Development of a miniaturised microchip capillary electrophoresis system and analytical methods for forensic applications

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Cao, Xi
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University College Cork
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The objective of this research was to manufacture a miniaturised microchip capillary electrophoresis (MCE) system, which can be used to analyse suspected chemical agents accurately and rapidly. In order to avoid contamination during sample transportation and to provide real-time data for forensic analysis, this system is ideally portable and can be used on-site at the crime scene. Microchip with cheap and disposable material is also preferred for the purpose of saving cost and simplifying decontamination process. In Chapter 1, the most toxic known chemical warfare agents, nerve agents, including G-series, V-series, Novichok agents, carbamates, and organophosphate (OP) pesticides were introduced. Their history, poisoning cases, and existed analysis methods were briefly described. Introduction of capillary electrophoresis (CE) and MCE were given. Their features like principle, history, modes, materials, fabrications, and detection techniques were also discussed. In Chapter 2, highly toxic OP pesticides were chosen as target analytes. In order to qualify and quantify the selected OP pesticides, including methyl paraoxon (MPX), ethyl paraoxon (EPX), methyl parathion (MPT), fenitrothion (FT), and ethyl parathion (EPT), micellar electrokinetic chromatography (MEKC) with UV/Vis detection and short-end injection were investigated. This was the first time that this combination has been used to separate OP pesticides. A capillary with 8.5 cm effective length was used and the analytes were separated rapidly within 2.1 min. Separation conditions including buffer (type, pH, and concentration), sodium dodecyl sulphate concentration, and separation voltage were optimised. The limit of detection was estimated in the range of 10 – 20 µM. The OP pesticides spiked artificial saliva, drinking water, and river waters gave superior peak profiles, and good average recoveries 95.6 %, 62.3 %, 61.2 %, and 54.6 % respectively. Overall, a rapid method with excellent resolution and efficiency was developed and successfully applied in the analysis of potential sample matrixes. In order to further reduce the separation time, the developed method was subsequently applied on a commercialised MCE system with linear imaging UV/Vis detector. Quartz microchip with 2.5 cm length separation channel was tested and the separation speed was improved to 45 sec. Although UV/Vis detection was commonly used in CE and MCE, capacitively coupled contactless conductivity detection (C4D), an electrochemical detection method, has attracted more and more attention, due to its compact size, low-cost, easy-fabrication, good compatibility, versatility, and low contamination risk. Because of these advantages, it offered great potential in the miniaturisation of the MCE system. In Chapter 3, a low-cost customised MCE system with polydimethylsiloxane (PDMS) microchip and printed circuit board (PCB) C4D electrodes cell was fabricated and tested. Sodium chloride (NaCl) and potassium chloride (KCl) were used as samples to test the system. Fabrication processes of the microchip and the electrodes cell were optimised. Detector parameters including frequency, voltage, gain, ADC, and filter were investigated. Impact of electrodes width was tested using PCB electrodes cells with 0.7 and 1 mm widths. Impact of microchip channel length was tested with 35 and 40 mm channels. Impact of microchip channel width was tested using microchips with 100, 75, and 50 µm width channels. The results indicated that this miniaturised, easy-fabrication, low-cost, and partly disposable MCE system provided very stable signal, flat baseline, satisfied signal-to-noise ratio (SNR), and robust detection ability in chemical analysis. The C4D detector was also attached to CE to collect comparable data, however noisier baseline, lower SNR and fronting peak shape were observed. In Chapter 4, the MCE system that was fabricated and tested in Chapter 3 was applied to analyse of OP pesticides (MPX, EPX, MPT, and FT), their degradation products (3-methyl-4-nitrophenol (3M4NP), p-nitrophenol (PNP), diethyl phosphate (DEP), and dimethyl phosphate (DMP)), and sarin simulant dimethyl methylphosphonate (DMMP). In the test of OP pesticides, native PDMS chip showed no visible peak when MPX was injected, due to the interaction between hydrophobic MPX and hydrophobic PDMS surface. After modification of the PDMS surface with plasma (stored in water) and poly (vinyl alcohol) (PVA), and combination of the PDMS with capillary, stable hydrophilic surfaces were obtained, and MPX and EPX were successfully detected. In the test of OP pesticides’ degradation products, four degradation products were separated with fully-resolved peaks in CE-C4D. In MCE-C4D, all of the degradation products were successfully detected, and 3M4NP, DEP and DMP were separated. As part of the FP7 Project GIFT (Generic Integrated Forensic Toolbox), a chemical exercise was planned. In this chemical exercise, the MCE system was sent to Belgium and set up in a mobile lab near the crime scene. An unknown liquid sample was collected in the kitchen of the crime scene and injected into preconditioned PDMS chip. Peak of unknown sample was obtained within 1.5 min and was confirmed as DMMP. In Chapter 5, the MCE system was applied to analyse other analytes including aspirin, tetrahydrocannabinol (THC), and Shigatoxin 1 antibody (13C4). Aspirin was detected using both CE-C4D and MCE-C4D. Because of its hydrophilic property, good repeatability was achieved. When THC was tested using MCE-C4D, it was found that peak height decreased when the injection was repeated. The reason was inferred as THC partially adsorbed on the surface of PDMS. For the antibody test, the antibody moved in the electric field and can be detected by the C4D detector at low concentration (0.25 ppm). However, it is more hydrophobic than THC and thus fully adsorbed on the surface of PDMS. The inference was proved by disappearance of the antibody peak in the repeated injections. From Chapter 4 and 5, the MCE system showed rapid, stable, and reliable analysis ability for all applications, especially for the hydrophilic compounds. For the hydrophobic compounds, surface modification methods were developed and effectively improved the detection. The system has also met all requirements (portable, low-cost, easy-fabrication, disposable, high-speed and real-time analysis) of the project, and gave satisfied outcomes in the field deployment. In Chapter 6, general conclusions of the work were given, possible future works were discussed, and final remarks was provided. In Appendix, publications, poster presentations, oral presentations, showcasing MCE system, other workshops and conferences, and awards were listed. Copies of published papers were also attached.
Capillary electrophoresis , Microchip capillary electrophoresis , Contactless conductivity detection , Organophosphate pesticides , Forensic applications
Cao, X. 2019. Development of a miniaturised microchip capillary electrophoresis system and analytical methods for forensic applications. PhD Thesis, University College Cork.