Heat-based procedure for detectability enhancement of colorimetric paper-based spot tests

Abstract

Colorimetry is an analytical procedure in which a colored compound is formed or degraded from a selective reaction between the analyte and a chromophore followed by color intensity measurement for quantification purposes. The employment of paper-based analytical devices as a solid substrate to perform chemical reactions enabled the miniaturization of the analytical system, thus requiring a minimum volume of sample (e.g., µL) and resulting in low cost, disposability, and ease of use. On the other hand, the detectability of such devices is usually not suitable for a variety of practical applications where the analyte concentration is present in either low parts-per-million or high parts-per-billion concentration range. In this study, we describe a simple procedure for analyte preconcentration (i.e., enrichment) based on a multiple and automated sample delivery of 15 µL per time on a heated paper surface. In this procedure, an aliquot of the sample solution diffuses through the center of the paper to form a circular spot, followed by quick solvent vaporization prior to the next volume injection. After the enrichment procedure,15 µL of the reagent is added to perform the colorimetric reaction. The applicability of the proposed procedure was demonstrated for the determination of nickel (II) in natural waters and iron (III) in ethanol fuel using dimethylglyoxime and 1,10 phenanthroline as colorimetric reagents, respectively. By relating the color intensity of the products formed to the concentration, the limits of detection were as low as 0.1 mg L−1 for nickel (II) and iron (III) using 20 and 10 aliquots of 15 µL of each metal ion solution. Under these conditions, the enrichment factors were 180- and 20-fold higher for Ni2+ and Fe3+, respectively, with a total time of analysis inferior to 10 min. We anticipate that the proposed procedure can be tailored for a wide range of analytical problems using colorimetric determinations in spot tests with low concentration detectability and ease of operation.