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Title: Temperature dependent Raman spectroscopy of melamine and structural analogs in milk powder

item Schmidt, Walter
item BROADHURST, C. LEIGH - University Of Maryland
item QIN, JIANWEI - University Of Maryland
item LEE, HOYOUNG - Seoul National University
item Nguyen, Julie
item Chao, Kuanglin - Kevin Chao
item Hapeman, Cathleen
item Shelton, Daniel
item Kim, Moon

Submitted to: Applied Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2014
Publication Date: 3/1/2015
Publication URL:
Citation: Schmidt, W.F., Broadhurst, C., Qin, J., Lee, H., Nguyen, J.K., Chao, K., Hapeman, C.J., Shelton, D.R., Kim, M.S. 2015. Temperature dependent Raman spectroscopy of melamine and structural analogs in milk powder. Applied Spectroscopy. 669:398-406.

Interpretive Summary: Incidents of intentional adulteration of agricultural products, such as melamine added to milk powder, have resulted in illnesses and even death for animals and humans. Melamine (and three other chemicals: urea, biuret, and cyanouric acid) contains high nitrogen levels which, when mixed into milk, deceptively appear as if they were protein. Previous research has demsontrated that hyperspectral Raman imaging allows for detection of 1% melamine in milk powder. Because all four compounds mentioned above are chemically similar in structure, all could be potential adulterants. Temperature Dependent Raman (TDR) spectroscopy is a simple method which can differentiate each of the above compounds from one another, as well as from the components of milk powder. By taking measurements along a temperature gradient, the Raman spectra of different chemicals can used to identify “flexible” molecular sites, which are unique to each chemical, as the chemicals melt. In this work, we describe the results of TDR experiments with milk powder contaminated with melamine and other nitrogen-rich chemicals, and provide interpretation of the Raman spectra. This research should be beneficial to other scientists and regulatory agencies responsible for ensuring the safety of various agricultural commodities.

Technical Abstract: Hyperspectral Raman imaging has the potential for rapid screening of solid-phase samples for potential adulterants. We found that the Raman spectra of melamine analogs changed dramatically and uniquely as a function of elevated temperature. Raman spectra were acquired for urea, biuret, cyanuric acid and melamine (pure and at 1% in dried milk powder) from 50–250ºC with a gradient of 1ºC min-1. Adulterants were clearly identified 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; however, each molecule reacted differently to thermal stress. Further, specific frequencies significantly increased/decreased in intensity within narrow temperature ranges independent of whether the amine was mixed in milk. Temperature-dependent Raman allows identification of individual chemical structure components that concur with and/or trigger the start of phase transitions.