Micro sequential injection: environmental monitoring of nitrogen and phosphate in water using a "Lab-on-Valve" system furnished with a microcolumn

A "Lab-on-Valve" manifold operated in the micro sequential injection (mu SI) mode was adopted to accommodate EPA-approved methods for spectrophotometric determinations of nitrate, nitrite and orthophosphate in the ppb (N or P) concentration range. A computer programmable mu SI protocol, utilizing stopped-flow within a copperized Cd-foil filled microcolumn was developed for nitrate reduction to nitrite with subsequent colorimetric measurement, yielding concentration ranges for nitrate of 100.0-4000.0 ppb (N) and for nitrite of 30.0-4000.0 ppb (N) and linear calibration responses of r(2) = 0.9999 for nitrate and 0.9995 for nitrite. Using a stopped-flow reaction rate measurement, phosphate was determined in the range 1.0-30.0 ppb (P) with a calibration response of r(2) = 0.9997. The technical improvement of this methodology, apart from micro miniaturization, is the use of the stopped-flow technique, that resulted in improved detection limits and allowed reagent consumption to be reduced 1500-fold compared with conventional procedure while the amount of metallic cadmium was reduced 20-fold compared with the EPA-approved continuous-flow assay.