Direct determination of phosphate in urine by sequential-injection analysis with single on-line dilution-calibration...

In this work we report a sequential-injection (SI) analysis method for the direct determination of phosphate in urine with on-line dilution and photometric detection. The developed SI manifold incorporated an additional dilution coil (DC) connected to one of the ports of the selection valve. On-line dilution was performed by aspirating a zone of sample with volume V(S) from the sample line into the holding coil (HC), transferring volume V(T) of that zone into the DC, and, finally, aspirating a portion of V(T) with volume V(A) back into the HC. This dilution introduces an additional dispersion of the initial sample zone represented by the effective dispersion coefficient D(eff) which can be tailored to the desired value by appropriate selection of the volumes V(S), V(T), and V(A). Actual detection of phosphates was performed by the molybdenum blue method with detection at 690 nm. By employing an on-line dilution step with a D(eff)= 100, the calibration curve for phosphate was linear in the range 0.05 x 10(-2) to 3 x 10(-2) mol L(-1) which covers the actual phosphate concentration in urine samples. The regression coefficient was r2 = 0.996, the relative standard deviation was sr = 3.9% at the 1 x 10(-2) mol L(-1) level (n = 10), and the injection frequency was 30 injections h(-1). The method was applied to urine samples with recoveries > 97%.