Novel programmable flow injection (pFI) technique for optimization of reagent-based assays was used to enhance performance of phosphomolybdenum blue method

The most challenging aspect of optimizing reagent-based techniques for determining trace levels of nutrients in sea water, is the investigation of the underlying chemical reaction kinetics. We demonstrate the use of programmable Flow Injection (pFi) to optimize the conventional phosphomolydate blue method for use on the lab-on-valve (LOV) platform. With the instrument programmed to operate in batch mode, the formation of phosphomolybdate (PMo) complex was studied. This was achieved by monitoring the kinetics of the reaction by collecting spectra during the formation of PMo within the flow cell. From this work it was found that the crucial parameter for increasing the sensitivity of the PMo based assay is the acidity of the reaction media. Our results show that high acidity suppresses formation of PMo, while low acidity promotes the formation of a reagent blank. The fully optimized method, performed using the pFI-LOV platform, had a detection limit of 30nM, comparable to that of traditional auto analyzer, and the sensitivity of the method was independent of the salinity of the samples. We anticipate that this pFI-LOV method for optimizing analytical methodology will be applicable to a wide range of reagent-based assays used in oceanography.