Simultaneous determination of amino acid mixtures in cereal by using terahertz time domain spectroscopy and chemometrics

Terahertz (THz) spectroscopy displays special features comparing to the commonly used spectroscopies like infrared and Raman and provides potential applications in broad areas. In this work, terahertz time-domain spectroscopy (THz-TDS) has been utilized for the qualitative and quantitative analyses of ternary mixtures of L-Glutamic acid, L-Glutamine, and L-Tyrosine which have similar chemical structures and properties. Mixtures of amino acids were prepared with yellow foxtail millet matrix to simulate more natural situation instead of polyethylene (PE). Partial least squares (PLS) and support vector machine (SVM) were compared for quantitative analysis in this work. Preprocessing methods, multiplicative scatter correction (MSC), Savitzky-Golay (S-G) smoothing, first derivative and wavelet transform, were investigated based on bootstrapped Latin partitions and external test. The SVM model with MSC preprocessing yielded a stable model and gave accurate prediction for the ternary amino acid in foxtail millet analyzed by THz-TDS. The THz absorption spectra and corresponding concentration profiles of three amino acids components from mixtures were resolved by multivariate curve resolution alternating least squares (MCR-ALS). The results show that THz spectroscopy could be applicable for analyzing more nutritional compounds in different cereals in the future.