length. Two reagent batches were prepared (December 1999 and April 2001) and were shown to still be highly comparable after 1 year in storage. Multiple calibrations have been performed on the microfluidic system over a 12-month period showing only minimal loss in performance and a standard orthophosphate-containing sample was analysed in the microfluidic manifold on a weekly basis with a relative standard deviation of <2.3%.