Analysis of anions in ambient aerosols by microchip capillary electrophoresis

We describe a microchip capillary electrophoresis method for the analysis of nitrate and sulfate in ambient aerosols. Investigating the chemical composition of ambient aerosol particles is essential for understanding their sources and effects. Significant progress has been made towards developing mass spectrometry-based instrumentation for rapid qualitative analysis of aerosols. Alternative methods for rapid quantification of selected high abundance compounds are needed to augment the capacity for widespread routine analysis. Such methods could provide much higher temporal and spatial resolution than can be achieved currently. Inorganic anions comprise a large percentage of particulate mass, with nitrate and sulfate among the most abundant species. While ion chromatography has proven very useful for analyzing extracts of time-integrated ambient aerosol samples collected on filters and for semi-continuous, on-line particle composition measurements, there is a growing need for development of new compact, inexpensive approaches to routine on-line aerosol ion analysis for deployment in spatially dense, atmospheric measurement networks. Microchip capillary electrophoresis provides the necessary speed and portability to address this need. In this report, on-column contact conductivity detection is used with hydrodynamic injection to create a simple microchip instrument for analysis of nitrate and sulfate. On-column contact conductivity detection was achieved using a Pd decoupler placed upstream from the working electrodes. Microchips containing two Au or Pd working electrodes showed a good linear range (5-500 microM) and low limits-of-detection for sulfate and nitrate, with Au providing the lowest detection limits (1 microM) for both ions. The completed microchip system was used to analyze ambient aerosol filter samples. Nitrate and sulfate concentrations measured by the microchip matched the concentrations measured by ion chromatography.     

Simultaneous stacking of cationic and anionic compounds in single run capillary zone electrophoresis by two-end field amplified sample injection

In order to extend the application of field amplified sample injection (FASI) in high throughput analysis, a convenient and simple procedure, namely two-end field amplified sample injection (TE-FASI), was developed for the simultaneous stacking of cationic and anionic compounds in a single run capillary zone electrophoresis (CZE). Following the capillary-filling with a buffer of high conductivity, water plug was loaded into each end of the capillary; and two high-field strength zones were generated at both heads of the column when high voltage was applied. Therefore, under suppressed EOF cations and anions can be selectively FASI stacked at anode and cathode head, respectively. After separation, the stacked anions and cations are detected by a common detector placed in the center of the capillary. Under the optimized conditions, the limits of detection for the model cationic (matrine and oxymatrine) and anionic (5-sulfosalicylic acid) compounds were determined as 0.2, 0.2 and 0.06 ng/mL, respectively. Compared with non-stacking conditions, the sensitivities of these compounds were enhanced 1003-, 1330- and 1380-fold, respectively. The results of reproducibility, linearity and real sample analysis show that the proposed procedure is promising to be applied for the simultaneous quantification detection of trace cationic and anionic analytes.     

Capillary electrophoresis with end-capillary potentiometric detection using a copper electrode.

Potentiometric end-capillary detection in capillary electrophoresis has the advantage of relatively easy miniaturisation without having to compromise the concentration sensitivity. Potentiometric end-capillary detection using a copper electrode is also attractive because of the sensitive detection of many inorganic and organic UV-transparent ions and the ability to work in both direct and indirect mode. In this work, detection of a number of common anions in a tartrate electrolyte at pH 3 was studied. The influence of the end-capillary detection geometry on the detection performance was investigated. An end-capillary detection cell allowing the separation capillary to be changed without the need to realign the detection electrode was constructed and fitted into a commercial CE apparatus. Under the optimal configuration, which was a 25 microm diameter copper electrode aligned coaxially with a 25 microm capillary and positioned at a distance of about 25 microm from the capillary end, excellent peak shapes were achieved and comparison with simultaneous on-capillary photometric detection showed no additional peak broadening. Good sensitivity was obtained, resulting in concentration limits of detection (LODs) in the low microM range and mass LODs in the low amol range. Examples of separations of inorganic and organic anions are presented and the analytical potential of the detection method is assessed.     

