Browsing by Author "Marshall, Graham"
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- Development of a flow analysis system for determination of phosphate in ground waterPublication . Mesquita, Leticia S; Mesquita, Raquel B. R.; Marshall, Graham; Rangel, António O. S. S.
- A greener alternative for inline nitrate reduction in the sequential injection determination of NOx in natural waters: replacement of cadmium reduction by UV radiationPublication . Machado, Ana; Marshall, Graham; Bordalo, Adriano A.; Mesquita, Raquel B. R.The use of sequential injection analysis (SIA) for key nutrient determination in water has been previously described using a copperised cadmium column to attain nitrate reduction to nitrite prior to determination, yielding hazardous waste. Here, a greener alternative is proposed for nitrate reduction using a UV lamp, aiming to avoid the use of cadmium. This method was based on the colorimetric Griess reaction for nitrite determination, after the UV reduction of nitrate. The reduction was performed inline during the determination of nitrite, minimizing both effluent production and the overall determination rate. Its application to natural waters, namely river water, proved to be effective as the results were comparable to those obtained with the reference procedure. Furthermore, certified reference samples were analysed, and an average of 3% relative deviation was observed. A limit of detection of 5.30 mM was achieved, enabling the nitrate determination in the dynamic range of 25-500 mM with a determination rate of 24 h(-1), and with a recovery percentage rate around 100%.
- Improvement of a sequential injection method for the determination of chromium (VI) in waters using a GlobalFIA platform.Publication . Teixeira, Graça M.; Mesquita, Letícia S.; Marshall, Graham; Mesquita, Raquel; Rangel, António O. S. S.
- A robust flow-based system for the spectrophotometric determination of Cr(VI) in recreational watersPublication . Ribas, Tânia C. F.; Mesquita, Raquel B. R.; Machado, Ana; Miranda, Joana L. A.; Marshall, Graham; Bordalo, Adriano; Rangel, António O. S. S.A flow-based method for the spectrophotometric determination of chromium (VI) in recreational waters with different salinities was developed. Chromium can occur in the environment in different oxidation states with different related physiological properties. With regard to chromium, the speciation is particularly important, as the hexavalent chromium is considered to be carcinogenic. To achieve that purpose, the use of the diphenylcarbazide (DPC) selective colored reaction with the hexavalent chromium was the chosen strategy. The main objective was to develop a direct and simple spectrophotometric method that could cope with the analysis of different types of environmental waters, within different salinity ranges (fresh to marine waters). The potential interference of metal ions, that can usually be present in environmental waters, was assessed and no significant interferences were observed (<10%). For a complete Cr(VI) determination (three replicas) cycle, the corresponding reagents consumption was 75 µg of DPC, 9 mg of ethanol and 54 mg of sulfuric acid. Each cycle takes about 5 min, including the system clean-up. The limit of detection was 6.9 and 12.2 µg L−1 for waters with low and high salt content, respectively. The method was applied for the quantification of chromium (VI) in both fresh and marine water, and the results were in agreement with the reference procedure.