Quantitative capillary zone electrophoresis of inorganic anions

Analysis and characterization of aluminum chlorohydrate oligocations by capillary electrophoresis

Aluminum chlorohydrates (ACH) are the active ingredients used in most antiperspirant products. ACH is a water soluble aluminum complex which contains several oligomeric polycations of aluminum with degrees of polymerization up to Al₁₃ or Al₃₀. The characterization and quantification of ACH oligo-cations remain a challenging issue of primary interest for developing structure/antiperspirant activity correlations, and for controlling the ACH ingredients. In this work, highly repeatable capillary electrophoresis (CE) separation of A_l3⁺, Al₁₃ and Al₃₀ oligomers contained in ACH samples was obtained at pH 4.8, owing to a careful choice of the background electrolyte counter-ion and chromophore, capillary I.D. and capillary coating. This is the first reported separation of Al₁₃ and Al₃₀ oligomers in conditions that are compatible with the aluminum speciation in ACH solution or in conditions of antiperspirant application/formulation. Al₁₃ and Al₃₀ effective charge numbers were also determined from the sensitivity of detection in indirect UV detection mode. The relative mass proportion of Al₁₃ compared to Al₁₃+Al₃₀ could be determined in different aluminum chlorohydrate samples. Due to its simplicity, repeatability/reproducibility, minimal sample preparation and mild analytical conditions, CE appears to be a promising analytical separation technique for the characterization of ACH materials and for the study of structure/antiperspirant activity correlations.

Improving chip-to-chip precision in disposable microchip capillary electrophoresis devices with internal standards

To realize portable systems for routine measurements in point-of-care settings, MCE methods are required to be robust across many single-use chips. While it is well-known internal standards (ISTDs) improve run-to-run precision, a systematic investigation is necessary to determine the significance of chip-to-chip imprecision in MCE and how ISTDs account for it. This paper addresses this question by exploring the reproducibility of Na quantification across six basic, in-house fabricated microchips. A dataset of 900 electrophoerograms was collected from analyzing five concentrations of NaCl with two ISTDs (CsCl and LiCl). While both improved the peak area reproducibility, the Na/Cs ratio was superior to the Na/Li ratio (improving the RSD by a factor of 2-4, depending on the Na concentration). We attribute this to the significant variation in microchannel surface properties, which was accounted for by cesium but not lithium. Microchip dimension and detector variations were only a few percent, and could be improved through commercial fabrication over in-house made microchips. These results demonstrate that ISTDs not only correct for intrachip imprecision, but are also a viable means to correct for chip-to-chip imprecision inherent in disposable, point-of-care MCE devices. However, as expected, the internal standard must be carefully chosen.

What can in situ ion chromatography offer for Mars exploration?

The successes of the Mars exploration program have led to our unprecedented knowledge of the geological, mineralogical, and elemental composition of the martian surface. To date, however, only one mission, the Phoenix lander, has specifically set out to determine the soluble chemistry of the martian surface. The surprising results, including the detection of perchlorate, demonstrated both the importance of performing soluble ion measurements and the need for improved instrumentation to unambiguously identify all the species present. Ion chromatography (IC) is the state-of-the-art technique for soluble ion analysis on Earth and would therefore be the ideal instrument to send to Mars. A flight IC system must necessarily be small, lightweight, low-power, and have low eluent consumption. We demonstrate here a breadboard system that addresses these issues by using capillary IC at low flow rates with an optimized eluent generator and suppressor. A mix of 12 ions known or plausible for the martian soil, including 4 (oxy)chlorine species, has been separated at flow rates ranging from 1 to 10 μL/min, requiring as little as 200 psi at 1.0 μL/min. This allowed the use of pneumatic displacement pumping from a pressurized aluminum eluent reservoir and the elimination of the high-pressure pump entirely (the single heaviest and most energy-intensive component). All ions could be separated and detected effectively from 0.5 to 100 μM, even when millimolar concentrations of perchlorate were present in the same mixtures.

Development of extraction and analytical methods of nitrite ion from food samples: microchip electrophoresis with a modified electrode

Two simple and fast methods for the extraction of the nitrite ion (NO(2)(-)) from food samples have been developed. The methods were characterized by UV-visible spectroscopic and electrochemical measurements, and their performance for NO(2)(-) extraction was compared with a standard method. The extraction methods yielded relative recoveries between 100 and 120% with good reproducibility of 3.9% (RSD, n = 4) in UV-visible experiments. Microchip electrophoresis with electrochemical detection (MCE-ED) coupled with a copper (3-mercaptopropyl)trimethoxysilane [Cu(II)-MPS] complex-modified carbon paste electrode (CPE) has been employed to detect NO(2)(-) in extracted samples. The Cu(II)-MPS complex was synthesized and characterized by voltammetry, XPS, and FT-IR analyses. Experimental parameters affecting the separation and detection performances of the MCE-ED method were assessed and optimized. The potential for the electrocatalytic reduction of NO(2)(-) for MCE-ED was found to be -190 mV (vs Ag/AgCl). When extracted food samples were analyzed by the MCE-ED method, a reproducible response for the NO(2)(-) reduction (RSD of 4.3%) at the modified-CPE reflected the negligible electrode fouling. A wide dynamic range of 1.0-160 ppm was observed for analyzing standard NO(2)(-) with a sensitivity of 0.05106 ± 0.00141, and the detection limit, based on S/N = 3, was found to be 0.35 ± 0.05 ppm. No apparent interference from NO(3)(-), other inorganic ions, and biological compounds was observed under the optimal experimental conditions. A standard addition method for real samples showed wide concentration ranges of 1.10-155 and 1.2-150 ppm for analyzing NO(2)(-) in ham and sausage samples, respectively.

