Use of experimental design to optimise a flow injection analysis assay for L-N-monomethylarginine

A flow injection analysis (FIA) method to determine L-N-monomethylarginine, based on the reaction with ortho-phthalaldehyde in the presence of a suitable thiol-group, was optimised using experimental design. Two different approaches were followed wherein, (i) critical factors were identified in a screening design, and (ii) the simplex algorithm was used for further optimisation. In the first approach, the chemical reaction was optimised off-line and the optimal chemical conditions were transferred to the FIA-system. In the second approach the reaction and the FIA-system parameters were optimised together. The on-line approach is preferred.