Sequential analyte removal in flow analysis: determination of nitrogen, phosphorus and potassium in fertilizers

A new approach for multiparametric determinations in flow analysis based on successive sequestering of analytes is proposed and exemplified in the determination of water soluble nitrogen, phosphorus and potassium in fertilizers. The analytes are partially and reproducibly removed from the sample zone in the main stream towards specific analytical channels without significant alterations in the flowing sample plug. By using gas diffusion, dialysis and asymmetric stream splitting, ammonium, potassium and orthophosphate ions are stripped out and determined by flow potentiometry, flame photometry and spectrophotometry, respectively. The system permits up to 75 samples to be examined per hour, Requires 90 mg tris(hydroxymethyl)aminomethane plus 75 mg ammonium heptamolybdate tetrahydrate plus 75 mg ascorbic acid per sample, and yields reproducible results (R.S.D. usually < 3%) in agreement with usual manual procedures. The feasibility of an analogous modular system with a random selection of procedures is discussed.