BUNSEKI KAGAKU Abstracts

This paper reviews the designs of novel physical and chemical separation fields principally based on the authors’ work. Usual physical fields simultaneously recognize materials and sizes of particles and may therefore result in unnecessarily complex separation in some cases. A physical field incapable of resolving a particular particle property, such as size, must be useful from this point of view. We have devised a coupled acoustic-gravity field, in which particles are aggregated at a particular position determined only by their acoustic properties. This field has been successfully applied to probe the interior structures of silica gel particles, separation of particles based on materials, the acoustic recognition of counterions in cation-exchange resins etc. Water-ice is related to various phenomena occurring in the global environment, and therefore the molecular processes taking place at its surface are of broad interest. If water-ice is used as a chromatographic stationary phase, we can effectively probe the molecular interactions at the interface between water-ice and a mobile phase. From this perspective, we have developed ice chromatography and have successfully verified various phenomena involved in retention processes. At temperatures lower than −3℃, solute retention can be explained by the hydrogen bonding of a solute with the -OH dangling bonds on the water-ice surface. However, the surface of water-ice becomes liquid-like as the temperature is raised ; this surface melting has been detected by a drastic change in the dominant retention mechanism from adsorption to partition. Ice chromatographic separation of various compounds, which is realized by the development of an efficient method for fine ice particle preparation, is also presented, involving estrogen, amino acid derivatives, ingredients in green tea leaves etc.

Pressurized-flow driven capillary electrochromatography (pCEC) is a hybrid separation method of liquid chromatography and capillary electrophoresis, in which an application of either positive or negative voltage, i.e., the electrophoretic migration of analytes toward either inlet or outlet, can be utilized to enhance the separation performance. By using intentional electrophoresis of analytes, an alteration of the separation selectivity and a reduction of the separation period through selective acceleration were achieved. The dual-gradient pCEC by varying the mobile phase composition, including a shift in the pH of the eluent and the applied voltage was effective to increase the separation performance. The combination of two different separation mechanisms of chromatography and electrophoresis in CEC was effective for comprehensive separation, and the separation efficiency was enhanced with an increase in the applied voltage. Furthermore, the electrophoresis of the analyte in a heterogeneous separation field under a gradient elution was effective to reduce the peak widths, i.e., zone sharpening.

An “ionic liquid chelate extraction system”, which is a novel liquid-liquid extraction method for the extractive separation of metal cations into the ionic liquid extraction phase with a chelate extractant, was investigated mainly from the point of difference from a normal chelate extraction system using a volatile organic solvent as the extraction phase. In this system, the extracted metals are easily recoverd by using back-extraction into an acidic solution. In the extraction of divalent metal cations (M^2＋) with 2-thenoyltrifluoroacetone (Htta) (or other β-diketone having trifluoromethyl group), some metals were extracted into the 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) extraction phase as coordination-unsaturated (hydrated) neutral complexes (M(tta)₂(H₂O)_n), whereas others were extracted as coordination-saturated anionic complexes (M(tta)₃⁻) with anion-exchange reactions. The extractability of the former species was reduced by using a more hydrophilic ionic liquid as the extraction phase, whereas that of the latter was enhanced. On the contrary, the use of 8-hydroxyquinoline (Hq) (or its derivative) as the extractant resulted in the extraction of M^2＋ into [bmim][PF₆] as neutral M(q)₂(H₂O)_n. Furthermore, a comparison of the extraction behavior of M^2＋ with 8-(trifluoromethanesulfonamido)quinoline (H(CF₃)sq) having a trifluoromethyl group with that with 8-(methanesulfonamido)quinoline (H(CH₃)sq) suggested that the existence of a trifluoromethyl group in an chelate extractant contributes to an enhancement of the extractability for an anionic complex into the ionic liquid extraction phase. In addition, a fundamental study was performed on an ionic liquid synergistic cation-exchange extraction system with co-using Htta and 18-crown-6 (18C6) as extractants for the selective extraction of trivalent light lanthanoids, such as lanthanum (La). In this system, La^3＋ was extracted into 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide as cationic ternary complexes, such as La(tta)₂(18C6)^＋ and La(tta)(18C6)^2＋.

