Melamine (IUPAC: 1,3,5-Triazine-2,4,6-triamine) has wide industrial importance, nowadays being used e.g., in the manufacture of polymers and resin [1
], concrete [2
], flame-resistant materials [3
], and it may be utilized in the production of nanomaterials (e.g., N
-doped carbon nanotubes) [4
]. In the past, it was even more widely applied in industry and agriculture [5
]. It was unfortunate that melamine has become infamous worldwide as a dairy adulterant after it caused a milk safety crisis in 2008 with severe casualties (290,000 people affected with 51,900 hospitalized in China only) [6
]. That event had a global impact on the food industry, food production, supply chains, and corresponding legal regulations [7
]. It strongly echoed in the field of food quality control, leading to a strong stimulus for development and the adaptation of adequate analytical routines [8
]. A number of other food safety incidents in recent years have induced particular pressure on this area of analytical chemistry [11
]. The methods that are based on vibrational spectroscopy [infrared (IR), Raman and near-infrared (NIR)] have become particularly important elements of this effort in controlling the food safety at every stage of its production and supply [13
Vibrational spectroscopy stands out as a non-invasive, widely applicable, low-cost, and quick time-to-result analytical method. Therefore, it combines advantages that are highly valued in analytical chemistry. Despite being grouped together, the key differences among these three kinds of techniques should be noted. IR (4000–400 cm−1
) spectroscopy elucidates chemical information from the fundamental vibrational transitions. In contrast, the signal that was measured in NIR (10,000–4000 cm−1
) spectroscopy originates from the excitations of higher quanta transitions, mostly first overtones and binary combinations [16
]. Raman spectroscopy also provides information regarding fundamental vibrations, but through a distinctly different working principle than IR spectroscopy. The differences in the wavelength regions and underlying physical background translate into distinct differences in the instrumentation and applicability of these methods. Each of these techniques offer unique advantages, but for the detection/quantification of melamine content in milk, NIR spectroscopy may be favored [9
]. A number of factors contribute to this fact [19
]. Higher sample volume resulting from low NIR absorptivity of matter in general, and water, in particular, allows for more straightforward measurement of transmittance or reflectance of the milk sample. The same may be achieved with ATR-IR (Attenuated Total Reflection IR) approach; however, the fiber probe compatibility of NIR instrumentation gives it superior flexibility in high-volume analysis. NIR spectroscopy also benefits from the largest tolerance for the sample inhomogeneity. Raman spectroscopy is suitable for the measurement of aqueous samples, but for similar analytical applications Raman instrumentation is often more expensive. Finally, in NIR spectroscopy, a strongly stimulated development for miniaturization [20
] has culminated in highly affordable micro-spectrometers, which are available under 300 USD nowadays [22
]. In a strict application to melamine detection/quantitation, all three techniques have been successfully used in the literature 9]. However, NIR spectroscopy demonstrates the best analytical performance in this case [9
], on top of its practical advantages [23
The nature of NIR spectra (overlaying overtones and combinations) [16
] largely limits their interpretability [25
]. Chemometric methods do not provide physical insights on the analyzed sample, and NIR spectroscopy is often used as a “black-box”. In contrast, for IR and Raman spectroscopy, this limitation is less severe due to its more simple spectra with milder fundamental band overlapping [27
]. Coincidently, quantum chemistry offers affordable methods (harmonic approximation) for the adequately accurate simulation of IR and Raman spectra [29
]. In contrast, prediction of NIR bands require resource intensive anharmonic methods [31
]. The difference in resource demand (harmonic vs. anharmonic approximation) is significant, and theoretical NIR studies of complex molecules have only recently appeared [33
]. Lately, we have reported the quantum mechanical calculations of NIR spectra of various molecules in solution, liquid, and solid state, including short- [34
], medium- [35
], and long-chain [36
] fatty acids. These studies could have been used, e.g., for the interpretation of the meaningful NIR bands that influence the chemometric models used in quantification of the content of phytopharmaceutical compounds in natural drugs [37
]. On the other hand, simulations of IR spectra in crystalline phase that use a proper representation of infinite crystal lattice by a three-dimensional (3D) periodic model remain equally rare.