Parameters influencing separation and detection of anions by capillary electrophoresis

Electrolyte composition is critical in optimizing separation and detection of ions by capillary electrophoresis. The parameters which must be considered when designing an electrolyte system for capillary electrophoresis include electrophoretic mobility of electrolyte constituents and analytes, detection mode, and compatibility of electrolyte constituents with one another. An electrolyte system based on pyromellitic acid is well suited for use with indirect photometric detection, and provides excellent separations of anions. The ability to modify the electrophoretic mobility of pyromellitic acid as a function of ph provides flexibility in matching electrophoretic mobilities of analytes. Additionally, the use of alkyl amines as electroosmotic flow modifiers allows the rapid separation of anions by reversing the direction of electroosmotic flow in a fused-silica capillary. The optimization of a capillary electrophoresis electrolyte for anion analysis is also discussed in terms of pH, ionic strength and applied voltage. The effect of organic solvent on separation selectivity is also discussed.     

Speciation analysis of arsenic and selenium compounds by capillary electrophoresis

High-performance capillary electrophoresis is applied to the separation of different inorganic and organic arsenic and selenium compounds. In comparison with UV-detection, an approach with conductivity detection is described expecting higher sensitivity and universality. In this case the capillary was statically modified with CTAB before the electromigration procedure. The separation was performed with an electrolyte system consisting of CHES and Triton X-100. Detection limits of 0.06 mg/L or lower were obtained for As(V) and Se(VI). Water samples of an arsenic-polluted tailing of tin mining processes were analysed for anions as well as arsenic and selenium species.     

Speciation analysis of arsenic and selenium compounds by capillary electrophoresis

High-performance capillary electrophoresis is applied to the separation of different inorganic and organic arsenic and selenium compounds. In comparison with UV-detection, an approach with conductivity detection is described expecting higher sensitivity and universality. In this case the capillary was statically modified with CTAB before the electromigration procedure. The separation was performed with an electrolyte system consisting of CHES and Triton X-100. Detection limits of 0.06 mg/L or lower were obtained for As(V) and Se(VI). Water samples of an arsenic-polluted tailing of tin mining processes were analysed for anions as well as arsenic and selenium species.     

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.     

Capillary zone electrophoresis for the determination of light-absorbing anions in environmental samples.

A rapid and simple method for separation and determination of inorganic anions by capillary zone electrophoresis was described. The detection was carried out directly with a diode array detector. The experimental conditions, such as concentration of carrier electrolyte, capillary length, voltage, and temperature were optimized. In order to improve selectivity, different organic modifiers were also investigated. The baseline separation of 10 light-absorbing anions was accomplished within 3.5 min with a background electrolyte consisting of 50 mM sodium tetraborate containing 5% MeOH. Linear plots were obtained in the concentration range of 0.1-10 microg/ml. With sample stacking injection, the quantitation limits of the anions were found to be in the range of 0.02-0.1 microg/ml. The proposed method was successfully applied to the determination of inorganic anions in environmental samples and in effluents of a power plant.     

Capillary zone electrophoresis separations of enantiomers present in complex ionic matrices with on-line isotachophoretic sample pretreatment.

