Flow-injection technique for the determination of low levels of phosphorus in natural waters

An inexpensive flow-injection instrument for determining low concentrations of dissolved reactive phosphorus in natural waters is reported. The unique feature is the use of an inexpensive detector consisting of a flow cell and a simple photometer that incorporates a super-bright light-emitting diode as the source and a photodiode as the detector. The tin(II) chloride-molybdate method was optimized using a modified simplex optimization method. Silicate interference up to 5 mg Si l^?1 was removed by addition of 0.10% (w/v) tartaric acid. Using the tartaric acid-modified optimized reagents, a detection limit of 0.6 μg P l^?1 was achieved. The method was linear over the range 0–100 μg P l^?1 with an excellent precision (r.s.d. 2.9% at 2.0 and 0.5% at 50 μg P l^?1). An in-line pre-concentration anion-exchange column was used to obtain an even lower detection limit of 0.1 μg P l^?1 and applied to the analysis of real samples.