Quantitative enrichment of trace levels of ions by electrodialysis

The effect of ionic strength on the rate of ion transport across ion-exchange membranes under either constant potential or constant current conditions in electrodialysis precludes direct application of this method to quantitative enrichment studies. Addition of an excess of electrolyte to a sample to normalize the ionic strength permits either dialysis mode to be employed. The rate of ion transport into a receiver electrolyte becomes directly proportional to concentration over at least 2.5 orders-of-magnitude. The concentration range for which linear enrichment is achieved for cations extends down to the 10^-7 M level with 30-min dialyses into a mixed 0.1 M MgCl₂—0.1 M HCl receiver. Alternatively, an internal standard approach can be used; however, the difficulty in matching the behaviour of membrane transport numbers as a function of ionic strength can limit the application of this approach.     

Separation of bromide, bromate, iodide, iodate, nitrite, nitrate and selenite anions by capillary zone electrophoresis

Summary Capillary zone electrophoresis (CZE) has been used for the separation of bromide, bromate, iodide, iodate, nitrite, nitrate and selenite anions. The separation was achieved using a fused silica capillary (72 cm long x 50 m i.d.) filled with an acidic phosphate buffer (pH=3, 25 mmol/l), and with on-column UV detection at 200 nm. The influence of different experimental parameters such as pH, ionic strength and voltage, was studied. The nitrate concentration of a Rhine water sample was then determined under selected conditions and the results were compared to those obtained by high performance ion chromatography (HPIC).On leave from the Universitat de Barcelona (postdoctoral fellowship from ECC-BCR).     

Investigation of the mechanism of pH-mediated stacking of anions for the analysis of physiological samples by capillary electrophoresis

Capillary electrophoresis has been widely used for the analysis of physiological samples such as plasma and microdialysate. However, sample destacking can occur during the analysis of these high-ionic strength samples, resulting in poor separation efficiency and reduced sensitivity. A technique termed pH-mediated stacking of anions (base stacking) has previously been developed to analyze microdialysate samples and achieve on-line preconcentration of analytes by following sample injection with an injection of sodium hydroxide. In this work, the mechanism of base stacking was investigated. Peak efficiency was shown to be a function of background electrolyte and sample ionic strength. Analytes representing several classes of compounds with a wide range of mobilities were used to study the effects of multiple parameters on sample stacking. The length of hydroxide injection required for stacking was shown to be dependent on analyte mobility and the type of amine background electrolyte used. Combinations of electrokinetic and hydrodynamic injections of sample and hydroxide were examined and it was concluded that although stacking could be achieved with several injection modes, electrokinetic injection of both sample and hydroxide was most effective for sample stacking. The mechanism of pH-mediated stacking for each of these modes is presented.     

Effects of sample matrix and injection plug on dsDNA migration in capillary gel electrophoresis

Reproducible DNA migration times are required for accurate basepair assignment in restriction fragment mapping and polymerase chain reaction product identification. Our data shows DNA migration time shifts with changes in sample ionic strength. Secondly, loss of resolution with replaceable polyacrylamide gels was observed when increasing the length of the sample plug with pressure injection. An easy way to correct for the migration time shifts is to incorporate an internal DNA standard directly into the separation process by consecutively injecting the DNA sample and the DNA standard. This allows for compensation of any possible migration time variation caused by high ionic strength sample matrices. Also high-resolution separations can be maintained with large injection volumes (long injection plug) by using consecutive injections of 0.1 M Tris-acetate buffer and the DNA sample.     

二维离子色谱法同时测定环境水样中的碘离子、硫氰酸根和高氯酸根

建立了二维离子色谱法同时测定环境水样中的碘离子、硫氰酸根离子和高氯酸根离子的方法。先采用常规阴离子色谱柱(IonPac AS16, 250 mm×4 mm)将水样中的碘离子、硫氰酸根离子和高氯酸根离子与干扰离子进行分离。样品溶液通过抑制器后,将含有碘离子、硫氰酸根离子和高氯酸根离子的淋洗液导入富集柱(MAC-200, 80 mm×0.75 mm),再通过毛细管阴离子色谱柱(IonPac AS20 Capillary, 250 mm×0.4 mm)进行分离和定量分析。方法的线性范围为0.05～100 μg/L,相关系数达到0.9999,检出限为0.02～0.05 μg/L。样品中碘离子、硫氰酸根离子和高氯酸根离子的加标回收率在85.1%～100.1%之间,回收率的相对标准偏差(RSD)(n=6)在1.7%～4.9%之间。该法试剂用量小,灵敏度比常规离子色谱提高30～40倍,同时去除了样液中的高浓度基体杂质,适用于水样中低含量碘离子、硫氰酸根离子和高氯酸根离子的检测。     

