Microextraction Techniques Combined With On-Column Preconcentration Methods For Elemental Speciation By Capillary Electrophoresis

It has been recognized that the toxicity, bioavailability, transportation, metabolism, and other specific biological functions of a given element are highly dependent on its physicochemical forms in environmental and biological samples. Therefore, identification and quantification of different elemental species, termed as elemental speciation, are more important than the determination of its total content of an element. Capillary electrophoresis (CE) has been demonstrated to be one of the effective analytical techniques for elemental speciation due to the merits of low cost, low-sample consumption, high resolution, rapid separation and no need for derivatization. However, insufficient detection sensitivity and low tolerance of sample matrix have limited the application of CE in real sample analysis. To overcome such limitations, effective sample pretreatment techniques and on-column preconcentration methods are highly demanded. An appropriate sample pretreatment technique could isolate the target analytes from the sample matrix and concentrate the analytes simultaneously. In recent years, a variety of novel sample pretreatment techniques such as liquid phase microextraction (LPME), solid phase microextraction (SPME) and stir bar sorptive extraction (SBSE) have been developing rapidly and gained great interest in analytical community. On the other hand, on-column preconcentration techniques such as staking, sweeping, dynamic pH junction and t-isotachophoresis are also excellent alternative methods to improve the detection sensitivity of CE, and they deserve special attention for their simple operation and high enrichment factor. However, to the best of our knowledge, the research on the combination of effective sample pretreatment techniques and/or on-column preconcentration methods with CE is very scarce until to present for elemental speciation.The aim of this dissertation is to explore efficient on-column methods for elemental speciation and investigate the concentration mechanism; to develop new microextraction methods for the direct extraction of polar elemental species; and to establish new methods by combining microextraction techniques and/or on-column preconcentration methods with CE/UV detection for elemental speciation in environmental and biological samples. The major contents of this dissertation are described as follows:(1) A novel method of polymer-coated hollow fiber microextraction (PC-HFME) combined with dynamic pH junction-CE/UV detection was developed for the speciation of four selenoamino acids. The enrichment resulted from dynamic pH junction is attributed to the migration velocity decreases of target analytes ions when migrating from a high-pH sample zone (pH 12.4) to a low-pH (pH 2.0) back ground electrolyte (BGE) buffer. In comparison to standard injection mode, the dynamic pH junction alone provided 430- to 590-fold improvement in terms of sensitivity. In PC-HFME, partially sulfonated polystyrene (PSP) coated hollow fibers were prepared for the simultaneous extraction of four selenoamino acids in acid media based on electrostatic interaction. With two-step enrichment procedure of dynamic pH junction and PC-HFME, enhancement factors (EFs) of 3171 to 19388 folds were obtained, and the limits of detections (LODs) (S/N=3) of 0.32-4.10μg/L were achieved for CE/UV determination of target selenoamino acids. The application potential of this method was successfully demonstrated by the analysis of selenoamino acids in environmental water samples.(2) Zirconia (ZrO2) coated stir bar sorptive extraction (SBSE) coupled with on-column large volume sample injection (LVSS)-CE with indirect UV detection was developed for the direct analysis of strong polar chemical warfare agent degradation products of methylphosphonic acid (MPA), ethyl methylphosphonic acid (EMPA) and pinacolyl methylphosphonate (PMPA). Nanometer-sized ZrO2 coated stir bars were prepared by a PDMS adhesion method, and successfully applied in the extraction of the target strongly polar analytes due to the high affinity of ZrO2 to electronegative phosphonate group containing compounds. By combination of ZrO2-SBSE with LVSS-CE/ indirect UV, the LODs were found to be 1.4,1.2 and 3.1μg/L for PMPA, EMPA, and MPA, respectively. The reproducibility (RSD) was in the range of 9.0-11.8%. The EFs were up to 1583-fold. The proposed ZrO2-SBSE-LVSS-CE/indirect UV method has been applied to the analysis of three target analytes in different environmental water samples with recoveries ranging from 93.8% to 105.3%.