Optimization of dynamic pH junction for the sensitive determination of amino acids in urine by capillary electrophoresis

Capillary Electrophoresis-Mass Spectrometry for the Direct Analysis of Metabolites in Highly Saline Samples Using In-Capillary Preconcentration

Capillary electrophoresis-mass spectrometry (CE-MS) is gaining interest for metabolomics studies because of its high separation efficiency, selectivity, and versatility. The ability to inject nanoliters from only a few microliters of sample in the injection vial makes this approach very suited for volume-limited applications. However, the low injection volumes could compromise the detection sensitivity of CE-MS, thereby potentially limiting its scope in metabolomics. To overcome this issue, online sample preconcentration methods have been developed to increase sample-loading volumes without hampering the intrinsic high separation efficiency of CE. In this protocol, online preconcentration with sample stacking based on pH junction was assessed for the direct profiling of endogenous metabolites in rat brain microdialysates. Sample stacking was realized by a pre-injection of ammonium hydroxide, followed by a large sample injection (i.e., about 17% of the total capillary volume). It is shown that this relatively simple and fast preconcentration procedure is fully compatible with the high-salt concentration in microdialysates and significantly improves the detection sensitivity of the CE-MS method.

Determination of sulfa antibiotic residues in river and particulate matter by field-amplified sample injection-capillary zone electrophoresis

In the present research, field-amplified sample injection-CZE (FASI-CZE) coupled with a diode array detector was established to determine trace level sulfa antibiotic. Sulfathiazole, sulfadiazine, sulfamethazine, sulfadimethoxine, sulfamethoxazole, and sulfisoxazole were selected as analytes for the experiments. The background electrolyte solution consisted of 70.0 mmol/L borax and 60.0 mmol/L boric acid (including 10% methanol, pH 9.1). The plug was 2.5 mmol/L borax, which was injected into the capillary at a pressure of 0.5 psi for 5 s. Then the sample was injected into the capillary at an injection voltage of -10 kV for 20 s. The electrophoretic separation was carried out under a voltage of +19 kV. The capillary temperature was maintained at 20˚C throughout the analysis, and six sulfonamides were completely separated within 35 min. Compared with pressure injection-CZE, the sensitivity of FASI-CZE was increased by 6.25-10.0 times, and the LODs were reduced from 0.2-0.5 to 0.02-0.05 μg/mL. The method was applied to the determination of sulfonamides in river water and particulate matter samples. The recoveries were 78.59-106.59%. The intraday and interday precisions were 2.89-7.35% and 2.77-7.09%, respectively. This provides a simpler and faster method for the analysis of sulfa antibiotic residues in environmental samples.

Establishing Tablet Dissolution Curve Through Determination of Underivatized Alendronate Sodium by Capillary Electrophoresis-UV Detector

Background:

Alendronate sodium is a common clinical osteoporosis drug for postmenopausal women; its determination is very important. However, there is no absorption of chromophores or fluorophores in the molecule, therefore, their direct determination is a challenge. Thus, establishing a common and direct method is very inspiring.

Methods:

According to the direct determination of alendronate sodium through the formation of a complex between alendronate sodium and divalent copper ion by capillary electrophoresis with ultraviolet detection, the dissolution profile of alendronate sodium tablet was established. The dissolution curves obtained from high-performance liquid chromatography method involving derivatization with 9- fluorenyl methylchloroformate and capillary electrophoresis with ultraviolet detector were found to be highly similar. Underivatized alendronate sodium can be determined by the capillary electrophoresis method.

Results:

Optimum conditions were as follows: background electrolyte including 25 mM CuSO4 at pH 4.59, 5 s injection time, 18 kV applied voltage, and 240 nm detected wavelength. Method validation indicated good linearity (r2>0.9993), precision of migration time with a relative standard deviation <1.5 % for intra-day and <3.6 % for inter-day, precision of peak areas <2.3 % for intra-day and <5.0 % for inter-day, limits of detection (0.01 μg/mL), limit of quantification (0.04 μg/mL) and recovery (90.6 %- 109.0 %).

Conclusion:

The proposed capillary electrophoresis method has been proved to be simpler, faster and more convenient to test dissolution profile of alendronate sodium tablet than that of high performance liquid chromatography.

Dynamic pH barrage junction focusing of amino acids, peptides, and digested monoclonal antibodies in capillary electrophoresis-mass spectrometry

Dynamic pH barrage junction focusing in CE enables effective signal enhancement, quantitative capture efficiencies, and straightforward optimization. The method is a technical variant of dynamic pH junction focusing. CE separation with dynamic pH barrage junction focusing is compatible with both optical and mass spectrometric detection. We developed a CE-MS/MS method using hydrophilic polyethyleneimine-coated capillaries and validated it for the qualitative analysis of amino acids, peptides, and tryptic peptides of digested monoclonal antibodies. The S/N of extracted ion electropherograms of zwitterionic analytes were enhanced by approximately two orders of magnitude with a tradeoff of a shortened separation window. Online focusing improved the MS signal intensity of a diluted antibody digest, enabling more precursor ions to be analyzed with subsequent tandem mass spectrometric identification. It also broadened the concentration range of protein digest samples for which adequate sequence coverage data can be obtained. With only 0.9 ng of digested infliximab sample loaded into the capillary, 76% and 100% sequence coverage was realized for antibody heavy and light chains, respectively, after online focusing. Full coverage was achieved with 9 ng of injected digest.

