Changes of concentrations of plasma free amino acids (PFAAs) is an essential feature of protein metabolic abnormalities in cancer patients. In this study, we aim to decompose Raman spectra of mixtures of amino acids. The effect of noise on the decomposition result is investigated. The experimentally measured spectra of amino acids and artificially simulated spectra of their mixtures are studied. As a decomposition method, Multivariate Curve Resolution–Alternating Least Squares (MCR-ALS) analysis is used. It is shown that one can evaluate the concentration of amino acids in the mixtures using the Raman spectra of the mixtures and the spectra of pure amino acids. The results can be used in further research on lung cancer.