Purpose. The article is devoted to brief exposition of methodology of calculation and research of cross-sections of the single ionising of complex molecules, e.g. amino acids, by an electron impact. The normalization of relative values of experimental complete cross-sections of ionization with the aim of obtaining its absolute values is proposed. Approximation procedure of these absolute cross-sections is briefly discussed. Methods. The well-known analytical expressions of description of ionization process of molecules are given in semiclassical (Binary – Encounter – Dipole and Binary – Encounter – Bethe) and classical (Gryzinski formula) approaches. On their basis, analytical expressions for realization of procedure of approximation of ionization cross-section of molecules are given. Results. Ionization potential of glutamine molecule in two approximations - adiabatic and molecular orbitals – were calculated. Thus, for the calculation of structure of molecule two approachеs  – density-functional theory and Hartree-Fock theory – were used. Calculated values were compared with experimental data. Using mentioned formulas, energy dependences of single ionization cross-sections of glutamine molecule by electron impact up to 60 eV energy were calculated. Two approaches – density-functional theory and Hartree-Fock – were used as well. Conclusions. Considered analytical expressions for the calculation of single ionization of cross-sections may be successfully used for the analysis of the ionization process of complex molecules by an electron impact. Energy behavior of calculated ionization cross-sections differs significantly between used approaches. Ionization potential of glutamine molecule that was calculated using density-functional theory coincides better with experimental data

Keywords: cross section, ionization, approach, approximation, molecule, glutamine