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Iron is an important micronutrient involved in several mechanisms in the human body and can be an important biomarker. In this work, a simple and disposable microfluidic paper-based analytical device (µPAD) was developed for the quantification of iron in urine samples. The detection was based on the colorimetric reaction between iron(II) and bathophenanthroline and the reduction of iron(III) to iron(II) with hydroxylamine. The developed µPAD enabled iron determination in the range 0.07–1.2 mg/L, with a limit of detection of 20 µg/L and a limit of quantification of 65 µg/L, thus suitable for the expected values in human urine. Additionally, targeting urine samples, the potential interference of the samples color was overcome by incorporating a sample blank assessment for absorbance subtraction. Stability studies revealed that the device was stable for 15 days prior to usage and that the formed colored product was stable for scanning up to 3 h. The accuracy of the developed device was established by analyzing urine samples (#26) with the developed µPAD and with the atomic absorption spectrometry method; the relative deviation between the two sets of results was below 9.5%. Graphical abstract: [Figure not available: see fulltext.]

In this work, two different microfluidic paper-based analytical devices (μPADs) were developed for the quantification of nitrite and nitrate in human saliva samples, in order to aid in the diagnosis of some diseases and health conditions associated with these ions. The development of these nitrite and nitrate μPADs involved several studies to optimize their design and construction, including an interference assessment and stability studies. These μPADs allowed a nitrite determination in a range of 5–250 μM with limits of detection and quantification of 0.05 μM and 0.17 μM, respectively, and a nitrate determination in the range 0.2–1.2 mM with limits of detection and quantification of 0.08 mM and 0.27 mM, respectively. As for the stability, both of the μPADs were stable when stored in vacuum at 4 °C (the nitrite μPAD for at least 60 days and the nitrate μPAD for at least of 14 days) and, after the sample placement, the nitrite and nitrate μPADs could be scanned within the first 4 and 2 h, respectively. The nitrite μPAD measurements were compared with the ones obtained from the standard colorimetric method and there were no statistically significant differences between these two methods. To evaluate the accuracy of nitrate μPAD measurements, 4 certified water samples were used and recovery studies using saliva samples were performed.

Phosphate is a well-known contaminant and its content is an indicator of water quality, so it is important to have an efficient monitoring of the soil leaching process. In this work, an automatic, low cost flow analysis method, capable of real-time monitoring of the soil leaching processes using spectrophotometric detection, based on the molybdenum blue reaction, was developed. The developed methodology for phosphate determination was based on the molybdenum blue reaction and includes an on-line solid phase extraction (SPE) step, involving an AG1-X8 anionic exchange sorbent. The described SI method enabled phosphate determination within a wide range, 1–80 µM, with a detection limit of 0.52 µM. It was successfully applied to leachates from laboratory scale soil columns (LSSC) and one sample analysis was carried out in triplicate, in less than 10 minutes.