A minimal sampling, in-line spectroscopic calibration method for unstable components during ammoniation of fatty acids

The ammoniation of fatty acid produces fatty amide, as well as unstable ammonium salt, whose composition profile can vary with off-line sampling. This makes the analysis and determination of reaction kinetics challenging. In-line FT-IR spectroscopy removes the need for sampling, but requires calibration of the reacting system at reaction conditions, which is complicated by the near-instant formation of ammonium salt. In this work, we demonstrate the development of a calibration method which overcomes the complexities posed by both off-line and in-line analysis. This is achieved by taking only eight off-line samples for determination of the more stable components and formulating mass balances of the reactive system for each in-line acquired FT-IR spectrum. This allows for the determination of the rapidly fluctuating salt content. The essential assumptions underlying the mass balances and the resulting calibration are based on a qualitative examination of the reaction system through principal component analysis. Put together, this enables in-line calibration of the system at reaction conditions, achieving relative errors below 10% using partial least squares regression.