Re-examination of the MAGIC method to determine low orthophosphate concentration in seawater

A new and detailed MAGIC25 procedure is proposed to determine low concentration of orthophosphate in seawater. By preconcentrating phosphate 25 times, the procedure allowed detection of nanomolar concentration in seawater with low concentration of particles (detection limit = 0.8 ± 0.5 nM). The centrifugation step was considerably reduced (from 60 to 10 min) in comparison to previous MAGIC method. The calibration coefficient was determined by performing procedural and extra calibration curves and a previously unknown matrix effect in the MAGIC procedure was revealed. Its omission leads to a 12% underestimation of the phosphate concentration. The necessity of pre-filtration was demonstrated and a turbidity blank was measured to avoid overestimation even in the oligotrophic Mediterranean Sea water. The absence of filter reactivity with phosphate (contamination or retention) was verified. A synthetic reagent blank was proposed to simplify procedure in comparison to previous MAGIC method. The slow down of phosphomolybdenum blue complex was observed in the MAGIC concentrate and this results in a minimum reaction time of 30 min. The sensitivity (ratio of signal/noise) showed a three-fold improvement in comparison to the more recent nanomolar methods. Interferences of arsenate and silicate, commonly observed in the classic phosphomolybdic blue spectrophotometry, were undetectable. The MAGIC25 method appears reliable, sensitive, accurate and relative easy to use during oceanographic cruises. Therefore, MAGIC25 procedure is a useful tool for oceanograph chemists to be used instead of the classic phosphomolybdic blue method when concentration is below 200 nM.