Site-specific natural isotópe fractionation in fruit juices determined by deuterium NMR an European inter-laboratory comparison study

An inter-laboratory comparison study by deuterium NMR spectrometry was organized by working group No. 1 of the European Commission of Standardization (CEN/TC 174) in order to define the repeatability (r) and the reproducibility (R) of the site-specific isotope ratio determinations in ethanol resulting from the fermentation of fruit juices. The fifteen participating laboratories from the European Union and from Israel received six different juices in blind duplicates, 100 g of beet sucrose and a convenient quantity of the carefully calibrated internal reference TMU (tetramethylurea) for the NMR determination. The different laboratories used the same experimental protocol under different conditions (operator, spectrometer, period of time) to ferment the fruit juices, distil the ethanol and record the 2H NMR spectra. The results were considered to be acceptable by specialists for the field of isotopic analysis and, according to the ISO Standard 5725/86, r = 3.6% (0.6 ppm) and R = 4.8% (0.8 ppm). It was also shown that isotope ratio (D/H)1 for CH2DCH2OH as determined by 2H NMR, correlates well with the normalized average ratio of the non exchangeable sites of glucose determined by mass spectrometry. However, good reproducibility was not obtained for these measurements and it was concluded that sample preparation led to significant systematic deviations.