After bacteriorhodopsin was successfully isolated from bacterial membranes and was hydrolyzed H.halobium to individual amino acids was necessary investigate the levels of enrichment of deuterium in the aromatic amino acids. Analysis of the degree of deuteration of amino acids of the hydrolyzate of bacteriorhodopsin is most conveniently carried out by mass spectrometry in electron impact EI instrument "MB-80 A" (Hitachi, Japan) at an energy of ionizing electrons 70 eV. Full mass spectrum of the electron-impact blend of methyl esters of N-DNS-amino acid derivatives, shown in Fig. 7 (scanning at m / z 50-640, the base peak m / z 527, 100%), differed continuity. The peaks in the range m / z 50 to 400 on the scale of the mass numbers are fragments of metastable ions, low molecular weight impurities, as well as products of chemical modification of amino acids. Analyzed 2H-labeled aromatic amino acids occupying the scale of the mass numbers of m / z from 415 to 456 were a mixture of molecules with different numbers of deuterium atoms incorporated, so that molecular ions (M) + polymorphonuclear were split into separate clusters with a statistical range of values of m / z depending on the number of hydrogen atoms in a molecule. Given the isotopic effect of polymorphism, counting the level of deuteration of the molecules of amino acids was carried out on the most popular peak of the molecular ion (M) + in each cluster with a mathematically averaged value (M) + (Fig. 7) – for the molecular ion peak of phenylalanine was determined (M) + at m / z 417, 14% (instead of (M) + at m / z 412, 20% for the unlabeled derivative (Peaks unlabeled amino acids are not shown)), tyrosine – (M) + at m / z 429, 15% (instead of (M) + at m / z 428, 13%), tryptophan – (M) + at m / z 456 11% (instead of (M) + at m / z 451, 17%).