Development, characterisation, and application of a novel polydimethylsiloxane-microdiamond composite sorbent for selective extraction and preconcentration

The thesis encompasses the development and characterisation of a novel polydimethylsiloxane-microdiamond (PDMS-˜í¬¿Diamond) composite sorbent for application within 'in-sample extraction' prior to either liquid desorption (LD) or thermal desorption (TD) and gas chromatography (GC) analysis. Chapter 1 introduces a review of the novel coatings (other than PDMS) developed over the last five years for stir bar sorptive extraction (SBSE). The review focuses on the selectivity of each, as well as the physical, chemical and thermal stabilities of these coatings, which are based on a range of carbon materials, functional polymers, metal organic frameworks, and various nanoparticles. The majority of these materials exhibit high thermal and chemical stability, along with unique selectivity profiles to extend the potential application of SBSE. The review also identifies how the extractive stir bars (SBs) modified with these coatings have been used to develop methods based more on LD-high performance liquid chromatography (HPLC) than on the TD-GC. Therefore, highlighting how future research should focus on how these novel coatings can be applied with TD-GC. Chapter 2 describes several systematic experiments involving preparation, characterisation, and final application of the non-porous and porous PDMS-˜í¬¿Diamond composite rods as sorptive phases for extracting model solutes from wine samples, followed by their quantitation using LD-GC-FID. Introducing ~60 wt.% of ˜í¬¿Diamond particles (2-4 ˜í¬¿m) within the PDMS matrix gave several advantages: (1) significant increase in density (up to ~170% increase compared to PDMS) while maintaining physical integrity, (2) significant improvement in mechanical stability, (3) increased thermal stability up to 450-500 ¬¨‚à´C, and (4) significantly higher thermal conductivity (~108% higher than that of the PDMS). The composite sorptive rods were applied in the development of a robust PDMS-˜í¬¿Diamond-LD-GC-FID method for the analysis of the model solutes (R\\(^2\\)>0.98, intra-day precision RSD ~1.3-19.4%, accuracy in terms of % recovery ~87->100% RSD 2.1-12.5%). The porous rods showed improved % recovery for most of the test solutes (>10-20%) compared to a commercial PDMS phase under identical analytical conditions. The method's limit of detection (LOD) (0.6-27.3 ˜í¬¿g L\\(^{-1}\\)) further confirmed its robustness and its applicability for the trace chemical analysis. In the Chapter 3, the development of a rapid and a solvent-free method for removing siloxane oligomers from the PDMS-˜í¬¿Diamond composite rods is reported. The method involved post-cure thermal conditioning of the composite rods within a GC liner at 350 ¬¨‚àûC for 12 h under a helium flow of 2.5 mL min\\(^{-1}\\). Significant reduction of siloxane oligomers, as seen in the chromatograms (obtained by placing the rods within the GC liner at 200-350 ¬¨‚à´C), compared to the control chromatograms confirmed the effectiveness of the method, which was ~8 times faster than an alternative solvent purification method (washing the rods in toluene for 72 h, followed by drying for 6 h). Chapter 4 presents the first demonstration of the non-porous PDMS-˜í¬¿Diamond composite rods with TD-GCMS. This study confirmed the structural suitability of the rods when used within a thermal desorption unit (TDU). It also confirmed the advantage of high thermal conductivity of the composite rods, enabling faster solute desorption rates, within 1 min of TD (~11-342% higher mean relative peak area), than a commercial PDMS phase, under similar analytical conditions. The higher %recovery (~84 to >100%, RSD ~3-19%) further confirmed the relationship between the faster desorption rates of the solutes and the improved heat dissipation within the rods. Using an individual rod, a PDMS-˜í¬¿Diamond extraction based TD-GCMS method was developed and validated (R2>0.99, intra-day precision RSD ~3.4-24.7%, and inter-day precision RSD ~5.2-9.3%). The method's limit of detection (LOD) values for ethyl hexanoate (0.6 ˜í¬¿g L-1, ~117% lower), ethyl octanoate (0.1 ˜í¬¿g L-1, ~400% lower), ethyl decanoate (0.4 ˜í¬¿g L-1, ~25% lower), and phenethyl acetate (0.8 ˜í¬¿g L-1, ~13% lower) were lower than that obtained with the commercial PDMS phase and TD-GCMS method, under identical analytical conditions. Application of the non-porous PDMS-˜í¬¿Diamond phase with TD-GCMS to white wine analysis was carried out to confirm suitability of the new sorptive phase when using a TDU. Chapter 5 includes concluding remarks and a summary of the thesis findings and provides a direction for future research, and discussion of potential future applications of the PDMS-˜í¬¿Diamond composite as a sorptive phase within various analytical devices.