Determination of silicate in freshwaters using flow injection with luminol chemiluminescence detection

A flow injection (FI) manifold based on luminol chemiluminescence (CL) detection for the determination of silicate in freshwaters is described. The molybdosilicic heteropoly acid formed by silicate and ammonium molybdate in acidic conditions generated CL response via the oxidation of luminol. The calibration graph was linear over the range 0.35–140 (mu)g Si L^-1 (r^2=0.9974) at 25 °C with relative standard deviations (n=5) in the range 1.0–3.8%. The practical limit of detection was 0.35 (mu)g Si L^-1 with a sample throughput of 80 h^-1. Potential interferences in freshwaters were removed by passing the sample stream through in-line iminodiacetate chelating (to remove cations) and anion exchange (to remove phosphate) micro-columns. The method was applied to freshwater samples from the upper Tamar catchment and the results obtained (140(plus-minus)9 to 1493(plus-minus)47 (mu)g Si L^-1) were in reasonable agreement with the results obtained using a segmented flow analyser with spectrophotometric detection (182(plus-minus)9 to 1652(plus-minus)23 (mu)g Si L^-1) as the reference method.