Determination of perchlorate in drinking water by ion chromatography using macrocycle-based concentration and separation methods

Macrocycle-based ion chromatography provides a convenient, reliable method for the determination of perchlorate ion, which is currently of great interest to the environmental community. This study shows that effective perchlorate determinations can be made using standard conductimetric detection by combining an 18-crown-6-based mobile phase with an underivatized reversed-phase mobile phase ion chromatography (MPIC) column. One unique feature of this method is the flexibility in column capacity that is achieved through simple variations in eluent concentrations of 18-crown-6 and KOH, facilitating the separation of target analyte anions such as perchlorate. Using a standard anion exchange column as concentrator makes possible the determination of perchlorate as low as 0.2 ug/L in low ionic strength matrices. Determination of perchlorate at the sub-ug/L level in pure water and in spiked local city hard water samples with high background ion concentrations can be achieved this way. However, like other IC techniques, this method is challenged to achieve analyses at the ug/L level in the demanding high ionic strength matrix described by the United States Environmental Protection Agency (EPA) (1,000 mg/L chloride, sulfate and carbonate). We approached this challenge by use of the Cryptand C1 concentrator column, provided by Dionex Corporation, to effectively preconcentrate perchlorate while reducing background ion concentrations in the high ionic strength matrix. The retention characteristics of the concentrator column were studied in order to maximize its effectiveness for perchlorate determinations. The method makes possible the determination of perchlorate at the 5 ug/L level in the highest ionic strength matrix described by the EPA.     

Rapid analysis of pesticide residues in drinking water samples by dispersive solid‐phase extraction based on multiwalled carbon nanotubes and pulse glow discharge ion source ion mobility spectrometry

An analytical method based on dispersive solid‐phase extraction with a multiwalled carbon nanotubes sorbent coupled with positive pulse glow discharge ion mobility spectrometry was developed for analysis of 30 pesticide residues in drinking water samples. Reduced ion mobilities and the mass–mobility correlation of 30 pesticides were measured. The pesticides were divided into five groups to verify the separation capability of pulse glow discharge in mobility spectrometry. The extraction conditions such as desorption solvent, ionic strength, conditions of adsorption and desorption, the amounts of multiwalled carbon nanotubes, and solution pH were optimized. The enrichment factors of pesticides were 5.4‐ to 48.7‐fold (theoretical enrichment factor was 50‐fold). The detection limits of pesticides were 0.01～0.77 μg/kg. The linear range was 0.005–0.2 mg/L for pesticide standard solutions, with determination coefficients from 0.9616 to 0.9999. The method was applied for the analysis of practical and spiked drinking water samples. All results were confirmed by high‐performance liquid chromatography with tandem mass spectrometry. The proposed method was proven to be a commendably rapid screening qualitative and semiquantitative technique for the analysis of pesticide residues in drinking water samples on site.     

Fast multiresidue screening of 300 pesticides in water for human consumption by LC-MS/MS

The study tested the determination of 300 pesticides in mineral water at levels of 0.1 and 1.0 μg/L. Measurements were conducted by direct sample injection into a liquid chromatograph coupled to a tandem mass spectrometer without any sample enrichment and/or cleanup. Two separate injections enabled the recording of two transitions per analyte (600 selected reaction monitoring transitions in total). For 285 analytes the sensitivity of direct sample injection (100 μL) was sufficient to quantify residues at 0.1 μg/L. All remaining pesticides were detected at 1.0 μg/L. Calibration functions were linear for more than 80% of analytes. Signal suppression or enhancement compared with signals in high-performance liquid chromatography water was equal to or smaller than 20% for 240 analytes. Even the largest matrix-induced suppression did not result in the disappearance of peaks. Combining the results of seven mineral waters, the relative standard deviation of “recovery” was 20% or less for 87% of the substances. A second transition for confirmatory purposes was often available. Consequently, the proposed direct injection of samples without any sample enrichment and/or cleanup is suitable for screening of many pesticides in mineral and drinking water.     

Separation techniques hyphenated to electrospray-tandem mass spectrometry in proteomics: capillary electrophoresis versus nanoliquid chromatography

Liquid chromatography (LC) nanoelectrospray-tandem mass spectrometry (MS/MS) is a key technology for the study of proteomics, with the main benefit to the characterization of sensitive peptides from complex mixtures. Capillary electrophoresis coupled to mass spectrometry (MS) has been taken into consideration sporadically due to the highly efficient separation and ability to handle low sample amount, yet classified as being less sensitive with respect to analyte concentration. The limitation in capillary zone electrophoresis (CZE) injection volumes can be overcome by on-line solid-phase extraction (SPE). Such an on-line SPE-CZE system was explored in combination with an ion trap (IT) mass spectrometer. Thus, it was possible to inject more than 100 microL sample solution on to the CZE capillary. Concentration limits of detection as low as 100 amol/microL were demonstrated for a peptide standard. This SPE-CZE-microelectrospray ionization (ESI)-MS/MS setup was compared directly to nanoLC/nanoESI using the same sample of a tryptic digest of bovine serum albumin (BSA) as a reference standard. Measurements were made on one IT mass spectrometer with identical acquisition parameters. Both chromatography systems enabled the separation and detection of low levels of peptides from a mixture of moderate complexity, with most peptides identified using both techniques; however, specific differences were obvious. The nanoLC-MS is about five times more sensitive than the CZE-MS, yet the difference was less pronounced than expected. The CZE-MS technique showed reduced loss of peptides, especially for larger peptides (missed cleavages) and is about four times faster than the nanoLC-MS approach.     

Determination of Silver in Human Hair by Ionic Liquid‐based Cold‐induced Aggregation Microextraction as a Solvent Extraction Method

We report on the determination of silver ion in human hair sample using ionic liquid cold‐induced aggregation combined with spectrophotometric detection. Parameters governing the extraction efficiency (such as concentration of ligand, volume of ionic liquid, pH, temperature, extraction time) were optimized and resulted in enrichment factor of 44.4 for silver ion. The calibration curve was linear with correlation coefficient (R²) of 0.999, in the concentration range from 1.0 to 20 ng mL^?1. The relative standard deviation is 2.3% (for n = 10), the limit of detection is 0.262 ng mL^?1, and relative recoveries in real samples were more than 93%.     

Determination of trace anions in liquefied petroleum gas using liquid absorption and electrokinetic migration for enrichment followed by ion chromatography

A simple sample enrichment technique, electrokinetic migration enrichment in single phase using a designed device, coupled with ion chromatography is presented for the determination of four anions (H(2)PO(4)(-), Cl(-), NO(3)(-), and SO(4)(2-)) in liquefied petroleum gas by liquid adsorption. The electrokinetic migration enrichment is based on the phenomenon of ion electrokinetic migration to the opposite electrode. When the anions migrated to the anode in a smaller volume chamber under the electric field, the concentration was realized. The main parameters affecting enrichment efficiency of applied voltage and enrichment time were investigated. The ion chromatography condition for anions separation was also studied. Under the optimal electrokinetic migration enrichment and ion chromatography conditions, the four anions were detected simultaneously with good linear relationship (r(2) = 0.9908-0.9968) and high precisions (less than 5% of the relative standard deviations of peak areas). The limits of detection of anions (S/N of 3) were in the range of 8-600 μg L(-1). The enrichment factors of the four anions ranged from 3.1 to 5.8. The established method was successfully applied to the analysis of the trace anions in liquefied petroleum gas by liquid adsorption with satisfactory results. The advantages of this method are simple operation and low cost.     

Determination of sulfur-containing inorganic anions by dual ion chromatography and capillary electrophoresis – application to the characterization of bacterial sulfur degradation

An analytical characterization of microbiological oxidation and reduction of sulfur anions has been performed with dual ion chromatography. The apparatus consisted of two chromatographic lines combined by a sample injection valve that allows the simultaneous introduction of a sample solution to both systems. With system 1 non-suppressed conductivity detection of sulfite and sulfate after separation in phthalate eluent was performed. For sulfide, thiosulfate and thiocyanate amperometric detection using a carbonate eluent was carried out on system 2. Parallel runs were possible for these species which could not be separated with one system. Optimizing separation and detection, limits of quantification of 0.02–0.3 mg/L could be obtained. The on-line coupling of a reaction vial to the injection valve was used to investigate the bacterial conversions, because the samples could be taken without contamination and air introduction. Thiosulfate was detected as a metabolite in both sulfur reduction and oxidation. Capillary electrophoresis with conductivity detection was applied as a complimentary technique for monitoring the bacterial sulfur oxidation. The results showed good correlation to the concentration values obtained by ion chromatography. Received: 14 May 1999 / Revised: 7 July 1999 / Accepted: 10 July 1999     

The impact of ionic strength and background electrolyte on pH measurements in metal ion adsorption experiments