Analytical capabilities of capillary zone electrophoresis (CZE) with on-line coupled capillary isotachophoresis (ITP) sample pretreatment in the column-coupling capillary electrophoresis equipment to separate and determine enantiomers present in multicomponent ionic matrices were studied. Tryptophan was used as a model analyte in the ITP-capillary zone electrophoresis experiments performed in this context while a 90-component model mixture of UV-light absorbing organic anions and urine served as multicomponent sample matrices. Various working modes in which the on-line coupled capillary isotachophoresis-capillary zone electrophoresis combination in the column-coupling separation system can operate were employed in the anionic regime of the separation with direct injections of the samples. Advantages and limitations of these working modes in the separations of enantiomers present in model and urine matrices were assessed. Experiments with model mixtures of tryptophan enantiomers revealed that the two were resolved in the capillary zone electrophoresis stage with the aid of alpha-cyclodextrin also when their concentration ratio in the sample was 1:200 while the concentration of L(-)-tryptophan was 25 nmol/l. The limits of detection for the enantiomers were at approximately 10 nmol/l (approximately 1.5 ng/ml) concentrations for a 220 nm detection wavelength of the UV detector employed in the capillary zone electrophoresis stage and for a 30 microliters sample load. A high sample load capacity of the on-line coupled capillary isotachophoresis stage was effective in separating the samples corresponding to 3-6 microliters volumes of undiluted urine. The results from the runs with urine samples showed that only the capillary isotachophoresis-capillary zone electrophoresis combination with a post-column on-line coupled capillary isotachophoresis sample clean-up (responsible for a removal of more than 99% of the sample anionic constituents migrating in the on-line coupled capillary isotachophoresis stack and detectable in the capillary zone electrophoresis stage) provided a universal alternative for the detection and quantitation of the model analyte (L(-)-tryptophan).     

Application of a contactless conductometric detector for the simultaneous determination of small anions and cations by capillary electrophoresis with dual-opposite end injection

A contactless conductometric detection (CCD) system for capillary electrophoresis (CE) with a flexible detection cell was applied for the simultaneous determination of small anions and/or cations in rain, surface and drainage water samples. The applied frequency, the amplitude of the input signal, the electrolyte conductivity and electrode distance were found to be the most significant factors affecting the detection sensitivity. 2-(N-Morpholino)ethanesulfonic acid/histidine-based (MES/His) electrolytes were used for direct conductivity detection of anions and cations, while ammonium acetate was selected for indirect conductivity determination of alkylammonium salts. For the simultaneous separation procedure, involving dual-opposite end injection, an electrolyte consisting of 20 mM MES/His, 1.5 mM 18-crown-6 and 20 microM cetyltrimethylammonium bromide provided baseline separation of 13 anions and cations in less than 6 min. The detection limits achieved were 7-30 micrograms/l for direct conductometric detection of various common inorganic cations and anions, excluding F- (62 micrograms/l) and H2PO4- (250 micrograms/l), and 35-178 micrograms/l for indirect conductometric detection of alkyl ammonium cations. The developed electrophoretic method with conductometric detection was compared to ion chromatography.     

Large-volume stacking in capillary electrophoresis using a methanol run buffer

Highly sensitive nonaqueous capillary electrophoresis of weakly acidic organic compounds has been performed using methanol as the run buffer solvent. Methanol provided appropriate suppression of the electroosmotic flow and an increase in the electrophoretic mobilities of anionic solutes compared to water. These two effects allowed large-volume stacking using the electroosmotic flow pump (LVSEP) to be achieved for larger anions using a bare fused-silica capillary under an electric field of reverse polarity, whereas only fast-moving small anions were previously known to be suitable for LVSEP in aqueous media. A field-enhanced sample injection of an additional amount of analytes during the solvent plug removal further enhanced the limits of detection to below the nanomolar range with conventional UV absorption detection. Under optimum conditions, excellent linear responses and reproducibility in the migration times together with the corrected peak areas for ten analytes were obtained in the concentration range of 10-100 nM.     

Method development for the analysis of particle phase substituted methoxy phenols and aromatic acids from biomass burning using capillary electrophoresis/electrospray ionization mass spectrometry (CE/ESI-MS)

