Research On The Hyphenation Of Microfluidic Chip-based Analysis And Inductively Coupled Plasma Mass Spectrometry

Owing to its figures of merit including low sample and reagent consumption, high analytical efficiency, miniaturization and portability, microfluidic chip-based analysis has become one of the hottest topics in the chemistry and biology fields. Inductively coupled plasma mass spectrometry (ICP-MS) is an outstanding technique applied to a wide range of applications in analytical chemistry, because of its exceptional detection power, multi-elemental and isotopic capabilities and wide dynamic range. Therefore, it is an excellent tool for detailed characterization of the elemental composition. Since the coupling of microchip-based analysis and ICP-MS combines the figures of merit themselves, total quantitative analysis of microsamples and elemental speciation analysis by ICP-MS may be feasible with the assistance of microfluidic chip-based analysis. However, their successful hyphenation relys on an effective interface, which can transfer the sample in chips in the nL/min level into the plasma with high efficiency. Such an interface should be on the basis of a microflow nebulizer operated in the nL-μL per min range.In the first chapter, factors affecting nebulising efficiency of pneumatic nebulizers, microflow nebulizers and mini spray chambers for inductively coupled plasma spectrometries, capillary electrophoresis-ICP-MS interfaces, short column capillary electrophoresis as well as microchip based electrophoresis hyphenated with ICP-MS were reviewed.In the second chapter, a simple and demountable capillary microflow nebulizer (d-CMN) was developed for inductively coupled plasma mass spectrometry (ICP-MS). It consisted of a nebulizer body, a fused-silica capillary with a tapered tip and a polytetrafluoroethylene (PTFE) adapter. The gas orifice i.d., the solution capillary tip i.d. and its wall thickness were200,30, and5μm, respectively. The experimental results indicated that its performances at low uptake rates were similar or better than those obtained with the conventional concentric nebulizer at820μL/min and the micromist nebulizer at200μL/min. The tapered capillaries can be easily fabricated from commercially available fused-silica capillaries by a flame-heated drawing method with good reproducibility. The low self-aspiration rate (4.77μL/min) and the analytical characteristics comparable to commercial microflow nebulizers made the d-CMN a good choice for coupling capillary electrophoresis and microbore high-performance liquid chromatograph to ICP-MS. The demountable construction of the d-CMN permitted that the blocked or broken solution capillary could be conveniently renewed.In the third chapter, a microfluidic sub-microlitre (0.1-0.8μL) sample introducing system was developed for direct analysis of Chinese rice wine by inductively coupled plasma mass spectrometry (ICP-MS). It consisted of a microfluidic chip, an eight-way multifunctional valve used in flow injection analysis (FIA), a syringe pump and a peristaltic pump of the Ar ICP-MS instrument.20%ethanol containing15g/L glucose,20%ethanol containing40g/L glucose and20%ethanol containing100g/L glucose were used to simulate Chinese rice wine of the dry type, the semidry type and the semisweet type, each. The effects of their volume introduced into ICP-MS on the plasma stability and ICP-MS intensities were studied. The experimental results showed that neither alteration of plasma stability nor carbon deposition was observed when the sampling volume of20%ethanol containing100g/L glucose was downscaled to0.8μL. Further reducing the sampling volume to0.4μL, no significant difference between the intensities of multi-element standard prepared in three rice wine matrices and those in aqueous solution. It indicated no negative effect of rice wine matrix on the ICP-MS intensities. The intensities of111Cd and208Pb nearly linearly ascended with the sample volume. A sample volume of0.4μL was considered to be a good compromise between sensitivity and matrix effect. Based on these observations, a microflow injection (μFI) method for the direct determination of cadmium and lead in Chinese rice wine by ICP-MS using an external aqueous calibration was developed. Owing to elimination of time-consuming pretreatment of Chinese rice wine, the total analysis time was dramatically reduced, inducing high analytical efficiency. The sample throughput was45h-1with the detection limit of19.8and10.4ng/L for Cd and Pb, respectively. The contents of Cd and Pb in10Chinese rice wine samples were measured. The results agreed well with those determined by ICP-MS with the conventional sampling system after microwave assisted digestion.In the forth chapter, a microfluidic system for introducing nanolitre sample in inductively coupled plasma mass spectrometry using electrokinetic flow combined with hydrodynamic flow was developed, which consisted of a microfluidic chip, a syringe pump, a high voltage power supply, a homemade microflow nebulizer and a heated single pass spray chamber. In the microfluidic chip, the sample in the reservoir S automatically flowed across the intersection P1to the reservoir SR under hydrodynamic pressure. Two porous polymer plugs C1and C2were created on the microchip to isolate electrokinetic flow from the pressure-driven flow. Once applying a high voltage between S and buffer reservoir B, the sample was driven through the porous polymer plug C1into the metering channel by electrokinetic force. Simply changing the sampling time can freely optimize the sampling volume down to0.18nL, which was injected into the nebulizer by the pressure-driven makeup solution. The nanolitre sample introducing system has the advantages of low sample consumption, satisfactory precision, low sampling dead volume (0.38nL) and low sampling dead time (0.42s). The proposed system was successfully applied to determine platinum content in a serum sample of a cancer patient on cisplatin chemotherapy. The sample consumption of1.8nL was about105times less than that with the conventional sampling system. The sample throughput up to112h-1could be achieved with the detection limit of64ng/L for195Pt. The content of Pt in the sample was8.4±0.3μg/L, agreeing well with that (8.5±0.3μg/L) determined by the conventional sampling system.In the fifth chapter, a suction-free interfacing system was developed for microchip electrophoresis hyphenated with inductively coupled plasma mass spectrometry (MCE-ICP-MS). To eliminate the nebulizer suction generated by the pneumatic nebulizer and to ensure that the makeup solution flows into the nebulizer, two porous polymer plugs were fabricated inside microchannels in the microchip. As a result, reasonably true electropherograms were obtained when compared to the CE separation performed in the traditional MCE-ICP-MS mode without porous polymer plugs. Electrophoretic separation of I-and IO3-was achieved within25s in a microchip with effective separation length of only15mm at857V/cm electric field using10mmol/L borate (pH9.2) as the running buffer. A resolution of1.3was obtained and the absolute detection limits for I-and IO3-were0.12and0.13fg, respectively. The precisions (RSD, n=10) of migration time and peak height for I-and IO3-were in the range of1.1-1.6%and2.5-2.8%, respectively. Two table salt samples were analyzed by an external calibration method. Contents of iodate were in accordance with their labeled values. The recoveries of I-and IO3-in the table salt samples were in the range of92-105%.