Study of organic impurities affecting ultraviolet transmittance in monoethylene glycol and their formation mechanism

Monoethylene glycol (1, 2-ethanediol; MEG) is an important monomer for producing polyester. It must be very pure, which is usually indicated by a high ultraviolet (UV) transmittance. Low UV transmittance, especially at wavelength of 220, 250 and 275 nm, usually indicates the presence of undesirable impurities that lower polyester quality. Without a practical analytical method, tracing these impurities is difficult, rather than improving the refining process. Therefore, a facile, fast and sensitive method based on ultra-high performance liquid chromatography (UHPLC) equipped with photodiode array (PDA) detector and coupled with quadrupole time-of-flight mass spectrometry (Q-TOF MS) was developed for qualification and quantification of UV impurities in this paper. The UV impurities were identified as 3-methyl-1, 2-cyclopentanedione, 3, 5-dimethyl-1, 2-cyclopentanedione, and 3-ethyl-1, 2-cyclopentanedione. For tracing these impurities, the samples were directly injected without pretreatment and the compounds were analyzed within 8 min. The limit of detection (LOD) of the proposed method was around 0.07 mg/L. This method has been used to trace these impurities in MEG plants for optimizing process parameters and developing purification techniques. A probable pathway of impurity formation by base (or acid)-catalyzed model was proposed. Hydroxypropanone and 1-hydroxy-2-butanone were identified after 2, 4-dinitrophenylhydrazine (DNPH) treatment and they were supposed to be the precursors of UV impurities.