Further studies into the development of a micro-FIA (μFIA) system based on electroosmotic flow for the determination of phosphate as orthophosphate Original Research Article

Photolithographic and wet etching techniques were used to fabricate a μFIA manifold in a borosilicate glass substrate for the spectrophotometric determination of orthophosphate based on the molybdenum blue reaction, employing electroosmotic flow for both the mobilization of reagents and sample injection. The manifold channel dimensions were 200 μm wide and 50 μm deep. An evaluation of the electroosmotic flow characteristics (i.e. voltage, solution concentration/pH ranges and current–voltage relationships) for ascorbic acid, ammonium molybdate and orthophosphate standards, with and without borate buffer present, has been carried out. In addition, the effect that reaction precursors may have on the in situ spectrophotometric detection system is also discussed. Using the optimized operating conditions of 1.75% ascorbic acid and 0.6% ammonium molybdate in 10 mM borate buffer, the calibration model for phosphate standards was found to be linear (y = 0.0036x + 0.0155; correlation coefficient, r2 = 0.9952) over the working range 1–10 μgml−1. A sampling rate of ca. 60 samples/h was achieved, with a detection limit of 0.1 μgml−1 for orthophosphate, with RSDs <5%. The reagent consumption was very low with the total reactant volume in the system estimated to be 0.6 μl and a sample volume of 0.1 μl being required.