We have developed methods for the separation analysis of highly reactive chemicals, such as organic radicals, by non-aqueous capillary zone electrophoresis (CZE). Anion radicals of quinoide compounds [benzoquinone, chloranil, phenanthrenequinone, anthraquinone, and tetracyanoquino- dimethane (TCNQ)] and a dianion of TCNQ, generated by electrolysis in an acetonitrile solution under anaerobic conditions, was successfully electrophoresed to be detected as single components using the acetonitrile solutions as a separation matrix. We studied the effects of solvated O₂ and water contents in running solutions on the detection of the reactive species to optimize the analysis conditions. We also investigated ways to transfer the reactive chemicals from the generation cell to the separation capillary so as not to be decomposed during the transfers. Finally, we developed an on-line generator that enables us fast completion of electrolysis and following quick transfer of the generated species to CE separation systems without any contact with outside compounds, such as air. By employing the present methods, we also studied hydrogen-bonding interactions between the radical anions and hydrogen-donating reagents occurring in the separation systems.

Solid phase extraction (SPE) is usually considered to be superior to liquid-liquid extraction as a pretreatment method with regard to its simplicity, rapidity, and the ability to attain a high concentration efficiency when the concentration levels of the analytes to be collected/concentrated are at trace levels of sub-ppb to ppm. The collection and concentration of analytes at trace levels in analytical or process-scale separation with solid phase materials have been performed by using styrene/divinylbenzene co-polymer (SDB) with functional groups, octadecyl silanized silica (ODS), and ion exchange resins etc.. Recently, the selective extraction and pre-concentration of analytes at trace levels for improving the detection limits and the accuracy of analysis for metals in such samples containing large amounts of specific matrices as seawater, soil extracts, biological fluids and industrial solutions have been studied using various kinds of SPE, in which highly selective functional ligands, such as chelating groups and molecular-recognition technology-based macrocyclic compounds have been introduced. The main SPE materials used were : (1) reversed-phase copolymers, (2) ion exchange resins, (3) aminocarboxylic acid-type chelating resins, and (4) molecular-recognition technology (MRT)-based resins. In this paper, several kinds of analytical applications of the SPE technique to metal analysis are summarized and demonstrated : they have been carried out by using Empore^TM Disk of Anion-SR, Cation-SR and Chelating resin, InertSep ME-1, and MetaSEP^® of Hg-01, Hg-02, Pb-01 and Pb-02, which show excellent selectivity towards certain metal ions. The adsorption/collection of target analyte elements and the removal of interference matrices in samples solutions by the SPE technique will reduce major spectral interferences, as well as physical interferences, and therefore such SPE pretreatment procedures will lead to improvements of the detection sensitivity in such analytical measurement methods as anodic stripping voltammetry (ASV), ICP-AES, ICP-MS, AAS, and XRF.

Novel stationary phases for the determination of inorganic anions in seawater have been developed in ion chromatography. Stationary phases, which allow the use of high-concentration eluents, have been examined in order to avoid the effects of matrix ions contained in seawater. The retention of analyte anions increased with increasing eluent concentration for poly (ethylene glycol) stationary phases, which was advantageous to determine trace iodide and thiocyanate in seawater samples. Quaternary ammonium surfactants could be directly introduced onto silica gel via ion exchange, followed by hydrophobic interaction. These stationary phases also allowed us to use high-concentration eluents for the determination of nitrate, bromide and nitrate in seawater samples. The use of conventional-size columns with 4.6 mm I.D. improved the concentration sensitivity, and achieved detection limits at ng/mL levels.

Microchip electrophoresis (MCE), which is based on capillary electrophoresis, is performed in a microchannel fabricated on a planner plate. MCE provides rapid and high-resolution analyses for a small amount of analytes. In MCE, however, a low concentration sensitivity is often problematic, which interferes further progress of MCE. To improve the sensitivity, several on-line sample preconcentration techniques have been applied to MCE. However, they sometimes cause a decrease in the separation efficiency and/or an increase in the analysis time. To overcome these problems, we applied a partial filling technique to MCE, and developed an on-line sample preconcentration technique, named “transient-trapping”. These novel approaches should provide a high-performance MCE analysis.

