Illustration of inversion-recovery and Carr-Purcell-Meiboom-Gill sequences by the determination of ethanol content in alcoholic beverages

In Nuclear Magnetic Resonance (NMR) education, the introduction of the relaxation phenomenon and the relaxation times (T₁ and T₂) is an important and compulsory step, as is the description of the Carr-Purcell-Meiboom-Gill (CPMG) and inversion-recovery (IR) measurement sequences. Indeed those sequences are still used nowadays for, respectively, the measurement of T₂ and T₁ but also in Magnetic Resonance Imaging (MRI) and NMR spectroscopy. Practical works with the students, performed for example with water, allow to illustrate this part of the teaching. In this work we propose an alternative and funny way to introduce these important topics. With a few microliters of a concentrated Gd³⁺ solution, a few milliliters of an alcoholic beverage and a low resolution and low field NMR device, it is possible, thanks to the relaxation phenomenon and using CPMG and IR sequences, to measure the alcohol content of the beverage provided that the alcohol proton exchange with water protons is taken into account. First the method is validated with synthetic water-ethanol mixtures, then it is used to study nine different alcoholic beverages. The correlation of the ethanol volume fractions determined by NMR with the actual ethanol content of the beverages is rather good, especially for the method based on T₂ relaxation, with a correlation coefficient r² = 0.994. However, it seems that the method developed in this work always underestimates the ethanol volume fraction at high ethanol content for a reason which remains to be found.