In this equation, X is the matrix of experimental data, C and S
are concentration profiles of the compounds and spectral profile,
respectively, and E is the error-related matrix. The decomposition
of the X matrix is attained through iterative optimization. The
result of this process is finding the C and S matrices that minimize
the error in the raw dataset at each iteration cycle. To start the
iterative ALS procedure, an initial estimation is required for the
concentration or spectral profiles of each species. Several methods
have been used for this purpose, such as simple-to-use interactive
self-modeling mixture analysis (SIMPLISMA) [20], which was utilized
for the initial estimation of spectral profile in this study. If the
initial estimates are the spectral profiles, the unrestricted least
squares solution for the concentration profiles can be calculated
according to following equation: