On-column complexation of metal ions using 2,6-pyridinedicarboxylic acid and separation of their anionic complexes by capillary electrophoresis with direct UV detection

On-column complexation of metal ions with 2,6-pyridinedicarboxylate (2,6-PDC) to form anionic complexes enabled their separation by capillary zone electrophoresis with direct UV detection at 214 nm. Nine metal ions, Cu2+, Zn2+, Ni2+, Cd2+ Mn2+, Pb2+, Fe3+, Al3+ and Ca2+, were determined in less than 7 min using 10 mM 2.6-PDC solution containing 0.75 mM tetradecyltrimethylammonium bromide at pH 4.0. Satisfactory working ranges (20-300 microM), detection limits (3-10 microM) and good repeatability of the peak areas (RSD 2.1-4.2%, n=5) were obtained using hydrodynamic injection (30 s). The proposed method was used successfully for the determination of Mn2+, Fe3+, Al3+ and Ca2+ in groundwaters.     

The synthesis and structure of a novel 1D copper(II) weak coordination polymer with 2,6-pyridinedicarboxylic acid

A novel copper complex, [Cu(dipic)(H₂O)₂] _n (H₂dipic?=?2,6-pyridinedicarboxylic acid), was synthesized and its crystal structure determined by X-ray diffraction. The complex has a polymeric structure of infinite one-dimensional (1D) zigzag chains, consisting of six-coordinate Cu(II) units. Each copper(II) ion is in a distorted octahedral environment with a CuNO₅ core: two oxygen atoms and one nitrogen atom from one dipic anion, one oxygen atom from an adjacent dipic ligand and two oxygen atoms from coordinated water. Each dipic anion connects two copper ions via a μ₂-oxygen atom. The zigzag 1D-chains are linked by extensive hydrogen bonds to form 2D infinite sheets.     

Synthesis of novel copper compounds containing isonicotinic acid and/or 2,6-pyridinedicarboxylic acid: third-order nonlinear optical properties

Two novel compounds, [Cu₂(pydc)₂(inta)₂(H₂O)₂]·3H₂O 1 (pydc?=?2,6-pyridinedicarboxylic acid, inta?= isonicotinic acid) and [Cu(pydc)₂][Cu(H₂O)₅]·2H₂O 2, have been synthesized in aqueous solution and characterized by single-crystal X-ray diffraction, elemental analyses and IR spectra. Compound 1 exhibits reverse saturable absorption and self-defocusing. X-ray structural analysis reveals that Compounds 1 and 2 both possess π–π stacking and hydrogen-bonding interactions forming three-dimensional (3D) networks. Crystal data for 1: a?=?7.2345(14), b?=?12.219(2), c?=?17.069(3)?Å, α?=?90.44(3), β?=?91.82(3), γ?=?93.56(3)°, Z?=?1, R1?=?0.0435, wR2?=?0.1216. Crystal data for 2: a?=?8.3708(17), b?=?27.386(6), c?=?9.6170(19)?Å, α?=?90.00, β?=?98.14(3), γ?=?90.00°, Z?=?3, R1?=?0.0742, wR2?=?0.2160.     

Improving sensitivity by large-volume sample stacking combined with sweeping without polarity switching by capillary electrophoresis coupled to photodiode array ultraviolet detection

An easy, simple, and highly efficient on-line preconcentration method for polyphenolic compounds in CE was developed. It combined two on-line concentration techniques, large-volume sample stacking (LVSS) and sweeping. The analytes preconcentration technique was carried out by pressure injection of large-volume sample followed by the EOF as a pump pushing the bulk of low-conductivity sample matrix out of the outlet of the capillary without the electrode polarity switching technique using five polyphenols as the model analytes. Identification and quantification of the analytes were performed by photodiode array UV (PDA) detection. The optimal BGE used for separation and preconcentration was a solution composed of 10 mM borate-90 mM sodium cholate (SC)-40% v/v ethylene glycol, without pH adjustment, the applied voltage was 27.5 kV. Under optimal preconcentration conditions (sample injection 99 s at 0.5 psi), the enhancement in the detection sensitivities of the peak height and peak area of the analytes using the on-line concentration technique was in the range of 18-26- and 23-44-fold comparing with the conventional injection mode (3 s). The detection limits for (-)-epigallocatechin (EGC), (-)-epicatechin (EC), (+)-catechin (C), (-)-epigallocatechin gallate (EGCG), and (-)-epicatechin gallate (ECG) were 4.3, 2.4, 2.2, 2.0, and 1.6 ng/mL, respectively. The five analytes were baseline-separated under the optimum conditions and the experimental results showed that preconcentration was well achieved.     

Determination of mercaptopurine and its four metabolites by large-volume sample stacking with polarity switching in capillary electrophoresis

This study describes approaches for stacking a large volume of sample solutions containing a mixture of mercaptopurine monohydrate, 6-methylmercaptopurine, thioguanine, thioguanosine, and thioxanthine in capillary electrophoresis (CE). After filling the run buffer (60 mM borate buffer, pH 8.5), a large sample volume was loaded by hydrodynamic injection (2.5 psi, 99.9 s), followed by the removal of the large plug of sample matrix from the capillary using polarity switching (-15 kV). Monitoring the current and reversing the polarity when 95% of current recovered, the separation of anionic analytes was performed in a run buffer < 20 kV. Around 44- to 90-fold improvement of sensitivity for five analytes was achieved by large-volume stacking with polarity switching when compared with CE without stacking. This method was feasible for determination of the analytes spiked in plasma. Removing most of electrolytes from plasma is a key step for performing large-volume sample stacking. Solid-phase extraction was used for pretreatment of biological samples. To our knowledge, this study is one of few applications showing the possibilities of this stacking procedure to analyze biological samples by large-volume sample stacking with polarity switching (LVSSPS) in CE.     

Separation of alkylbenzyl quaternary ammonium compounds by capillary zone electrophoresis with sample stacking injection

Summary A systematic investigation of operational buffer systems, sample preparation and instrument parameters for achieving the best possible performance for determinating an homologous series of N-benzyl-N-alkyl-N,N-dimethylammonium chloride compounds by capillary zone electrophoresis with direct UV detection. The most effective separation was achieved within 3.5 min with the addition of acetonitrile (40%) in a phosphate buffer (20 mM pH 5.2) using a 40 cm fused-silica capillary operating at 25 KV and 20°C. Degassing of all electrolyte solutions and samples was very important. The linearity and repeatability for each compounds were satisfactory. To improve detection limits, on-column sample preconcentration, sample stacking, was investigated achieving a tenfold enrichment factor and quantitation limits about 10⁻⁷M.     

Synthesis, characterization and biological evaluation of a novel Cu(II) complex with the mixed ligands 2,6-pyridinedicarboxylic acid and 2-aminopyridine

A novel bridged binuclear Cu(II) complex with mixed ligands, di-μ-(2-aminopyridine(N,N′))-bis[(2,6-pyridinedicarboxylate)aquacopper(II)] tetrahydrate, formulated as [Cu(μ-ap)(dipic)(H₂O)]₂·4H₂O (1) (dipic = 2,6-pyridinedicarboxylate, ap = 2-aminopyridine), has been synthesized and characterized by elemental, spectral (IR and UV–Vis.), thermal analysis, magnetic measurements and single crystal X-ray diffraction analysis. The central Cu(II) ion resides on a centre of symmetry in a distorted square-pyramid coordination environment comprising of two N atoms, one from dipic and one from the ap ring, two carboxylate O atoms from dipic, and one O atom from water. Intermolecular N–HO and O–HO hydrogen bonds and π–π stacking interactions seem to be effective in the stabilization of the crystal structure. The free ligands and the complex were also evaluated for their antimicrobial and radical scavenging activities (DPPH = 1,1-diphenyl-2-picrylhydrazyl hydrate) using in vitro microdilution methods. Antimicrobial screening of the free ligands and their complex showed that the free ligands and the complex possess antifungal activity against Candida sp.     

Confirmation of vanadium complex formation using electrospray mass spectrometry and determination of vanadium speciation by sample stacking capillary electrophoresis

Capillary zone electrophoresis (CZE) with UV detection was used to determine vanadium species. Nitrilotriacetic acid (NTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), ethylene glycol-bis(2-aminoethylether)-tetraacetic acid (EGTA) and 2,6-pyridinedicarboxylic acid (PDCA) were investigated to determine whether these ligands formed stable anionic complexes with vanadium. Of all the ligands studied HEDTA was the most suitable ligand because it gave the largest UV response with reasonable migration time. Electrospray mass spectrometry (ES-MS) was used to confirm the formation of [VO₂(HEDTA)]²⁻ and [VO(HEDTA)]¹⁻ in solution. An electrolyte containing 25 mM phosphate, 0.25 mM tetradecyltrimethylammonium bromide (TTAB) at pH 5.5 was optimum for the separation of these anionic vanadium complexes. Sample stacking techniques, including large-volume sample stacking (LVSS) and field-amplified sample injection (FASI), were tested to improve the sensitivity. Best sensitivity was obtained using FASI, with detection limits of 0.001 μM, equivalent to 0.4 μg L⁻¹, for [VO₂(HEDTA)]²⁻ and 0.01 μM, equivalent to 3.4 μg L⁻¹ for [VO(HEDTA)]¹⁻. The utility of the method for the speciation of V(IV) and V(V) was demonstrated using ground water samples.     

Comparative study for the analysis of parabens by micellar electrokinetic capillary chromatography with and without large-volume sample stacking technique

The separation and determination of four parabens (methyl, ethyl, propyl, and butyl p-hydroxybenzoate) which are commonly used as preservatives in cosmetic products, by micellar electrokinetic capillary chromatography (MEKC) with and without large-volume sample stacking (LVSS) technique were compared. As an effective on-line concentration technique, LVSS was successfully combined with MEKC to determine neutral parabens in an acidic media. The effects of some typical parameters such as sample volume, buffer pH, temperature, and concentration of surfactant were examined. The detection limits for this LVSS-MEKC method were found to be 3.0 × 10⁻⁷ M for each of the parabens based on the signal-to-noise ratio of 3, which were around 300 times lower than normal MEKC technique. The curves of peak response versus concentration were linear from 1.0 × 10⁻⁶ to 5.0 × 10⁻⁵ M with regression coefficients of 0.9987, 0.9960, 0.9925 and 0.9864, respectively. A simple and easy-manipulative sample preparation method was developed and validated by analyzing commercially available cosmetic samples. It was found that with current sample preparation process and instrumentation system, 0.5 g of sample is enough for the analysis of parabens preservatives in cosmetic product with satisfactory results.     

A DNA electrochemical sensor prepared by electrodepositing zirconia on composite films of single-walled carbon nanotubes and poly(2,6-pyridinedicarboxylic acid), and its application to detection of the PAT gene fragment

Carboxyl group-functionalized single-walled carbon nanotubes (SWNTs) and 2,6-pyridinedicarboxylic acid (PDC) were electropolymerized by cyclic voltammetry on a glassy-carbon electrode (GCE) surface to form composite films (SWNTs/PDC). Zirconia was then electrodeposited on the SWNTs/PDC/GCE from an aqueous electrolyte containing ZrOCl₂ and KCl by cycling the potential between −1.1 V and +0.7 V at a scan rate of 20 mV s⁻¹. DNA probes with a phosphate group at the 5′ end were easily immobilized on the zirconia thin films, because of the strong affinity between zirconia and phosphate groups. The sensors were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EIS was used for label-free detection of the target DNA by measuring the increase of the electron transfer resistance (R _et) of the electrode surface after the hybridization of the probe DNA with the target DNA. The PAT gene fragment and polymerase chain reaction (PCR) amplification of the NOS gene from transgenically modified beans were satisfactorily detected by use of this DNA electrochemical sensor. The dynamic range of detection of the sensor for the PAT gene fragment was from 1.0 × 10⁻¹¹ to 1.0 × 10⁻⁶ mol L⁻¹ and the detection limit was 1.38 × 10⁻¹² mol L⁻¹.     

Comparative study on sample stacking by moving reaction boundary formed with weak acid and weak or strong base in capillary electrophoresis: II. Experiments

To demonstrate the theoretic method on the stacking of zwitterion with moving reaction boundary (MRB) in the accompanying paper, the relevant experiments were performed. The experimental results quantitatively show that (1) MRB velocity, including the comparisons between MRB and zwitterionic velocities, possesses key importance to the design of MRB stacking; (2) a much long front alkaline plug without sample should be injected before the sample injection for a complete stacking of zwitterion if sample buffer is prepared with strong base, conversely no such plug is needed if using a weak base as the sample buffer with proper concentration and pH value; (3) the presence of salt in MRB system holds dramatic effect on the MRB stacking if sample solution is a strong base, but has no effect if a weak alkali is used as sample solution; (4) all of the experiments of this paper, including the previous work, quantitatively manifest the theory and predictions shown in the accompanying paper. In addition, the so-called derivative MRB-induced re-stacking and transient FASI-induced re-stacking were also observed during the experiments, and the relevant mechanisms were briefly demonstrated with the results. The theory and its calculation procedures developed in the accompanying paper can be well used for the predictions to the MRB stacking of zwitterion in CE.     

Comparative study on sample stacking by moving reaction boundary formed with weak acid and weak or strong alkali in capillary electrophoresis: I. Theory

The condensation of low abundance zwitterion substance, such as protein and peptide, has great significance to the study on proteomics. This paper develops the theory on design of online stacking conditions of zwitterion by a moving reaction boundary (MRB) in capillary electrophoresis (CE). This concerns the choice of running and sample buffers, velocity design of MRB, and salt effect on the stacking. The theoretical results unveil that: (1) the velocity of MRB formed with weak acidic buffer and strong alkali should be set between zero and the velocity of zwitterion in the alkali phase, or no stacking occurs; (2) if a strong alkali is used to prepare the sample, a much long front plug of strong base must be injected before the alkaline sample plug for complete stacking, whereas no such front plug is needed if a weak alkali with enough high concentration and pH value is used to prepare the sample buffer; (3) the existence of salt in sample matrix has a weak effect on the stacking of zwitterion if sample is prepared with weak alkaline buffer, while has a dramatic effect on the same stacking if with a strong base buffer. In addition, the concentration of weak alkali used for preparation of sample should be set at the point, at which the velocity of MRB is as much as possible close to that of negative zwitterion. The developed theory and its computation are quantitatively proved by the experiments of zwitterion stacking by the MRB as shown in the previous and the accompanying papers. The proposed theoretic results hold obvious significances on-column stacking of low abundance zwitterions, such as amino acid, or peptides or proteins, in CE.     

Application of capillary zone electrophoresis with large-volume sample stacking to the sensitive determination of sulfonamides in meat and ground water

A CZE method with UV-Vis detection has been established and validated for the determination of nine sulfonamides: sulfapyridine, sulfamethazine, sulfamerazine, sulfamether, sulfadiazine, sulfadimethoxine, sulfamethoxazole, sulfachlorpyridazine, and sulfamethizole. Optimum separation was obtained on a 64.5 cm x 75 microm bubble cell capillary using a buffer containing 45 mM sodium phosphate and 10% methanol at pH 7.3, with temperature and voltage of 27 degrees C and 25 kV, respectively. p-Aminobenzoic acid was used as an internal standard . Taking into account the lack of sensitivity of the UV-Vis detection, the application of an on-line preconcentration methodology, such as large-volume sample stacking with polarity switching has been proposed. This procedure combined with a solvent extraction/SPE method applied for off-line preconcentration and cleanup provides a significant improvement in the LODs, ranging from 2.59 to 22.95 mug/L for the studied compounds; the quantification of these residues being possible below the levels established by EU legislation in animal food products, such as meat. Satisfactory recoveries were also obtained in the analysis of these compounds in ground water.     

毛细管电泳-场强放大样品堆积法检测染发剂中的7种苯胺类物质

建立了毛细管电泳-场强放大样品堆积测定染发剂中4,4′-二氨基二苯甲烷、苯胺、邻甲氧基苯胺、对氨基苯甲醚、3,4-二甲基苯胺、间氨基苯酚、1-萘胺7种苯胺类物质的分析方法。在优化的缓冲溶液体系(0.15 mol/L NaH2PO4,0.015 mol/L 三乙醇胺, pH 2.3)下7种分析物在6.5 min内实现基线分离。考察了样品中添加的磷酸浓度和乙腈浓度、水柱长度、电动进样时间与电压对场强放大富集效率及重现性的影响。最佳的富集条件为: 水柱注入3.45 kPa(0.5 psi)×6 s,样品中添加40%(v/v)乙腈和0.6×10~3mol/L磷酸,进样电压与进样时间为10 kV×10 s。线性范围为3～1000 μg/L(R2＞0.996),检出限为0.26～2.75 μg/L,将已有方法的检测灵敏度提高了1～3个数量级。在2种市售黑色染发剂中均检测到间氨基苯酚,含量分别为7.32 mg/g和1.34 mg/g。平均加标回收率为74%～108%。该方法灵敏度高、快速、重现性好、成本低,可供多种样品基质中痕量苯胺类污染物及其他阳离子物质的测定借鉴使用。     

Quantification of imatinib and related compounds using capillary electrophoresis-tandem mass spectrometry with field-amplified sample stacking

A quantification method for imatinib (IM), its major metabolite N-desmethyl imatinib (NDI), and a degradation by-product was developed using CE–MS combined with an online concentration technique. The use of multiple reaction monitoring (MRM)–MS/MS further improved the sensitivity of this technology. Liquid–liquid extraction (LLE) using tertiary butyl methyl ether yielded high recovery and reproducibility for the pretreatment of serum samples. The recovery rate exceeded 83% for all three analytes, and was 90% for IM. To improve quantification results, a conductivity-induced online analyte concentration technique, field-amplified sample stacking (FASS), was used. The S/N ratios were improved at least 10-fold when compared with conventional capillary zone electrophoresis. The detection limits were 0.2 ng/mL for IM, 0.4 ng/mL for NDI, and 4 ng/mL for the degradation by-product. These results are superior to those previously obtained by other reported methods. The new method was validated in terms of its selectivity, intra- and interday repeatability and accuracy, and sample storage stability, following the guidelines issued by the European Medicines Agency. Considering the convenient pretreatment procedure (LLE), superior sensitivity, and fast analysis speed (<15 min), this method can be useful in the determination of imatinib levels in blood.     

Simultaneous capillary electrophoretic separation of small anions and cations after complexation with ethylenediaminetetraacetic acid

A new approach for simultaneous separation of small inorganic and organic anions and metal cations by capillary electrophoresis is demonstrated. Metal cations in the sample are transformed into their chelates with EDTA and are separated together with the anions using an anionic separation mode. Simultaneous separation of 19 common anions and cations was achieved in about 6 min with the electrolyte containing 5 mM K2CrO4, 3 mM boric acid, 35 microM cetyltrimethylammonium bromide and 12 microM EDTA at pH 8. Limits of detection (s/n = 3) were in the range from 4 ppb for Cl- up to 1250 ppb for Cu-EDTA and RSDs of peak areas ranged from 1.4% for Cl- up to 8.5% for Mn-EDTA chelate. The practical applicability of the method was demonstrated on the analysis of anions and cations in various water samples.     

Determination of glyphosate and aminomethylphosphonic acid by capillary electrophoresis with indirect detection using pyridine-2,6-dicarboxylic acid or 3,5-dinitrobenzoic acid

Glyphosate (GlyP), a widely used nonselective herbicide, and aminomethylphosphonic acid (AMPA), GlyP’s most common product of degradation in the environment, seem not to pose any major health threats. However, due to their persistence and the large quantities applied worldwide, they have become a source of concern. This justifies that their simple and swift determination is of considerable relevance. This work presents two indirect capillary electrophoretic methodologies using as chromophores 3,5-dinitrobenzoic acid (DNB) and pyridine-2,6-dicarboxylic acid (PDC), both associated with hexadecyltrimethylammonium bromide (CTAB). Although both methods showed suitable analytical parameters, DNB evidenced to be slightly superior. The recoveries values were close to 100%, correlation coefficients were above 0.99 and the limits of detection (LODs) values were below 0.5 mg L^?1 (2.9 µmol L^?1) and 0.4 mg L^?1 (3.6 µmol L^?1) for GlyP and AMPA, respectively.     

Confirmation of iron complex formation using electrospray ionization mass spectrometry (ESI-MS) and sample stacking for analysis of iron polycarboxylate speciation by capillary electrophoresis

The use of capillary electrophoresis (CE) and UV detection for the determination of metal speciation is based on the standards via matching migration time. Consequently, it requires that the metal species are stable during electrophoresis. Migration time of the metal species is dependent on the electrolyte composition. However, the stability for such metal complexes is also dependent on electrolyte composition and electrolyte-specific stability is not always well known. In this paper, the stability of iron (Fe) polycarboxylate complex formation was determined using electrospray ionization mass spectrometry (ESI-MS). Mass spectra indicates that Fe[DTPA]²⁻, Fe[EDTA]⁻, Fe[HEDTA] and Fe[NTA] were present in solution but the mass spectrum for Fe[NTA] was found to be weak. An electrolyte containing 25 mM NaH₂PO₄ and 0.25 mM tetradecyltrimethylammonium bromide (TTAB) at pH 6.0 was used to successfully separate Fe[DTPA]²⁻, Fe[EDTA]¹⁻ and Fe[HEDTA]. The instability of Fe[NTA] meant it was not observed due to its instability during electrophoresis. To improve UV detection limits sample stacking techniques, such as large volume sample stacking (LVSS) without polarity switching and filed-amplified sample injection (FASI), were investigated. The results show that less than 0.01 μM detection limits for the Fe complexes were obtained using FASI. The calibration plots were linear from 0.05-3.0 with good reproducibility (peak area: 6.5-8.1%) when a water plug was used. Finally, the proposed method was demonstrated for the determination of trace Fe complexes in river waters.     

Fabrication of poly(2,6-pyridinedicarboxylic acid)/MWNTs modified electrode for simultaneous determination of guanine and adenine in DNA

The fabrication of poly(2,6-pyridinedicarboxylic acid)/MWNTs modified glass electrode(PPDA/MWNTs/GCE) was proposed and used for individual or simultaneous determination of guanine and adenine.The performances of the PPDA/MWNTs/GCE were characterized with cyclic voltammetry(CV).The modified electrode exhibited enhanced electrocatalytic behavior and good stability for the detection of guanine and adenine.Differential pulse voltammetry(DPV) was used to determine the concentration of guanine,adenine.The detection limit(S/N = 3) for guanine and adenine was 0.045μmol/L and 0.05μmol/L,respectively.The electrochemical method for the measurement of guanine and adenine in calf thymus DNA was also developed with this modified electrode and the result was satisfactory.     

High-sensitivity capillary electrophoresis for speciation of organomercury in biological samples using hollow fiber-based liquid-liquid-liquid microextraction combined with on-line preconcentration by large-volume sample stacking

A hollow fiber-based liquid-liquid-liquid microextraction (HF-LLLME) combined with on-line large-volume sample stacking (LVSS) has been developed for the speciation of organomercury in biological samples by CE with UV detection. Separation was achieved in less than 11 min with an electrolyte consisting of 35 mM sodium tetraborate at pH 9.1. In LVSS, a reverse electrode polarity-stacking mode (REPSM) was applied as on-line preconcentration strategy. In HF-LLLME, the analytes were extracted from 12 mL volume of sample solution (pH adjusted to 3.0) into bromobenzene impregnated in the pores of the hollow fiber, and into an acceptor solution of L-cysteine (15 microL, 0.02% w/v) inside the hollow fiber. Under the optimized conditions, concentration factors of 2610-4580 were achieved and LODs in the range of 0.03-0.14 microg/L were feasible. The linearity was found to be over two orders of magnitude with correlation coefficient of 0.9991-0.9996. 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 method was also applied to the speciation of organomercury in three kinds of fish samples and human hair samples.