Our understanding of metal ion adsorption to clay minerals has progressed significantly over the past several decades, and theories have been promulgated to describe and predict the impacts of pH, ionic strength, and background solution composition on the extent of adsorption. Studies evaluating the effects of ionic strength on adsorption typically employ a broad range of background electrolyte concentrations. Measurement of pH in these systems can be inaccurate when pH values are measured with liquid junction pH probes calibrated with standard buffers due to changes in the liquid junction potential between standard, low ionic strength (0.05 M) buffers and high ionic strength solutions (>0.1 M). The objective of this research is to determine the extent of the error in pH values measured at high ionic strength, and to develop an approach for accurately measuring pH over a range of ionic strengths using a combined pH electrode. To achieve this objective, the adsorption of cobalt (10(-5) M) onto gibbsite (10 g/L) from various electrolyte solutions (0.01-1 M) was studied. The pH measurements were determined from calibrations with standard buffers and ionic strength corrected buffer calibrations. The results show a significant effect of the aqueous solution background electrolyte anion and ionic strength on pH measurement. The 0.5 and 1 M ionic strength metal ion adsorption edges shifted to lower pH with increasing ionic strength when pH was calibrated with standard buffers whereas no shift in the adsorption edges was observed when calibrated with ionic strength corrected buffers. Therefore, to obtain an accurate pH measurement, pH calibration should contain the same electrolyte and ionic strength as the samples.     

Headspace solvent microextraction and gas chromatographic determination of some polycyclic aromatic hydrocarbons in water samples

Headspace solvent microextraction (HSME) was shown to be an efficient preconcentration method for extraction of some polycyclic aromatic hydrocarbons (PAHs) from aqueous sample solutions. A microdrop of 1-butanol (as extracting solvent) containing biphenyl (as internal standard) was used in this investigation. Extraction occurred by suspending a 3 μl drop of 1-butanol from the tip of a microsyringe fixed above the surface of solution in a sealed vial. After extraction for a preset time, the microdrop was retracted back into the syringe and injected directly into a GC injection port. The effects of nature of extracting solvent, microdrop and sample temperatures, stirring rate, microdrop and sample volumes, ionic strength and extraction time on HSME efficiency were investigated and optimized. Finally, the enrichment factor, dynamic linear range (DLR), limit of detection (LOD) and precision of the method were evaluated by water samples spiked with PAHs. The optimized procedure was successfully applied to the extraction and determination of PAHs in different water samples.     

在线固相萃取-离子色谱法测定4种芳环磺酸盐中的硫酸根离子

建立一种在线固相萃取-离子色谱测定4种芳环磺酸盐中硫酸根离子含量的新方法。将自装填的多孔石墨化碳固相萃取柱应用于离子色谱系统,对样品进行在线前处理。样品经过多孔石墨化碳固相萃取柱基体消除后进入收集环,通过阀切换方式使待测硫酸根离子转入阴离子分析柱和检测系统。固相萃取流路用1.5 mmol/L碳酸钠以0.8 mL/min的流速对基体在线富集,进样量为20μL,分析柱为SH-AC-3(250 mm×4.0 mm)+SH-AG-3(50 mm×4.0 mm)色谱柱,柱温为35℃,在6 mmol/L碳酸钠-4 mmol/L碳酸氢钠条件下等度洗脱,流速为0.8 mL/min。结果表明:硫酸根离子在0.50~20.00 mg/L范围内呈良好的线性关系,线性相关系数为0.998 3,保留时间、峰高和峰面积的相对标准偏差均在0.28%~2.86%之间,方法检出限为0.010 6 mg/L,回收率为91.01%~109.3%,具有良好的线性关系和重复性。整个在线分析过程在25 min之内完成。该方法进样量少、快速、高效。     

Pressure pinched injection of nanolitre volumes in planar micro-analytical devices

A new method for injecting and driving fluids by means of a multi-port injection valve and syringe pumps in a micro-channel network is described. A structure composed of two micro-channels arranged as a cross is connected with capillary tubes to an external multi-port injection valve. The fluid flows are driven by pressure and the multi-port valve controls the direction of the flow within the different sections of the structure. The first position of the multi-port valve allows the preparation of the loading of the sample, which is pinched in the cross section of the two micro-channels. The second position allows the precise injection of nL volumes. No dead volume exists between injection and separation modes. The system can be used to prepare a sample plug by pressure in order to perform chromatography with a broad range of buffered or non-buffered solutions. Thanks to the insensitivity to the ionic strength of the sample, this injection method is useful for the injection of complex biological samples in microchip analysis. In order to demonstrate the feasibility of the method, different solutions of ionic or fluorescent molecules were injected and detected in a photoablated planar polymer device.     