A method is developed for the determination of substituted methoxy phenols and aromatic acids in biomass burning aerosol using capillary electrophoresis (CE) coupled to an electrospray ionization mass spectrometer. Background electrolytes (BGEs) containing ammonium acetate, ammonium hydroxide and 10% (v/v) methanol at pH 9.1 and ammonium hydroxide at pH 11 are investigated for their suitability. A good linearity is found for all analytes in the range of 1-50 microM for the ammonium acetate based BGE and 1-40 microM for the ammonium hydroxide BGE. The detection limit ranged from 0.1 to 1.0 microM for the ammonium acetate based BGE and 0.3 to 0.7 microM for the ammonium hydroxide BGE. The relative standard deviation (R.S.D.) is typically less than 0.5% (ammonium acetate based BGE) and 4.2% (ammonium hydroxide BGE) for the migration time and 3-9% (ammonium acetate based BGE) and 2.5-8% (ammonium hydroxide BGE) for the peak area (n = 5). The analytical time was less than 10 min for both methods. The proposed methods are fast, sensitive and quantitative and can be applied to the analysis of complex biomass burning aerosol samples without complex pre-treatment. The results from the analysis of real biomass burning samples demonstrate the suitability of the proposed methods to the analysis of low concentration water soluble organic carbon (WSOC) in biomass burning samples. The fast analytical time and high sensitivity of the proposed methods enables the analysis of a large number of size segregated impactor samples from biomass burning aerosols.     

On-line preconcentration of organic anions in capillary electrophoresis by solid-phase extraction using latex-coated monolithic stationary phases

Quaternary ammonium functionalised polymeric latex particles were coated onto the wall of a fused-silica capillary or onto a methacrylate monolithic bed synthesised inside the capillary in order to create ion-exchange stationary phases of varying ion-exchange capacity. These capillaries were coupled in-line to a separation capillary and used for the solid-phase extraction (SPE), preconcentration and subsequent separation of organic anions by capillary electrophoresis. A transient isotachophoretic gradient was used for the elution of bound analytes from the SPE phase using two modes of separation. The first comprised a low capacity SPE column combined with a fluoride/octanesulfonate discontinuous electrolyte system in which peak compression occurred at the isotachophoretic gradient front. The compressed anions were separated electrophoretically after elution from the SPE preconcentration phase and resolution was achieved by altering the pH of the electrolyte in which the separation was performed. In the second approach, a latex-coated monolithic SPE preconcentration stationary phase was used in combination with a fluoride/perchlorate electrolyte system, which allowed capillary electrochromatographic separation to occur behind the isotachophoretic gradient front. This method permitted the removal of weakly bound anions from the SPE phase, thereby establishing the possibility of sample clean-up. The effect of the nature of the strong electrolyte forming the isotachophoretic gradient on the separation and also on the preconcentration step was investigated. Capillary electrochromatography of inorganic and organic species performed on the latex-coated monolithic methacrylate column highlighted the presence of mixed-mode interactions resulting from the incomplete coverage of latex particles onto the monolithic surface. Analyte preconcentration prior to separation resulted in compression of the analyte zone by a factor of 300. Improvement in the limit of detection of up to 10400 times could be achieved when performing the preconcentration step and the presented methods had limits of detection (S/N=3) ranging between 1.5 and 12 nM for the organic anions studied.     

Study of the selectivity of inorganic anions in hydro-organic solvents using indirect capillary electrophoresis

In capillary electrophoresis (CE) analysis of small inorganic anions, the ability to control the electroosmotic flow (EOF) and the ability to alter the electrophoretic mobility of the ions are essential to improve resolution and separation speed. In this work, a CE method for separation of small inorganic anions using indirect detection in mixed methanol/water buffers is presented. The suitability of different UV absorbing probes commonly used for indirect detection including chromate, iodide, phthalate, benzoate, trimellitate, and pyromellitate, in mixed methanol/water buffers is examined. The effect of the electrolyte buffer system, including the pH, buffer concentration and the organic solvent on the electrophoretic mobility of the probes and analytes are also investigated. The EOF was reversed using cationic surfactant, cetyltrimethylammonium bromide (CTAB) so ions were separated under co-EOF mode. The organic solvent alters the electrophoretic mobility of the probes and the analytes differently and hence choice of the appropriate probe is essential to achieve high degree of detection sensitivity. Separations of six anions in less than 2.5 min were accomplished in buffers containing up to 30% MeOH. Adjustment of the methanol content helps to improve the selectivity and resolution of inorganic anions. Limit of detection, reproducibility and application of the method for quantification of anions in water samples will also be discussed.     