Integration of three-phase microelectroextraction sample preparation into capillary electrophoresis

A major strength of capillary electrophoresis (CE) is its ability to inject small sample volumes. However, there is a great mismatch between injection volume (typically <100 nL) and sample volumes (typically 20-1500 µL). Electromigration-based sample preparation methods are based on similar principles as CE. The combination of these methods with capillary electrophoresis could tackle obstacles in the analysis of dilute samples. This study demonstrates coupling of three-phase microelectroextraction (3PEE) to CE for sample preparation and preconcentration of large volume samples while requiring minimal adaptation of CE equipment. In this set-up, electroextraction takes place from an aqueous phase, through an organic filter phase, into an aqueous droplet that is hanging at the capillary inlet. The first visual proof-of-concept for this set-up showed successful extraction using the cationic dye crystal violet (CV). The potential of 3PEE for bioanalysis was demonstrated by successful extraction of the biogenic amines serotonin (5-HT), tyrosine (Tyr) and tryptophan (Trp). Under optimized conditions limits of detection (LOD) were 15 nM and 33 nM for 5-HT and Tyr respectively (with Trp as an internal standard). These LODs are comparable to other similar preconcentration methods that have been reported in conjunction with CE. Good linearity (R² > 0.9967) was observed for both model analytes. RSDs for peak areas in technical replicates, interday and intraday variability were all satisfactory, i.e., below 14%. 5-HT, Tyr and Trp spiked to human urine were successfully extracted and separated. These results underline the great potential of 3PEE as an integrated enrichment technique from biological samples and subsequent sensitive metabolomics analysis.

Determination of underivatized amino acids to evaluate quality of beer by capillary electrophoresis with online sweeping technique

Capillary electrophoresis (CE) with ultraviolet detection was applied to determine underivatized amino acids in beer, based on the coordination interaction of copper ions and amino acids. An online sweeping technique was combined with CE to improve detection sensitivity. Using the United Nations Food Agriculture Organization/World Health Organization model of essential amino acid pattern and flavor of amino acids, the quality and taste in three kinds of beer were evaluated. It was found that Beer2 had higher quality than the other two kinds and the content of phenylalanine, proline, serine, and isoleucine was relatively large in all three kinds of beers with a great influence on beer flavor. Optimal conditions for separation were as follows: 50mM CuSO₄ at pH 4.40 as buffer; total length of fused silica capillary, 73 cm; effective length, 65 cm; separation voltage, 22.5 kV; and optimized sweeping condition, 70 seconds. In the appropriate range, linearity (r² > 0.9989), precision with a relative standard deviation < 8.05% (n = 5), limits of detection (0.13–0.25 μg/mL), limit of quantification (0.43–0.83 μg/mL), and recovery (80.5–115.8%) were measured. This method was shown to be applicable to the separation of amino acids in beer and to perform quantitative analysis directly without derivatization for the first time.

Analysis of Three Compounds in Flos Farfarae by Capillary Electrophoresis with Large-Volume Sample Stacking

The aim of this study was to develop a method combining an online concentration and high-efficiency capillary electrophoresis separation to analyze and detect three compounds (rutin, hyperoside, and chlorogenic acid) in Flos Farfarae. In order to get good resolution and enrichment, several parameters such as the choice of running buffer, pH and concentration of the running buffer, organic modifier, temperature, and separation voltage were all investigated. The optimized conditions were obtained as follows: the buffer of 40 mM NaH₂P0₄-40 mM Borax-30% v/v methanol (pH 9.0); the sample hydrodynamic injection of up to 4 s at 0.5 psi; 20 kV applied voltage. The diode-array detector was used, and the detection wavelength was 364 nm. Based on peak area, higher levels of selective and sensitive improvements in analysis were observed and about 14-, 26-, and 5-fold enrichment of rutin, hyperoside, and chlorogenic acid were achieved, respectively. This method was successfully applied to determine the three compounds in Flos Farfarae. The linear curve of peak response versus concentration was from 20 to 400 µg/ml, 16.5 to 330 µg/mL, and 25 to 500 µg/mL, respectively. The regression coefficients were 0.9998, 0.9999, and 0.9991, respectively.