(3) Off-line hollow fiber liquid liquid liquid microextraction (HF-LLLME) combined with on-column anion selective exhaustive injection (ASEI)-CE/UV detection was proposed for the speciation of phenylarsenic compounds. By the use of tributyl phosphate (TBP) as the organic phase and 0.8 mmol/L Tris solution as acceptor phase in HF-LLLME, a simultaneous preconcentration of target analytes in pH 2.15 H2SO4 medium as the donor phase was realized. In ASEI, a large plug of water (91% length of total capillary) was introduced into the separation capillary before sample injection in order to prolong the sample injection time and thus enhance the stacking efficiency of ASEI. Under the optimized conditions, up to 236-fold of EF was obtained for the ASEI-CE/UV determination of target phenylarsenic compounds. Compared with reported LVSS technique, ASEI is more effective. By combining HF-LLLME with ASEI-CE/UV, the obtained LODs were in the range of 0.68-6.90μg/L for five phenylarsenic compounds with RSDs (n=5) of 5.6-11.8%. The proposed HF-LLLME-ASEI-CE/UV method was applied for the determination of target phenylarsenic compounds in pig feed, stored pig litter, soil and water samples obtained in a local pig farm.(4) Taking four thyroid hormones including thyroxine (T4),3,5,3-triiodo-L-thyronine (T3), reverse 3,3,5-triiodo-L-thyronine (rT3) and 3,5-diiodo-L-thyronine dihydrate (T2), and two other related compounds of 3,5-diiodo-L-tyrosine (DIT) and 3-iodo-L-tyrosine (MIT) as the target analytes, a new method of ion pair based hollow fiber liquid-liquid-liquid microextraction (IP-HF-LLLME) combined with CE-UV detection was developed for the simultaneous determination of six target analytes in pharmaceutical formulations. The extraction was facilitated by adding sodium dodecyl sulfate (SDS) in the donor phase to form ion pairs with target analytes. The factors affecting the extraction efficiency of IP-HF-LLLME were optimized. Under the optimum conditions, the LODs (S/N=3) for six target analytes were in the range of 0.54-1.43μg/L with RSDs (n=7) ranging from 3.19 to 8.98%. The enrichment factors obtained by this method ranged from 183 to 366 folds. The proposed method is simple, inexpensive, high selective and sensitive for the simultaneous analysis of thyroid hormones and related compounds.(5) U-shaped hollow fiber-based liquid-liquid-liquid microextraction (HF-LLLME) combined with large-volume sample stacking (LVSS)-CE/UV detection has been proposed for the speciation of organomercury in biological samples. In LVSS, a polarity switch mode was applied. In HF-LLLME, the analytes were extracted from 12 mL sample solution (pH 3.0) into an acceptor solution of L-cysteine (15μL,0.02% w/v) inside the hollow fiber through bromobenzene impregnated in the pores of the hollow fiber. Under the optimized conditions, EFs of 2610-4580 were achieved with the LODs in the range of 0.03-0.14μg/L. The developed method has been validated using a certified reference material (DORM-2, dogfish muscle), and the determined values coincided very well with the certified values. The developed method was also applied to the speciation of organomercury in real fish samples and human hair samples.(6) A simple automatic dynamic hollow fiber based liquid liquid liquid (AD-HF-LLLME) device was designed for the mercury speciation by using a programmable flow injection analyser. With 18-crown-6 as the complexing reagent, mercury species including methyl-, ethyl-, phenyl- and inorganic mercury were first extracted into the organic phase (chlorobenzene), and then transferred into 0.1%(m/v) 3-mercapto-l-propanesulfonic acid (MPS) aqueous solution in the hollow fiber lumen as acceptor phase. However, ethylmercury was found to be partially decomposed during the AD-HF-LLLME process, and was not included in the developed method. Based on it, a new method of AD-HF-LLLME combined with LVSS-CE/UV detection was established for the simultaneous analysis of methyl-, phenyl- and inorganic mercury species in water and human hair samples.(7) A novel extraction technique termed phase transfer based liquid-liquid-liquid microextraction (PT-LLLME) was proposed for the simultaneous extraction of inorganic mercury and three organic mercury species. To ensure a maximum contact between the target mercury species and the complexing reagent of dodecylamine (DDA) in donor phase, an intermediate solvent, which is miscible with water, was added into the sample solution. Furthermore, a membrane supported liquid-liquid-liquid phase microextraction (MS-LLLME) unit was designed. By using nylon membrane as supporting carrier, larger than 50μL of acceptor solution could be hung up in MS-LLLME unit. Parameters affecting the extraction efficiency of PT/MS-LLLME were investigated in details. Under the optimized conditions, EFs ranging from 160- to 478-fold were obtained for the mercury species by PT/MS-LLLME. The acceptor phase was directly injected into CE for LVSS-CE/UV analysis. By combination of PT/MS-LLLME with LVSS-CE/UV, the limits of detection (LODs) at lowμg/L level were achieved with the EFs up to 12138- fold. This newly established approach of PT/MS-LVSS-LLLME-CE/UV was successfully applied to simultaneous determination of inorganic and organic mercury species in biological samples and environmental water samples.