Flow injection analysis of nanomolar silicate using long pathlength absorbance spectroscopy

Determination of silicate at low concentrations (i.e., nanomolar levels) is an important analytical objective for both marine science and the semiconductor industry. Here we report the use of flow injection analysis (FIA) in combination with long pathlength liquid core waveguide (LCW) spectrometry to achieve detection limits for dissolved silica on the order of 10 nM. Sample throughput for the simple, automated analytical apparatus used in this work is 12 h−1 at low levels of dissolved silica; this rate can be increased by a factor of three for higher (micromolar) levels of dissolved silica. The analytical protocol is based on the reaction of silicate with ammonium molybdate to form a yellow silicomolybdate complex, which is subsequently reduced to silicomolybdenum blue by ascorbic acid. Optimization of the FIA procedure included consideration of the compositions and concentrations of reagents, volume of the injection loop, flow rate conditions, and lengths of mixing coils. The interference by phosphate was examined and eliminated through addition of oxalic acid. The dissolved silica detection limit of 7.2 nM in pure water is consistent with the strictest standard for the semiconductor industry, and the 9.0 nM detection limit for seawater shows that this analytical method is also suitable for oligotrophic ocean waters. The targeted analytical range of 10 nM to 5 μM can be easily extended to higher concentrations without altering the experimental hardware—i.e., by simply changing flow rates or selecting alternative analytical wavelengths. Compared to previously published LCW-based spectrophotometric methods, this analytical system exhibits improved sensitivity, reduced sample consumption, and higher sample throughput.