Enantioselective analysis of proteinogenic amino acids in cerebrospinal fluid by capillary electrophoresis-mass spectrometry

d-Amino acids (AAs) are increasingly being recognized as essential molecules in biological systems. Enantioselective analysis of proteinogenic AAs in biological samples was accomplished by CE-MS employing β-CD as chiral selector and ESI via sheath-liquid (SL) interfacing. Prior to analysis, AAs were fully derivatized with FMOC, improving AA-enantiomer separation and ESI efficiency. In order to optimize the separation and MS detection of FMOC-AAs, the effects of type and concentration of CD in the BGE, the composition of the SL, and MS-interfacing parameters were evaluated. Using a BGE of 10 mM β-CD in 50 mM ammonium bicarbonate (pH 8) containing 15% v/v isopropanol, a SL of isopropanol-water-1 M ammonium bicarbonate (50:50:1, v/v/v) at a flow rate of 3 μL/min, and a nebulizer gas pressure of 2 psi, 15 proteinogenic AAs could be detected with enantioresolutions up to 3.5 and detection limits down to 0.9 μM (equivalent to less than 3 pg AA injected). The selectivity of the method was demonstrated by the analysis of spiked cerebrospinal fluid, allowing specific detection of d-AAs. Repeatability and linearity obtained for cerebrospinal fluid were similar to standard solutions, with peak area and migration-time RSDs (n = 5) below 16.2 and 1.6%, respectively, and a linear response (R(2) ≥ 0.977) in the 3-90 μM range.

Bioanalytical Application of Amino Acid Detection by Capillary Electrophoresis

This chapter illustrates the usefulness of capillary electrophoresis (CE) for the analysis of amino acids, and both normal and chiral separations are covered. In order to provide a general description of the main results and challenges in the biomedical field, some relevant applications and reviews on CE of amino acids are tabulated. Furthermore, some detailed experimental procedures are shown, regarding the CE analysis of amino acids in body fluids, in microdialysate, and released upon hydrolysis of proteins. In particular, the protocols will deal with the following compounds: (1) underivatized aminoacids in blood; (2) γ-Aminobutyric acid, glutamate, and L-Aspartate derivatized with Naphthalene-2,3-dicarboxaldehyde; (3) hydrolysate from bovine serum albumine derivatized with phenylisothiocyanate. By examining these applications on real matrices, the capillary electrophoresis efficiency as tool for Amino Acid analysis can be ascertained.

High-sensitivity analysis of anionic sulfonamides by capillary electrophoresis using a synergistic stacking approach

A synergistic stacking approach whereby field-enhanced sample injection and micelle-to-solvent stacking in capillary zone electrophoresis are combined has been developed and has been applied to the separation and quantification of anionic sulfonamides. Electrokinetic injection of the sample in a low conductivity alkaline diluent was performed for 90s at -15kV. Micelle-to-solvent stacking was then undertaken by hydrodynamic injection of micellar cetyltrimethylammonium bromide solution prior to the electrokinetic injection of sample that also contained 50% methanol. This combined stacking approach, when compared to a typical hydrostatic injection, provided improvements in peak height and corrected peak area in the range of 397-1024 and 758-1246, respectively. Limits of quantification in the range of 0.01-0.03μg/mL were obtained for sulfamerazine, sulfamethazine and sulfamethizole and were sufficient for the determination of these analytes in river water. The percentage recovery and accuracy values obtained for a fortified river water sample that had been subjected to sample preparation by evaporation and reconstitution with diluent were 74-135%. Intra-day and inter-day repeatabilities for migration time, peak height, and corrected peak area were in the range 0.5-5.0% (percentage relative standard deviation, n=8) and these relatively low values were attributed to the use of a stable capillary coating established by the successive multiple ionic-polymer layer technique.

Contemporary sample stacking in analytical electrophoresis

Sample stacking is a term denoting a multifarious class of methods and their names that are used daily in CE for online concentration of diluted samples to enhance separation efficiency and sensitivity of analyses. The essence of these methods is that analytes present at low concentrations in a large injected sample zone are concentrated into a short and sharp zone (stack) in the separation capillary. Then the stacked analytes are separated and detected. Regardless of the diversity of the stacking electromigration methods, one can distinguish four main principles that form the bases of nearly all of them: (i) Kohlrausch adjustment of concentrations, (ii) pH step, (iii) micellar methods, and (iv) transient ITP. This contribution is a continuation of our previous reviews on the topic and brings an overview of papers published during 2010-2012 and relevant to the mentioned principles (except the last one which is covered by another review in this issue).

Determination of essential amino acids in human serum by a targeting method based on automated SPE-LC-MS/MS: discrimination between artherosclerotic patients

An automated method based on a hyphenated SPE-LC-MS/MS configuration has been optimized for the determination of essential amino acids (threonine, valine, methionine, leucine, isoleucine, lysine, tryptophan, and phenylalanine) in human serum, with the aim of discriminating between different states of coronary artery disease. Validation in terms of sensitivity (detection limits below 28.0 ng on column) and precision (repeatability expressed as relative standard deviation below 6.0%) supports the suitability of the method for application to a cohort of 122 atherosclerosis patients confirmed by a catheterization test. The cohort was composed by 80 individuals diagnosed with stable angina and 42 patients who suffered from acute myocardial infarction (AMI). Both groups of individuals are differentiated by the occurrence of ischemia in AMI patients due to the formation of thrombi. The chemometric treatment of the data obtained by multivariate analysis of variance (MANOVA) allowed comparison between both groups of diagnosed patients. Therefore, amino acids whose serum levels were affected by ischemia have been identified. The contribution of risk factors such as obesity and hypercholesterolemia as well as the individuals' gender to the concentration of essential amino acids has also been studied.