Determination of nanomolar concentrations of phosphate in natural waters using flow injection with a long path length liquid waveguide capillary cell and solid-state spectrophotometric detection

A flow injection manifold incorporating a 1 m liquid waveguide capillary cell and a miniature fibre-optic spectrometer for the determination of low phosphorus concentrations in natural waters is reported. The limit of detection (blank + 3 S.D.) was 10 nM using the molybdenum blue chemistry with tin(II) chloride reduction. The sensitivity of the flow injection manifold was improved by 100-fold compared with a conventional 1 cm flow cell. The response was measured at 710 nm and background corrected by subtracting the absorbance at 447 nm. Interference from silicate was effectively masked by the addition of 0.1% (m/v) tartaric acid and results were in good agreement (P = 0.05) with a segmented flow analyser reference method for freshwater samples containing 1 μM phosphate.