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Comprehensive analysis of alkylphenols and their polyethoxylates by using hyphenated chromatographic techniques
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posted on 14.02.2017by Wu, Zeying
Non-ionic surfactants, alkylphenol polyethoxylates (APnEOs, where n designates the number of ethoxy (EO) units) are primarily available as nonyl- or octylphenol polyethoxylates (NPnEOs and OPnEOs). NPnEOs are often a mixture of oligomers with various isomeric and branched nonyl side chains, while OPnEOs consist of only 4-tert-OP ethoxymers. Their starting materials, APs are formed in the environment as their final degradation products. The ability of APs to bioaccumulate and exhibit toxicity and estrogenic activity is strongly tied to their structures. To address the analysis of APnEOs homologues, oligomers and NPs and NPnEOs isomers, hyphenated chromatographic techniques have been developed in the current study.
The principles and mechanisms of comprehensive 2D separations (C2DS) were understood using GC×GC-FPD as a model. The applicability of GC×GC for the complete resolution of NPs isomers was investigated although co-elution was always observed. Thus, comparative study of the identification of NPs isomers was performed using EIC, AMDIS and HELP chemometric resolution approaches based on the GC-MS results. Chemometric method provided the pure chromatograms and mass spectra of 15 NPs isomers determined, which failed to be obtained by EIC or AMDIS methods.
Since the degradation of NPnEOs does not change the structure of the nonyl side chain, the isomeric composition of commercial NPnEOs was studied. NPLC first separated NPnEOs into individual oligomers. Preparative collection of each early eluting ethoxymer fraction allowed further separation of different isomeric nonyl group components by using analytical GC-MS. The GC-MS results suggest that each fraction has the same isomeric composition and each corresponding peak has the same isomeric structure. Prep-GC coupled with GC-MS and NMR elucidated the molecular structure of one resolved NP2EOs isomer. This is described in Chapter 3.
Chapter 4 consists of two studies, which focus on the HILIC technology. The separation mechanism was studied on a zwitterionic HILIC column. Adsorption was shown to be the dominant mechanism while hydrophilic and electrostatic interactions also contributed to the retention under different conditions. On the bare silica packing materials, both adsorption and partition were responsible for retention. The baseline resolution of NPnEOs ethoxymers demonstrated the applicability of HILIC-ESI-MS method.
For the simultaneous determination of APnEOs homologues and oligomers, an LC×LC system was established, as reported in Chapter 5. Different separation modes - NPLC, RPLC and HILIC - were compared in terms of separation for alkyl and EO distributions. NPLC offered better resolution than HILIC; however, the immiscibility of non-polar NPLC solvent with RPLC mobile phases leads to HILIC being chosen as 1D. The need and meaning of comprehensiveness and orthogonality were understood through the use and suggestions for definitions, nomenclature and symbols. A HILIC×RPLC system was then set up and its capability was demonstrated by complete simultaneous separation of APnEOs into individual oligomers, with each alkyl end group resolved. Different descriptors and metrics for assessing system orthogonality were investigated with the obtainment of a relatively high dimensionality of 1.76.
Finally, Chapter 6 provides general conclusions, highlighting the key outcomes in each study and proposes areas for attention and future work.