Hybrid flow system integrating a miniaturized optoelectronic detector for on-line dynamic fractionation and fluorometric determination of bioaccessible orthophosphate in soils

• Paired emitter detector diode as miniaturized fluorometric detector for phosphate.
• Sufficient sensitivity of FPEDD for detection of phosphate in soil leachates.
• Hybrid flow system for reliable dynamic bioaccessibility tests of phosphate in soils.
• On-line analysis of extracts without delay after dynamic extraction.
• On-line manipulation of sample/extracts using the secondary flow injection setup.

An integrated Sequential Injection (SI)/Flow Injection (FI) system furnished with a miniaturized LED-based fluorometric detector is presented in this work for expedient bioaccessibility tests of orthophosphate in soils. Equipped with a microcolumn of conical shape containing 50 mg of soil, the hybrid flow system was resorted to on-line dynamic leaching and real-time quantification of pools of mobilizable orthophosphate using a bi-directional syringe pump and multiposition valve. The flexibility of the flow manifold was harnessed to explore both bi-directional and uni-directional flow extraction modes with the added degree of freedom of on-line dilution of extracts whenever needed. Bioaccessible orthophosphate was split in three fractions, the so-called NH4Cl fraction containing labile exchangeable phosphates, the alkaline fraction with Fe and Al-bound phosphates and the acidic fraction containing Ca-bound phosphates.The prevailing molybdenum blue photometric detection method is replaced by spectrofluorometric detection based on the ion pair formation between the phosphomolybdate heteropolyacid and rhodamine B with the subsequent quenching of the dye fluorescence. The dedicated optoelectronic detector was integrated in a secondary FI manifold and operated according to the fluorometric paired emitter–detector diode (FPEDD) principle involving two light emitting diodes as fluorescence inductors and one as detector of LED-induced fluorescence.Demonstrated with the analysis of a standard reference material (SRM 2711) and a real agricultural soil, the developed FI/SI fractionation system with FPEDD detection is proven reliable against the standard molybdenum blue method (p>0.05), and useful for investigation of the leaching kinetics of orthophosphate in bioaccessibility tests through in-line recording of the extraction profiles.

Automatic bioaccessibility tests of orthophosphate in soils using dynamic sequential extraction combined with a miniaturized fluorometric detector.Figure optionsDownload as PowerPoint slide