Tetramethylurea (TMU) - Isotopic ratio (D/H) Used as internal standard for the determination of D/H isotope ratios of ethanol by 2H-NMR method (SNIF-NMR®) according to the formula 5-1 of Regulation EEC 2676/90. Certified value Uncertainty (D/H) by NMR ........................................ 141.9 x 10-6 .................. 0.9 x 10-6