Determination of dissolved silica in waters by a flow-based analysis system composed of a laser diode and a thin long flow-through cell

A highly sensitive detection system composed of a laser diode (LD) and a thin long flow-through cell (TLFTC) for flow-injection analysis (FIA) has been studied. The new detection system was mainly composed of a GaAlAs LD emitting at 780 nm (5 mW) as a light source and a polytetrafluoroethylene rod having a thin long hole (100 × 1.4 mm i.d.) as the light path. The sensitivity was enhanced about 10 times compared with a normal 10 mm flow cell. For the determination of dissolved silica in waters using the molybdenum blue method, the relative standard deviation, detection limit (3σ) and linear range were 0.6%, 0.0019 and 0.02–0.3 mg Si l−1, respectively. This detection system may also be useful for other flow-based analytical techniques such as liquid chromatography.