In the literature so far, quantum chemical calculation vibrational studies of melamine have been limited to finite models and harmonic approximation [40
]. Mircescu et al. have harmonically calculated IR and Raman spectra of melamine [40
]. They have used the single molecule model, and cluster of 10 melamine molecules. They have compared the spectra that were calculated with these two approaches and judged that the 10 molecule cluster model leads to a better quality of the calculated IR and Raman spectra of melamine. They concluded that the hydrogen-bonding of melamine in crystal needs to be taken into account in order to yield accurate calculated vibrational spectra. Accordingly, the calculations that are based on the 10 molecule cluster have led to much improved simulated IR and Raman spectra. Yuan et al. have drawn similar conclusions [41
] in their quantum mechanical calculations of IR spectra of melamine. They have used a single molecule model, a four-molecule cluster with two hydrogen-bonds, and a large cluster consisting of 32 molecules of melamine featuring 30 intermolecular hydrogen-bonds. They have compared the IR spectra that were calculated on the basis of these models and concluded that proper representation of the hydrogen-bonded structure of melamine is essential in improving the quality of the calculated IR spectrum [40
Therefore, the earlier studies [40
] have recognized the importance for spectra calculation of the proper description of the chemical neighborhood in crystalline melamine, in particular, the hydrogen bonding network. However, the methodology in these studies has been limited to finite models, clusters of melamine molecules (10 to 32 molecules). Although improved vs. single molecule models, the finite boundary of these model clusters has led to the distortions of the molecular structure as compared with the structure of the crystal lattice of melamine. This has resulted in a number of “phantom bands” appearing in the calculated IR and Raman spectra, which could not be observed in the experimental spectra [40
]. Additionally, these previous studies have been limited to harmonic approximation, which made any calculations of NIR bands unavailable. Accordingly, there are no NIR spectra simulations of melamine of any kind reported so far. On the other hand, melamine was intensively focused on in analytical near-infrared reflectance spectroscopy (NIRS) [9
]; however, these studies have not been able to derive insightful physicochemical information on melamine.
The purpose and novelty of the present study is to explore NIR vs. IR spectra correspondences in crystalline melamine. To achieve this, for the first time, we employ anharmonic quantum mechanical calculations of NIR spectra of melamine, by using two different approaches, which we directly compare. Moreover, we improve the previous investigations of IR spectra in a well-defined crystalline lattice by employing an infinite three-dimensional (3D) periodic model of the crystalline melamine, for the first time as well. This yields more accurate calculated IR spectrum, but it also is essential in obtaining good comprehension of a number of relevant effects. In example, the impact of anharmonicity may become well separated from the influence of the chemical neighborhood. The distinct difference in the importance of inter-molecular interactions for the accurate reproduction of IR and NIR transitions of melamine is found and explained. This means that, in contrast to IR bands, the accurate reproduction of NIR bands requires significantly less attention in describing the long-range, and in particular, inter-plane, interactions in crystalline melamine.
Quantum mechanical calculations of IR spectra of crystalline melamine were successfully carried out for an infinite (3D periodic) lattice model. All of the experimental IR bands were accurately reproduced, even when using reasonably affordable harmonic approximation. On the other hand, finite models gave very inaccurate calculated IR spectra. In contrast, the NIR spectrum of crystalline melamine was accurately modeled by anharmonic calculations that are based on the model of a single molecule. It is a striking difference in the dependence of the quality vs. model complexity, between these two spectral regions. This result may be explained based on direct comparison of the molecular motions corresponding to the vibrations being influential for the IR and NIR regions.
Vibrations involving out-of-plane atomic motions strongly affect the IR region of melamine. Therefore, it is essential to incorporate a proper description of the inter-plane interactions of melamine molecules as they appear in the crystalline lattice. The neglecting of the neighboring molecules in proximity planes leads to a completely incorrect calculated IR spectrum. In contrast, the in-plane vibrations of melamine are less affected by inter-molecular interactions. The vibrations involving in-plane atomic displacements are the most essential for the NIR spectrum of melamine. On the other hand, the out-of-plane motions are either suppressed in intensity or they are located outside of the NIR region. Hence, the calculated NIR spectrum of melamine is not significantly affected by a radical simplification of the molecular model. Even a single molecule model provides accurate reproduction of NIR spectrum of crystalline melamine. It may be concluded that the long-range ordering and, in particular, inter-plane interactions in the crystal lattice of melamine are significantly less important factors for NIR modes than for IR modes. From this observation, another important conclusion may be drawn. Due to the very significant computational cost of anharmonic calculations, the possibility to reduce the complexity of the molecular model (i.e., non-necessity to use 3D infinite model, which is also computationally costly) in the simulation of NIR spectra offers promising possibilities for similar studies of other crystalline materials.
Our findings shed light on the spectral features of melamine that have been reported in analytical spectroscopic studies of melamine as contaminant. In particular, the concluded in literature superiority of NIR spectroscopy in the analysis of the samples with relatively higher content of melamine was explained.