Determination of Carbon Chain Lengths of Fatty Acid Mixtures by Time Domain NMR

Abstract

Average carbon chain length is a key parameter that defines the quality of liquid biofuels. In this paper, a method for the determination of carbon chain lengths of fatty acid mixtures is presented. The approach is based on proton relaxation rates measured by time domain nuclear magnetic resonance. The spin–spin relaxation rates R 2 were used for the estimation of the carbon chain lengths. The method was examined for the set of samples with different mean lengths of the main linear carbon chain. Samples were prepared using four different fatty acids and mixtures of two, three or four of these fatty acids. The correlation coefficient between the known and measured values was equal to 0.994. Based on the relaxation theory, a linear-like dependence between the relaxation rate and carbon chain length was briefly shown, which endorses the experimental results. The developed methodology for determining carbon chain lengths offers robustness and rapidity, which are significant advantages when it comes to online use of the method in real industrial environments.

Molecular Properties of Fatty Acid Mixtures Estimated by Online Time-Domain NMR

“…In addition, this relationship was briefly confirmed by NMR relaxation theory. The values R 2 and T 2 from a previous paper [24] were also used in the current paper to prove the validity of the presented approach for estimation of μ, δ, and CN. Linear relationships were obtained between R 2 or T 2 [24] and the reference values µ mix , δ mix, and CN mix calculated by Eqs.…”

“…In the previous works [30,31], the presence of two components in the transverse magnetization decays of oleic acid and the methyl ester of oleic acid was connected to the different mobilities of protons on the carbon chain. However, in our earlier paper [24], magnetization decays measured for mixtures of different fatty acids showed only one peak in the Laplace transformation.…”

“…It has been previously shown [24] that TD-NMR can be used to determine the CLs of fatty acid mixtures, which is an important quality parameter of liquid fuels. In addition, this relationship was briefly confirmed by NMR relaxation theory.…”

“…Calculation of the carbon chain length CL mix was based on an approach explained in a previous paper [24]:…”

“…The transesterification reaction of oil was monitored in situ by measuring the transverse relaxation decay in the work by Cabeça et al [23]. Moreover, TD-NMR can be used to determine the CLs of fatty acid mixtures [24]. Because the CL strongly affects the µ, δ, and CN of liquid fuels [2,5], TD-NMR could be applied to evaluate the quality parameters of liquid fuels.…”