Graphene-modified Monolithic Capillary Column for the Microextraction of Trace Benzodiazepines in Biological Samples

A graphene monolithic column was fabricated in a capillary using π-electron-rich poly(N-vinylcarbazole-divinylbenzene) as the supporter through in situ one-step polymerization for the enrichment of trace benzodiazepines in biological samples. This new three-dimensional monolith showed uniformity and a continuous column bed; more importantly, it retained the unique properties of graphene that are typically associated with individual graphene sheets. Based on the large delocalized π-electron system, graphene forms π–π stacking interactions with benzodiazepines and benzene rings of poly(N-vinylcarbazole-divinylbenzene), which not only enhance the extraction performance for benzodiazepines compared to the neat polymer but also provide chemical stability of the graphene monolith. Moreover, several factors likely to affect the extraction, including ionic strength, sample pH, sample volume, and eluant volume were studied in detail. The optimized method gave a linear range of 0.005–1?ng?mL^?1, and detection limits of 1.12–2.35?ng?L^?1. Finally, the graphene monolith was successfully applied to the separation and enrichment of benzodiazepines from urine and hair samples coupled with high-performance liquid chromatography–mass spectrometry. The recoveries were in the range of 78.6–85.6% for urine and 87.2–94.3% for hair with relative standard deviations of 3.4–6.9 and 2.9–8.3%, respectively.     

Determination of aluminium and its fluoro complexes in natural waters by ion chromatography

Ion chromatography was applied to the determination of aluminium and its fluoro complexes in natural waters. The separation was carried out on a cation-exchange column. The aluminium species were detected by postcolumn reaction with Tiron followed by UV spectrophotometry. The method requires the adjustment of the pH and ionic strength of the sample to those of the mobile phase immediately prior to injection. Al³⁺ , AlF²⁺ and AlF₂⁺ are eluted separately while all hydroxo complexes are readily dissociated and eluted along with Al³⁺ under these conditions. The sum of peak areas, which represents the total aluminium concentration, was conserved whatever the amount of fluoride in the sample. Linearity of calibration was observed over the range 20–2000 μg 1⁻¹. Further, the speciation of fluoro-aluminium complexes as determined experimentally by ion chromatography is in good agreement with calculations based on complexation constants. The applications and limitations of the method are discussed.     

Measurement of pH by flow injection over a wide dynamic range with PVC/neutral carrier electrodes

Two pH-sensitive neutral carrier/PVC electrodes are used simultaneously for the measurement of pH by flow injection. One of these is based on the neutral carrier tridodecylamine and the other on octadecyl isonicotinate, and together they allow the pH range 1–13 to be covered. These electrodes have been used in a very low dispersion miniature potentiometric flow cell designed specifically for use in flow injection in conjunction with a multi-channel data acquisition system. The effects of the solution ionic strength and buffer capacity on the pH measurement are discussed. A flow-injection manifold is proposed which can be used for high-accuracy pH measurements without ionic strength adjustment of the sample and for simultaneous pH and ion concentration measurements with ionic strength adjustment. This has been tested on some synthetic samples for the simultaneous determination of potassium and calcium and the measurement of pH.     

Rapid Determination of DDT and Its Metabolites in Environmental Water Samples with Mixed Ionic Liquids Dispersive Liquid–Liquid Microextraction Prior to HPLC-UV

In this paper, a novel mixed ionic liquids-dispersive liquid–liquid microextraction method was developed for rapid enrichment and determination of environmental pollutants in water samples. In this method, two kinds of ionic liquids, hydrophobic ionic liquid and hydrophilic ionic liquid, were used as extraction solvent and disperser solvent, respectively. DDT and its metabolites were used as model analytes and high-performance liquid chromatography with ultraviolet detector for the analysis. Factors that may affect the extraction recoveries, such as type and volume of extraction solvent (hydrophobic ionic liquid) and disperser solvent (hydrophilic ionic liquid), extraction time, sample pH and ionic strength, were investigated and optimized. Under the optimum conditions, the linear range was 1–100 μg L⁻¹, limits of detection could reach 0.21–0.49 μg L⁻¹, and relative standard deviation was 6.01–8.48 % (n = 7) for the analytes. Satisfactory results were achieved when the method was applied to analyze the target pollutants in environmental water samples with spiked recoveries over the range of 85.7–106.8 %.