An optimised flow injection (FI) method suitable for the determination of silicate in the presence of high concentrations of arsenate (216 µg As L
-1) in anthropogenically impacted natural waters is presented. The proposed method has a practical limit of detection of 10 µg Si L
-1 and typical RSDs of ≈1.5 % (n = 3) and a sample throughput of 40 samples h
-1. Strategies are presented for the removal of matrix interferences. The manifold incorporates a thiosulfate stream to remove arsenate (AsO
43-) by reduction to arsenite (AsO
33-) and two micro-columns containing a chelating resin (iminodiacetate) to remove metal ions and an anion exchange resin (Dowex 1X8, 200–400 mesh, Cl
- form) to remove phosphate. The chelating column successfully removed matrix cations; Cu(II) at 0.1 mg L
-1, Ni(II), Co(II), Fe(II) and Fe(III) at 1.0 mg L
-1 and Mn(II), Zn(II) and Pb(II) at 10 mg L
-1. The anion exchange column effectively removed phosphate interference by complexing up to 1.5 mg P L
-1 and had no effect on the silicate response. A linear calibration was obtained with the optimised manifold in the range 10 - 1000 µg Si L
-1 (R
2 = 0.998). This method was applied to the determination of Si in the Tamar Estuary, an area of historical mining activity, and the results compared well with those from a segmented flow analyser standard method with spectrophotometric detection (P = 0.05; t
cal = 1.44 and t
crit = 2.14).
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