CE-ESI-MS of biological anions in plastic capillaries at high pH

In this study, the potential of poly(methylmethacrylate) (PMMA, Plexiglas) and polyether ether ketone (PEEK) tubing for CE-ESI-MS separations of anions at high pH values was examined. A set of model compounds of biological interest was used to investigate the main operational parameters for CE-ESI-MS, such as the sheath-flow interface design, the polarity of the ionization voltage, the use of ammonia-based separation electrolytes, and the sheath liquid composition. Optimum separations and detection sensitivities in negative ESI mode were obtained using a running electrolyte of 75 mM of ammonia at pH 11 and a sheath liquid of 60:40 v/v or 75:25 v/v isopropanol/water with 0.5% v/v of ammonia. At these experimental conditions, PMMA and PEEK capillaries show good hydrolytic stabilities and lower EOF values than fused-silica columns. Better separation resolutions were obtained with PMMA capillary, but this plastic rapidly swelled and bled because of its limited chemical resistance to the sheath liquid. PMMA columns equipped with a fused-silica tip were used for a safer exposure to the sheath liquid, but the inner surface of the fused-silica tips had limited stability at pH 11. On the other hand, good separations and reproducibility on migration times and peak areas were obtained using PEEK capillaries without capillary column deterioration.

Simultaneous analysis of nitrate and nitrite in a microfluidic device with a Cu-complex-modified electrode

A CE microsystem coupled with a microchip and a copper-(3-mercaptopropyl) trimethoxysilane (Cu-MPS) complex-modified carbon paste electrode (CPE) was developed for the simultaneous analysis of nitrite and nitrate. The method is based on the electrocatalytic reduction of both analytes with the modified electrode. The Cu-MPS complex was characterized by voltammetric, XPS, and FT-IR analyses. Experimental parameters affecting the sensitivity of the modified electrode were assessed and optimized. The best separation was achieved in a 60 mm separation channel filled with a 20 mM acetate buffer of pH 5.0 containing 3.0 mM CTAB at separation field strength of -250 V/cm within 90 s. The detection potential for the simultaneous analysis of nitrite and nitrate was found to be -225 mV versus Ag/AgCl. A reproducible response (RSD of 3.2% (nitrite) and 2.8% (nitrate), n = 8) for repetitive sample injections reflected the negligible electrode fouling at the modified CPE. The interference effect was examined for other inorganic ions and biological compounds. A wide hydrodynamic range between 0.25 and 120 microM was observed for analyzing nitrite and nitrate with the sensitivities of 0.069 +/- 0.003 and 0.065 +/- 0.002 nA/microM, and the detection limits, based on S/N = 3, were found to be 0.09 +/- 0.007 and 0.08 +/- 0.009 microM, respectively. The applicability of the method to water and urine samples analyses was demonstrated.

Measuring the length of hydrodynamically injected plugs in capillary electrophoresis using the electrical current monitoring

Although CE is nowadays a worldwide separation technique, it is generally recognized that one of its main limitations is its poor robustness for quantitative analysis. Although this limitation can partially be surpassed using internal standards (ISs), it is well known that to find adequate standards is a very difficult task when too complex mixtures have to be analyzed. In this work, an alternative method to improve quantitation by CE is presented using the electrical current profile monitored during any CE run. Thus, an abrupt step in the current monitoring is observed when a hydrodynamically injected plug of conductivity different from the BGE leaves the capillary under the influence of the EOF. It is demonstrated that under these conditions, the relative amplitude of this step can be used to measure experimentally the injection length. This measure can not only be used for calibration, but also to correct variations of the length injected which is demonstrated to improve significantly the quantitative accuracy and reproducibility of CE. Thus, RSD values for interday quantification (five experiments a day for 5 days) were improved from 10.5 to 4.2%. Moreover, it is also demonstrated that accuracy of quantitative determinations by CE can greatly be improved by using this procedure. The method can also be implemented in other separation techniques where the EOF is used as driving force (e.g., CEC, MEKC or chip-based separations). Advantages and limitations of this approach in comparison to the use of ISs are also discussed.

