Determination of boron in water and pharmaceuticals by sequential-injection analysis and fluorimetric detection

This work reports the application of a sequential-injection analysis (SIA) method for the determination of boron. The method relies on the enhancement of the fluorescence (λ_ex=313 nm, λ_em=360 nm) of chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulphonic acid-CA) as a result of its complexation with boric acid (BA). Individual zones of the sample, the CA solution in a suitable buffer and a NaOH solution were aspirated in the holding coil of the SIA apparatus. As the zones were propelled towards the detector, zone penetration in the sample–CA interfaces occurred resulting in the formation of the strongly fluorescent BA–CA complex. The native fluorescence of the CA was quenched by the alkaline environment established as a result of the mixing at the CA–NaOH interface. The chemical and instrumental parameters affecting the fluorescence intensity were investigated and the influence of potential interferents was investigated. After selecting the most suitable conditions, the calibration plot for boron was linear in the range of 8–350 μg l⁻¹ with a 3σ limit of detection of 3 μg l⁻¹ and a relative standard deviation of 2.7% at the 90 μg l⁻¹ boron level (n=8). Finally, the method was applied to the determination of boron in natural waters and pharmaceutical products with revoveries in the range of 96–106%.