Continuous Temperature-Dependent Raman Spectroscopy of Melamine and Structural Analog Detection in Milk Powder

Abstract

Hyperspectral Raman imaging has the potential for rapid screening of solid-phase samples for potential adulterants. We can improve mixture analysis algorithms by defining a temperature range in which the contaminant spectrum changes dramatically and uniquely compared with unadulterated material. Raman spectra were acquired for urea, biuret, cyanuric acid, and melamine (pure and at 1% in dried milk powder) from 50 to 310 °C with a gradient of 1 °C min⁻¹. Adulterants were clearly indentified in the milk powder. Specific frequencies that were mainly associated with ring breathing, stretching, and in-plane deformation shifted with respect to temperature up to 12 cm⁻¹ in all four molecules. Specific frequencies significantly increased/decreased in intensity within narrow temperature ranges independent of whether the amine was mixed in milk. Correlation of Raman and differential scanning calorimetry data identified structural components and vibrational modes, which concur with or trigger phase transitions.