Novel materials for VOC analysis

[摘要] ENGLISH ABSTRACT: The need to analyse and detect volatile organic compounds (VOCs) at trace levels has led to thedevelopment of specialized sample preparation techniques. The requirement for trace analysis ofVOCs stems from the negative effects they have on the environmental and human health. Methodsfor the analysis of non-polar VOCs commonly found as trace contaminants in water and analysis ofmore polar oxygenated compounds commonly found in zero-VOC water-based paints weredeveloped. Solid phase micro extraction (SPME) was employed and extraction of the majority ofthe target analytes could be achieved at levels below 0.3 μg.l-1. In an attempt to further improve thedetection of these two target analyte groups, novel materials based on poly(dimethyl siloxane)(PDMS) were investigated as possible extraction phases for VOCs, with the focus specifically onthe analysis of the polar analytes in paint. Conventional free radical polymerization was used tosynthesize poly(methyl methacrylate-graft-poly(dimethyl siloxane) (PMMA-g-PDMS),poly(methacrylic acid)-graft-poly(dimethyl siloxane) (PMAA-g-PDMS), polystyrene-graftpoly(dimethyl siloxane) (PSty-g-PDMS) and poly(butyl acrylate)-graft–poly(dimethyl siloxane)(PBA-g-PDMS). These polymers have a copolymer functionality which presents a series ofdifferent polarities. The MMA-g-PDMS and MAA-g-PDMS as well as the homopolymers wereelectrospun into nanofibers. The low glass transition temperature and molecular weight of the PBAg-PDMS meant that this polymer could not be electrospun. Scanning electron microscopy (SEM)was used to study the fiber morphology of the electrospun fibers and the non-beaded fibers werefurther investigated. Polyacrylonitrile-graft-poly(dimethyl siloxane) (PAN-g-PDMS) previouslysynthesized and electrospun by another member of the group were also investigated for use aspossible extraction material in volatile analysis. The thermal stability of the nanofibers at 200°Cwas studied using thermal gravimetric analysis (TGA). This property is important since after thetarget analytes are extracted using the nanofibers, elevated temperatures are used to thermallydesorp the volatile analytes from the extraction materials prior to GC analysis. The PAN-g-PDMS,MMA-g-PDMS and PMMA showed no significant weight loss during thermal evaluation, however,it was observed that the PMMA and PMMA-g-PDMS nanofibers looses their nanostructure and thatthe PAN-g-PDMS nanofibers changes colour from white to yellow to rust brown. The polymersbased on MAA showed weight losses of more than 10% after one hour of exposure to the elevatedtemperatures, but the nanostructure remained intact. The PAN-g-PDMS, PMAA-g-PDMS andPMAA nanofibers were evaluated as possible extraction materials for VOC analysis. Thenanofibers were evaluated using a similar approach to that of stir bar sorptive extraction (SBSE).Headspace sorptive extraction (HSSE) using a commercially available PDMS stir bar and the novelmaterials were used to evaluate the extraction efficiency of the different materials. The optimized extraction method developed using SPME were employed for the extraction using the nanofibersand PDMS stir bar. It was noted that the nanofibers lose their extraction capabilities during the firstextraction/desorption cycle possibly due to thermal degradation therefore each of the materials canonly be used in a single extraction. The majority of the non-polar analytes were extracted using thenanofibers at levels of 500 μg.l-1, however it was noted that the commercially available SPMEextraction materials and the PDMS stir bar had superior extraction efficiencies for the specifictarget analytes. In the evaluation of the nanofibers for extraction of the more polar oxygenatedanalytes it was noted that 2-Ethylhexylacrylate was the only analyte to be extracted by all of thematerials. The PAN-g-PDMS extracted three of the four analytes at levels of 100 μg.l-1. At loweranalyte concentrations of 10 μg.l-1 only two of the four acrylate compounds were detected using thePAN-g-PDMS nanofibers. Ethyl acrylate was not extracted by any of the novel materials, whereasin SPME using the CAR/PDMS fiber, the LOD was determined to be below 1 μg.l-1. Althoughthese materials were not superior to the commercially available phases, this is only the case for thespecific target analytes analyzed.

[发布机构] Stellenbosch University