Capillary electrophoresis using high ionic strength background electrolytes containing zwitterionic and non-ionic surfactants and its application to direct determination of bromide and nitrate in seawater

In capillary electrophoresis, it is commonly considered that even a moderately high ionic concentration in the background electrolyte (BGE) leads to high currents, resulting in Joule heating and serious peak distortion. As a new approach to overcome this problem, zwitterionic (Zwittergent-3-14) and/or non-ionic (Tween 20) surfactants have been added to BGEs containing high salt concentrations (e.g. 0.3 M NaCl) and have been shown to result in acceptable separation currents (<200 microA). In turn, these BGEs could be applied to the separation of samples containing high salt concentrations (such as undiluted seawater) without the occurrence of any significant peak broadening due to electrodispersion of the sample. For example, a BGE comprising 10 mM Zwittergent-3-14, 50 mM Tween 20, 0.3 M NaCl and 5 mM phosphate (ph 7) could be used for the determination of UV-absorbing anions in seawater, giving good peak shapes and detection limits of 0.8 microM and 0.6 microM for nitrate and bromide, respectively. The beneficial effects of the non-ionic surfactant on the separation were attributed largely to suppression of the electro-osmotic flow. On the other hand, the zwitterionic surfactant was found to be capable of the incorporation of some anions in accordance with the behaviour of these same surfactants in electrostatic ion chromatography. This incorporation resulted in a decreased conductivity of the BGE and also a change in the separation selectivity of the system.     

Sensitive indirect photometric detection of inorganic and small organic anions by capillary electrophoresis using Orange G as a probe ion

This study addresses the two major problems in the use of dyes as highly absorbing probes for indirect photometric detection in capillary electrophoresis (CE). First, effective electroosmotic flow (EOF) modification or suppression to allow separation and detection of a wide mobility range of analytes is not straightforward when electrolytes containing increased dye concentrations are used. The suppression of EOF to less than + 5x10(-9) m(2)V(-1)s(-1) was achieved with a combination of a poly(ethylenimine) (PEI)-coated capillary and the addition of the neutral polymer hydroxypropylmethylcellulose (HPMC) to the background electrolyte. Second, the deterioration of baselines due to adsorption of the dye probe to the capillary wall is generally a problem. In this work, baseline quality at higher probe concentrations was significantly improved by a rather unusual but highly effective combination of a simultaneous application of a slight overpressure (25 mbar) at the injection end during the separation, and the use of a relatively narrow capillary of 50 microm inner diameter. Both measures would appear to be counterproductive. Optimisation of the probe concentration with regard to signal-to-noise ratio resulted in an electrolyte of 4 mM Orange G, 0.05% HPMC buffered at pH 7.7 by the addition of 10.0 mM histidine isoelectric buffer. Very high separation efficiencies of 128 000-297 000 plates were made possible by the relatively high probe concentration. Combined with excellent detection sensitivity, even with the introduction of hydrodynamic flow and a reduced optical path length, these measures resulted in limits of detection ranging from 0.216 to 0.912 microM with a deuterium lamp light source (248 nm) and from 0.147 to 0.834 microM with a 476 nm blue light-emitting diode (LED) light source. Reproducibility over 30 consecutive runs without changing the electrolyte was excellent, with relative standard deviation (RSD) values of 0.14-0.80% for migration time, 1.27-3.36% for peak area and 0.88-5.12% for peak heights. The optimised electrolyte was used for the analysis of inorganic anions in air filter samples, providing good agreement with results obtained by ion chromatography.     

