Quantitative Raman Spectroscopy of Urea and Thiourea in the Reaction Mixtures of Allantoin and 4,5-Dihydroxyimidazolidine-2-Tione Formation

The present work introduces a new approach to quantitative determine the reagent concentrations (thiourea and urea) using in situ Raman spectroscopy in the reaction mixture exemplified by the reactions of formation of allantoin and 4,5-dihydroxyimidazolidine-2-thione. The approach comprises the use of a commercially available immersion probe (MarqMetrix Process Elite BallProbe with sapphire lens) as a standard for the immersion probe band at 790 cm⁻¹ of optical glass. This leads to a linear dependence of the ratio of the intensity of non-overlapping analyte bands at 1003 cm⁻¹ of -C-N- vibrations in urea to the one of the 790 cm⁻¹ immersion probe band on the concentration of urea in the test solution. The definable method parameters such as precision (repeatability and reproducibility), linearity, limit of detection (LoD), limit of quantitation (LoQ), and accuracy were determined. The quantitative Raman spectroscopy method is linear, precise within the range of determined concentrations from 0.75 to 2.00 M and can be used to calculate the kinetics of allantoin formation. Using thiourea as an example, it is shown that despite the partial overlap of the analyte band at 730 cm⁻¹ of the -C=S vibrations of thiourea with the immersion probe band at 790 cm⁻¹, the method to determine thiourea in the reaction of 4,5-dihydroxyimidazolidine-2-thione preparation is also linear and precise in the range of determined concentrations from 0.10 to 1.57 M.