Micropumping multicommutation turbidimetric analysis of waters

A micropumping multicommutation manifold to perform turbidity determinations in waters is described. The procedure is based on the use of a combination of hydrazine sulfate and hexamethylenetetramine, to obtain an external standard of nephelometric turbidity units (NTU), which could compare the absorbance measurements at high wavelengths for samples with a calibration line obtained from a concentrated formazine standard diluted on-line. To minimize sample and reagent consumption and waste generation, the flow system was designed with two solenoid micro-pumps, one of them for the alternative introduction of the formazine standard and samples and the other one for the water carrier. The multicommutation approach makes possible the on-line dilution of a single standard to obtain the external calibration. The linear response was ranged up to 160 NTU. The coefficient of variation was estimated as 1.6 and 3.2% for 10 and 100 mm flow cell, respectively, for solutions containing 40 NTU (n = 10). Approximately, 60 determinations can be carried out per hour with limit of detection values of 1 and 0.1 NTU, consuming only 160 or 240 μL formazine solution and generating 1.8 or 2.0 mL waste per determination, using measurement cells of 10 and 100 mm optical pathlength, respectively. The procedure was successfully applied to 11 different water samples. Recovery studies were carried out and results obtained were between 97.5 ± 0.2 and 100 ± 1%. The development of a homebuilt light emitting diode (LED)-based portable flow analysis instrument was checked for in situ turbidimetric measurements, providing this equipment a LOD value of 0.09 NTU working with a blue LED at 464 nm and a LOD value of 0.1 NTU working with an IR LED.