Portable two-color photometer based on paired light emitter detector diodes and its application to the determination of paraquat and diquat

Here we describe a methodology for the determination of paraquat and diquat using a newly developed portable photometer equipped with two colors of paired light emitter detector diodes (PEDD). The colorimetric measurements employed in this work include the redox reactions between 1) dithiothreitol and diquat to produce the red color characteristic of a diquat radical and 2) between sodium dithionite and either diquat or paraquat that results in the green and blue colors of diquat and paraquat radicals, respectively. The addition of sodium dithionite or dithiothreitol in a solid-state provides reproducible absorbance of the radicals, prevents decomposition of the reagents in a solution, and simplifies handling of the reagents. The diquat radical produced by dithiothreitol (λ_max = 495 nm) was successfully detected by using a pair of blue LEDs with a maximum emission wavelength at 472 nm while the radicals of paraquat (λ_max = 603 nm) and diquat (λ_max = 771 nm) reduced by sodium dithionite were measured by a pair of orange LEDs with a maximum emission wavelength of 609 nm. The proposed method consists of measuring diquat radicals at 472 nm, estimating the absorbance of diquat radicals at 609 nm, and subtracting the estimated absorbance of diquat radicals from the total absorbance at 609 nm to determine paraquat radicals. The developed method yielded examples of excellent linear regression (r²) of more than 0.99 in three calibration curves of the radicals measured at 472 nm for diquat radicals and measured at 609 nm for both diquat and paraquat radicals. The intra-day (n = 3) and inter-day (n = 3) precision of three calibration curves were less than or equal to 5%. By comparison with the standard method of high-performance liquid chromatography, the reliability of the proposed method was proven via the analysis of paraquat and diquat radicals in a commercially available herbicide.