On-line simultaneous and rapid separation of anions and cations from a single sample using dual-capillary sequential injection-capillary electrophoresis

A novel capillary electrophoresis (CE) approach has been developed for the simultaneous rapid separation and identification of common environmental inorganic anions and cations from a single sample injection. The method utilised a sequential injection-capillary electrophoresis instrument (SI-CE) with capacitively-coupled contactless conductivity detection (C⁴D) constructed in-house from commercial-off-the-shelf components. Oppositely charged analytes from a single sample plug were simultaneously injected electrokinetically onto two separate capillaries for independent separation and detection. Injection was automated and may occur from a syringe or be directly coupled to an external source in a continuous manner. Software control enabled high sample throughput (17 runs per hour for the target analyte set) and the inclusion of an isolation valve allowed the separation capillaries to be flushed, increasing throughput by removing slow migrating species as well as improving repeatability. Various environmental and industrial samples (subjected only to filtering) were analysed in the laboratory with a 3 min analysis time which allowed the separation of 23 inorganic and small organic anions and cations. Finally, the system was applied to an extended automated analysis of Hobart Southern Water tap water for a period of 48 h. The overall repeatability of the migration times of a 14 analyte standard sample was less than 0.74% under laboratory conditions. LODs ranged from 5 to 61 μg L⁻¹. The combination of automation, high confidence of peak identification, and low limits of detection make this a useful system for the simultaneous identification of a range of common inorganic anions and cations for discrete or continuous monitoring applications.     

Quantitative determination of sulfur-containing anions in complex matrices with capillary electrophoresis and conductivity detection.

The analysis of sulfur species in complex matrices, like environmentally related samples, requires selective and sensitive as well as robust determination methods. As many as possible different anions need to be quantified in a reasonable analysis time. Besides ion chromatography, capillary electrophoresis has proven to be a very efficient technique for the separation and determination of ionic compounds. With the advantages of less sample and solvent consumption compared to conventional LC, short separation time, inexpensive and robust capillaries, CE was used to separate the anions sulfate, sulfite, thiosulfate, thiocyanate and sulfide. Detection and injection modes and the composition of the separation buffers have been varied to find the most suitable conditions. Conductivity detection after electrokinetic sample injection and improvement of calibration linearity allowed the determination of sulfur containing anions with low limits of detection (8 to 50 micrograms/l). The developed CE method was applied to the analysis of water from an open-pit mining lake.     

Separation and determination of the macrolide antibiotics (erythromycin, spiramycin and oleandomycin) by capillary electrophoresis coupled with fast reductive voltammetric detection

Separation and determination of erythromycin, spiramycin and oleandomycin by capillary zone electrophoresis coupled with fast reductive voltammetric detection using an Hg-film electrode was investigated in a simple aqueous phosphate buffer system. The influence of pH, concentration of phosphate, applied voltage, capillary length and dimension on the separation was examined and optimized. The entire separation of erythromycin, spiramycin, and oleandomycin was achieved in a 0.2 mol/L phosphate buffer system without organic modifiers. The electrochemical detection parameters, such as electrode material, applied waveform, scan rate, preconcentration potentials and preconcentration times, were investigated and discussed. This approach provides high separation efficiency and high sensitivity for all compounds, with detection limits (3 x peak-to-peak baseline noise) of 7.5 x 10(-8) mol/L for spiramycin, and 3 x 10(-7) mol/ L for erythromycin and oleandomycin. The calibration plot of peak areas for each separated peak vs. concentration of analyte was found to be linear over three orders of magnitude.     

The Use of Cationic Polymer for the Separation of Inorganic Anions by Capillary Electrophoresis

A capillary zone electrophoresis (CZE) method for the simultaneous assay of bromide, iodide, nitrite, nitrate, and thiocyanate using direct UV detection is reported. The method is based on the separation of anions in a capillary coated with a cationic polymer, polyethyleneimine (PEI). The minimum detection limits, reproducibility of peak areas, and migration times were determined at the optimal condition. The method was applied to the analysis of tap water and human urine. The changes in separation selectivity of the anions resulting from addition of the cationic polymer to the buffer were investigated.