A â Lab-on-Valveâ manifold operated in the micro sequential injection (µSI) 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 µSI 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 100.0â 4000.0 ppb (N) and for nitrite of 30.0â 4000.0 ppb (N) and linear calibration responses of r² = 0.9999 for nitrate and 0.9995 for nitrite. Using a stopped-flow reaction rate measurement, phosphate was determined in the range 1.0â 30.0 ppb (P) with a calibration response of r² = 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.