Potential of CE for the determination of inorganic and acidic anions in cyanoacrylate adhesives

In this work, a CZE method with indirect UV detection was developed for the simultaneous determination of the inorganic and acidic anions, chloride, sulfate, nitrate, fluoride, formate, phosphate, diethylphosphate, methyl sulfonate, cyanoacetate, and methacrylate present in cyanoacrylate adhesives. Chromate was employed as the probe ion, and the EOF was reversed by incorporating CTAB into BGE. Detection limits of 0.7-4.6 microg/mL were obtained for all the anions studied. The CE method developed is a significant improvement on traditionally used chromatographic methods such as ion chromatography, as it resulted in shorter analysis times with enhanced separation efficiencies. This method was successfully employed for the analysis of inorganic and acidic anions in cyanoacrylate adhesive samples.

Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

Recent advances in the CE and CEC separation, detection, and sample preparation methodologies applied to the determination of a variety of compounds having current or potential environmental relevance have been overviewed. The reviewed literature has illustrated the wide range of CE applications, indicating the continuing interest in CE and CEC in the environmental field.

Separation and quantitation of the four stereoisomers of itraconazole in pharmaceutical formulations by electrokinetic chromatography

The four stereoisomers of itraconazole were resolved for the first time by EKC using a CD as chiral selector. A study on the enantiomeric separation ability of different neutral CDs was carried out. Heptakis-2,3,6-tri-O-methyl-beta-CD was shown to provide the highest values for the enantiomeric resolution. The influence of some experimental conditions, such as pH, chiral selector concentration, and temperature, on the enantiomeric separation was also studied. The use of a 100 mM phosphate buffer (pH 2.5), 30 mM in heptakis-2,3,6-tri-O-methyl-beta-CD together with an applied voltage of 30 kV and a temperature of 20 degrees C enabled the separation of the enantiomers of itraconazole with high resolutions (Rs > 3.0). Finally, the method was validated and successfully applied to the quantitation of itraconazole in three pharmaceutical formulations.

Evaluation of capillary ion exchange stationary phase coating distribution and stability using radial capillary column contactless conductivity detection

The use of radial (across) capillary column capacitively coupled contactless conductivity detection is demonstrated as a simple and rapid technique for visualisation of stationary phase longitudinal coverage and coating stability in capillary ion exchange chromatography.

CE determination of small ions: methods and techniques

This paper provides an overview on the current status of capillary electrophoresis (CE) in the analysis of inorganic and charged small organic species. The various CE strategies used to improve the separation of ionic analytes are summarized. Technical developments in the design of improved detection systems are described. A brief account of their advantages and limitations is given. The potential use of these devices for miniaturized CE systems is also described. Finally, special attention is focused on the on-capillary preconcentration techniques developed in attempts to overcome the poor detectability of CE. Recent review articles are frequently cited to provide readers with a source of information about pioneering work, theoretical treatments, and specific applications.

Enantiomeric separation of ketoconazole and terconazole antifungals by electrokinetic chromatography: Rapid quantitative analysis of ketoconazole in pharmaceutical formulations

EKC using a neutral CD as chiral selector was applied in this work to the development of a method enabling the enantiomeric separation of ketoconazole and terconazole antifungals. The influence of different experimental conditions such as temperature, CD concentration, pH, and nature and concentration of the buffer on the enantiomeric resolution of the compounds studied was investigated. The use of 10 mM heptakis-(2,3,6-tri-O-methyl)-beta-CD in a 100 mM phosphate buffer (pH 3.5) with a temperature of 15 degrees C allowed the separation of the enantiomers of ketoconazole and terconazole with high resolution (R(s) > 2.0). The rapid separation of ketoconazole enantiomers with an analysis time less than 3 min was carried out after fitting some experimental parameters. The developed method was applied to the determination of ketoconazole in different pharmaceutical formulations.

Application of capillary zone electrophoresis to the analysis and to a stability study of nitrite and nitrate in saliva

The applicability of capillary zone electrophoresis for the determinations of nitrite and nitrate was studied. Using direct UV detection the limit of detection values of the analytes were 0.14 and 0.21 microg/mL, respectively. The developed method was found to be useful to directly determine nitrite, nitrate and thiocyanate in saliva. It was found that adjusting the pH of the sample to 11 and storing the saliva at 4 degrees C was adequate to make constant the nitrite/nitrate ratio in saliva samples at least 7 days.

Capillary electrophoresis of inorganic ions: An update

This review as a sequel of three earlier similar reports gives a summary of the progress and significant methodological developments, starting from 2002, in the use of capillary electrophoresis (CE) for inorganic ion analysis. As substantiated by the illustrative number of relevant references, improvements in sensitivity achieved both in and outside a CE system, advances in manipulating the separation selectivity, novel hardware configurations, and system performance innovations are continually being reported over the review period. Specifically viewed are the recent advancements in elemental (bio)speciation analysis, which remains one of the most fertile areas of CE research, as well as in three recently booming research topics: contactless conductivity detection, separations on microchips, and transient isotachophoretic preconcentration. A state-of-the-art picture of technique's potentialities within the field of interest presented here demonstrates that CE has become recognized and is growing in acceptance as a reliable alternative to traditional analytical methods such as high-performance liquid chromatography (HPLC).