In mass spectrometry (MS)-based proteome analysis, peptides derived from proteins by proteolytic cleavage were measured. Prior to MS measurements, peptide mixtures must be cleaned, concentrated and often pre-fractionated and selectively enriched. We developed SPE mini-columns, named stop-and-go-extraction tips (StageTips), using disposable pipette tips containing very small disks made of various types of chromatographic beads embedded in a Teflon mesh. The fixed nature of the beads allows flexible combination of disks with different surfaces to obtain multi-functional tips. Based on it, we have developed StageTip-based devices for desalting, filtration, removal of contaminants, pre-fractionation with multi-dimensional format, specific enrichment of phosphopeptides and storage of peptides. Since StageTips are simple, self-made and extremely economical, they are expected to be widely used for sample pretreatment for MALDI-MS, nanoESI-MS/MS and nanoLC-MS/MS analysis in proteomics.

We studied the separation mechanism of polychlorinated biphenyls (PCB) from insulating oil by using an acetone mobile-phase on polyvinyl alcohol (PVA) gel, which is used as a high-performance PCB clean-up material for gel permeation chromatography. Based on the results of measurements on each elution characteristic for various organic compounds, such as aliphatic hydrocarbons, alkylbenzenes, polycyclic aromatic hydrocarbons, and their chlorinated derivatives, it has been confirmed that the partition mode and the size exclusion mode work exclusively for the elution of PCB and insulating oil, respectively, from PVA gel. It has also been observed that the elution position for each organic compound examined here varies roughly linearly with its inorganic value/organic value, used as an index in an organic conceptional diagram. Thus, PCB can be completely separated from insulating oil by its strong partition on PVA gel, because the index of the inorganic value/organic value of PCB is much higher than that of insulating oil. This separation mechanism is also explained well by using thermodynamic data (ΔH) estimated from the slope of a van’t Hoff plot. As a result, the separation characteristic on PVA gel can be evaluated by using the index of the inorganic value/organic value as well as a thermodynamic data.

A chelating cellulose (TLA) functionalized with thiolactic acid was synthesized, and its sorption properties were investigated for the preconcentration of Ni^2＋ and Cd^2＋ in aqueous samples. The analyte ions were quantitatively retained on the new sorbent at pH＞6.0. The adsorption of one mole of Ni^2＋ or Cd^2＋ was accompanied by the release of two moles of H^＋, as given by pH-log K_d plots. The effects of the flow rate of the sample solution through a column were studied over the range of 4〜26 mL/min. For up to 26 mL/min, the retention of both metals on the adsorbent was not affected by the flow rate of the sample solution. The effects of common metal ions on the sorption of analyte ions were also examined. The adsorbent was chemically stable and the adsorption-desorption cycles could be repeated at least 20 times. The maximum sorption capacities were found to be 0.38 mmol/g for Cd and 0.40 mmol/g for Ni, respectively. The proposed method could be successfully applied to the determination of Ni and Cd in aqueous samples.

EDTA-type chelating cellulose (abbr. : EDTA-Cell) has been synthesized and studied for its adsorption properties with respect to Cu, Ni and Co from aqueous solutions in both batch and column experiments. This adsorbent has a structure such that one of the carboxyl groups of EDTA directly binds with cellulose fiber, and it is considered that the remaining three carboxyl groups form a chelate with metal ions. This structure is suggested to be favorable to adsorb metals more strongly as compared to other analogous adsorbents. The above-mentioned three metal ions were quantitatively retained on the adsorbent at pH＞2.9. No effect of the flow rate on the adsorption performance was observed for at least by a rate of 26 mL/min. The retained metal ions could easily be desorbed with 10 mL of 1 M nitric acid. The recovery values were greater than 96％ and the preconcantration factor was 200 for all analyte ions. The adsorption isotherm of an element when it adsorbs by a monolayer from a diluted aqueous solution can be understood by the Langmuir method. The maximum sorption capacities were found to be 0.25 mmol/g for Cu, 0.30 mmol/g for Co and 0.29 mmol/g for Ni, respectively.