Speciation of chromium in water samples using microfluidic paper-based analytical devices with online oxidation of trivalent chromium

Speciation of chromium (Cr) was demonstrated using microfluidic paper-based analytical devices (μ-PADs) that permit the colorimetric determination of hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) via online oxidation. The μ-PADs consist of left and right channels that allow the simultaneous measurements of Cr(VI) and total Cr based on the colorimetric reaction of Cr(VI) with 1,5-diphenylcarbazide (DPC). For the determination of Cr(VI), a sample solution was directly reacted with DPC in the left channels whereas total Cr was determined in the right channels, which permitted online oxidation in the pretreatment zone containing cerium (IV) (Ce(IV)) followed by a colorimetric reaction with DPC. We found that the online oxidation of Cr(III) proceeded 100% whereas Ce(IV) inhibited the reaction of Cr(VI) with DPC. Therefore, speciation can be achieved by measuring the Cr(VI) and total Cr in the left and right channels followed by the subtraction of Cr(VI) from total Cr. The limits of detection and quantification were 0.008 and 0.02 mg L⁻¹ for Cr(VI) and 0.07 and 0.1 mg L⁻¹ for Cr(III) or total Cr, respectively. The linear dynamic ranges were 0.02–100 mg L⁻¹ and 0.1–60 mg L⁻¹ for Cr(VI) and Cr(III), respectively. The RSDs were less than 7.5%. The results obtained using μ-PADs were in good agreement with those obtained via ICP-OES with recoveries of 92–108% for Cr(III) and 108–110% for Cr (VI) using μ-PADs, and 106–110% for total Cr using ICP-OES. Thus, the μ-PADs could potentially be utilized for the speciation of chromium in developing countries where environmental pollution and the availability of sophisticated instruments are significant problems.