Loop flow analysis of dissolved reactive phosphorus in aqueous samples

The current flow based method for the determination of dissolved reactive phosphorus (DRP) suffers interference from salinity (e.g. index refractive difference) and the incidentally formed bubbles, which can be a problem for optical detection. Here we reported a simple and robust loop flow analysis (LFA) method for accurate measurement of DRP in different aqueous samples. The chemistry is based on the classic phosphomolybdenum blue (PMB) reaction and the PMB formed in a novel cross-shaped flow cell was detected at 700 nm using a miniature spectrophotometer. The effects of reagents on the kinetic formation of PMB were evaluated. The detection limit was 32 nM with an optical pathlength of 1 cm and the relative standard deviations for repetitive determinations of 1, 2 and 8 µM phosphate solutions were 1.8% (n=113, without any stoppage during repeating analysis for >7 h), 1.0% (n=49) and 0.39% (n=9), respectively. The analysis time was 4 min sample−1. The effects of salinity and interfering ions (silicate and arsenate) were evaluated and showed no interference under the proposed protocol for DRP analysis. Using the LFA method, different aqueous samples with a salinity range of 0–34 were analyzed and the results showed excellent agreement with the reference method (slope 0.9982±0.0063, R2=0.9987, n=34). Recoveries for spiked samples varied from 95.4% to 103.7%. The proposed method showed insignificant interference from salinity, silicate and arsenate, higher reproducibility, easier operation and was free of the bubble problem.