Strategies to Improve Solid Phase Microextraction Sensitivity: Temperature, Geometry and Sorbent Effects
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Solid phase microextraction (SPME) has been widely used in a variety of sample matrices and proven to be a simple, fast and solvent-free sample preparation technique. A challenging limitation in the further development of this technique has been the insufficient sensitivity for some trace applications. This limitation lies mainly in the small volume of the extraction phase. According to the fundamentals of SPME, different strategies can be employed to achieve higher sensitivity for SPME sampling. These include cooling down the extraction phase, preparing a high capacity particle-loading extraction phase, as well as using a thin film with high surface area-to-volume ratio as the extraction phase. In this thesis, four sampling approaches were developed for high sensitivity sampling by employing cold fiber, thin film, cooling membrane and particle loading membrane as sampling tools. These proposed methods were applied to liquid, solid and particularly trace gas analysis. First, a fully automated cold fiber device that improves the sensitivity of the technique by cooling down the extraction phase was developed. This device was coupled to a GERSTEL® MultiPurpose Sampler (MPS 2), and applied to the analysis of volatiles and semi-volatiles in aqueous and solid matrices. The proposed device was thoroughly evaluated for its extraction performance, robustness, reproducibility and reliability by gas chromatograph/mass spectrometer (GC/MS). The evaluation of the automated cold fiber device was carried out using a group of compounds characterized by different volatilities and polarities. Extraction efficiency and analytical figures of merit were compared to commercial SPME fibers. In the analysis of aqueous standard samples, the automated cold fiber device showed a significant improvement in extraction efficiency when compared to commercial polydimethylsiloxane (PDMS) and non-cooled cold fiber. This was achieved due to the low temperature of the coating during sampling. Results from the cold fiber and commercial divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber analysis of solid sample matrices were obtained and compared. Results demonstrated that the temperature gap between the sample matrix and the coating significantly improved the distribution coefficient, and consequently, the extraction amount. The newly automated cold fiber device presents a platform for headspace analysis of volatiles and semi-volatiles for a large number of samples, with improved throughput and sensitivity. Thin film microextraction (TFME) improves the sensitivity by employing a membrane with a high surface area-to-volume ratio as the extraction phase. In Chapter 3, a simple non-invasive sample preparation method using TFME is proposed for sampling volatile skin emissions. Evaluation experiments were conducted to test the reproducibility of the sampling device, the effect of the membrane size, and the method for storage. Results supported the reproducibility of multi-membrane sampling, and demonstrated that sampling efficiency can be improved using a larger membrane. However, ability to control the sampling environment and time was proved to be critical in order to obtain reliable information; the in vivo skin emission sampling was also influenced by skin metabolism and environmental conditions. Next, the method of storage was fully investigated for the membrane device before and after sampling. This investigation of storage permitted the sampling and instrument analysis to be conducted at different locations. Finally, the developed skin sampling device was applied in the identification of dietary biomarkers after garlic and alcohol ingestion. In this experiment, the previously reported potential biomarkers dimethyl sulphone, allyl methyl sulfide and allyl mercaptan were detected after garlic intake, and ethanol was detected after the ingestion of alcohol. Experiments were also conducted in the analysis of volatile organic compounds (VOCs) from upper back, forearm and back thigh of the body on the same individual. Results showed that 27 compounds can be detected from all of the 3 locations. However, these compounds were quantitatively different. In addition, sampling of the upper back, where the density of sebaceous glands is relatively high, detected more compounds than the other regions. In Chapter 4, a novel sample preparation method that combines the advantages of cold fiber and thin film was developed to achieve the high extraction efficiency necessary for high sensitivity gas sampling. A cooling sampling device was developed for the thin film microextraction. Method development for this sampling approach included evaluation of membrane temperature effect, membrane size effect, air flow rate and humidity effect. Results showed that high sensitivity for equilibrium sampling can be achieved by either cooling down the membrane and/or using a large volume extraction phase. On the other hand, for pre-equilibrium extraction, in which the extracted amount was mainly determined by membrane surface area and diffusion coefficient, high sensitivity was obtained by thin membranes with a large surface area and/or high sampling flow rate. In addition, humidity evaluations showed no significant effect on extraction efficiency due to the absorption property of the liquid extraction phase. Next, the limit of detection (LOD) and reproducibility of the developed cooling membrane gas sampling method were evaluated. LOD with a membrane radius of 1 cm at room temperature sampling were 9.24 ng/L, 0.12 ng/L, 0.10 ng/L for limonene, cinnamaldehyde and 2-pentadecanone, respectively. Intra- and inter-membrane sampling reproducibility had a relative standard deviation (RSD%) lower than 8% and 13%, respectively. Results uniformly demonstrated that the proposed cooling membrane device could serve as a powerful tool for gas in trace analysis. In Chapter 5, a particle-loading membrane was developed to combine advantages of high distribution coefficient and high surface area geometry, and applied in trace gas sampling. Bar coating, a simple and easy preparation method was applied in the preparation of the DVB/PDMS membrane. Membrane morphology, particle ratio, membrane size and extraction efficiency were fully evaluated for the prepared membrane. Results show that the DVB particles are uniformly distributed in the PDMS base. The addition of a DVB particle enhanced the stiffness of the membrane to some extent, and improved the extraction capacity of the membrane. Extraction capacity for benzene was enhanced by a factor of 100 when the membrane DVB particle ratio increased from 0% to 30%. Additionally, the prepared DVB/PDMS membrane provided higher extraction efficiency than pure PDMS membrane and DVB/PDMS fiber, especially for highly volatile and polar compounds. The high reproducibility of the prepared DVB/PDMS membrane in air sampling demonstrated the advantage of the bar coating preparation method, and also permitted quantitative analysis. Last, the prepared particle-loading membrane was applied to semi-quantitative and quantitative analysis of indoor and outdoor air, respectively. Both the equilibrium calibration method and diffusion-based calibration method were proposed for the quantitative analysis. Results showed that the high capacity particle-loading membrane can be used for monitoring trace analytes such as perfume components and air pollutants.