Specific Features of the Gas Chromatography–Mass Spectrometry Identification of Monoalkyl Phthalates

Abstract

A similarity of electron ionization mass spectra of various alkyl esters of benzene-1,2-dicarboxylic (phthalic) acid places special demands on the reliability of the identification of monoalkyl phthalates, often recognized as dialkyl phthalates and vice versa. An original mass spectrometric criterion is proposed for the group identification of monoalkyl phthalates (assignment to a subgroup of homologues), which assumes the calculation of the total relative intensity of ion peaks at m/z > 170. In the mass spectra of monoalkyl esters, this value does not exceed 1%, whereas for dialkyl ethers, it varies from 1 to 172%. Within the set subgroups of isomers, it becomes effective to use the homologous increments of the retention indices, i_RI = RI – 100x, where x = int (M/14), to refine the identification results. Using predefined i_RI values and the ratio М ≈ 0.14(RI – i_RI) + y, we can estimate the molecular weights of compounds in the mass spectra of which the molecular ion signals are not detected or are of low intensity (including monoalkyl phthalates). In addition, i_RI values are in many cases informative for estimating the number of branches of the sp³ carbon skeleton of molecules (Z) based on regression relations of the form of i_RI = aZ + b. The examination of homologous increments of the retention indices of dialkyl phthalates revealed an anomalous dependence of their values on the number of carbon atoms in the alkyl fragments of the molecules: from +125 (СН₃) to –33 (С₁₀Н₂₁). In the subgroup of monoalkyl phthalates, such anomalies were not observed.