Determination of nitrite and nitrate in cerebrospinal fluid by microchip electrophoresis with microsolid phase extraction pre-treatment

Zwitterionic surfactant as an additive for efficient electrophoretic separation of easily absorbed rhodamine dyes on plastic microchips

Plastic microchips possess the advantages of easy fabrication and low-cost, but their surface properties are frequently incompatible with electrophoretic separation without proper surface modification. Meanwhile, the separation microchannels on typical microchips are usually only a few centimeters long, the pressurized flow may significantly affect the electrophoretic separation if their inner diameters (id) are relatively larger (approximately > 50 μm), viscous separation medium is therefore required for efficient separation. Herein, a zwitterionic surfactant, N-hexadecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (HDAPS), was used as a multifunctional additive to inhibit the analyte adsorption, improve the surface status, control Joule heating and modulate the resolution on cyclic olefin copolymer microchips with 80 μm id, 5 cm long separation microchannels, eliminating the necessity of viscous polymeric additives. The effectiveness of HDAPS was compared with an ionic polymeric additive, poly(diallydimethylammonium chloride). The streaming potential and electroosmotic flow measurements indicated an effective inhibition of the adsorption of rhodamine B and a stable negative surface charge with zwitterionic HDAPS. Using 15 mmol/L HDAPS, 40% (v/v) methanol, and 10 mmol/L boric acid (pH 3.2) as the running buffer, rapid separation of four rhodamines was achieved within 90 s under a separation electric field of 520 V/cm. The theoretical plate numbers were in a range of 5.0×10⁵-6.9×10⁵/m. The relative standard deviations were no more than 0.9% for retention time and 1.5% for peak area. The proposed system was verified by the determination of rhodamines in eyeshadow and wolfberry, with standard recoveries in a range of 98.2%-101.4%.

Nitrite Determination in Environmental Water Samples Using Microchip Electrophoresis Coupled with Amperometric Detection

Nitrite is considered an important target analyte for environmental monitoring. In water resources, nitrite is the result of the nitrogen cycle and the leaching processes of pesticides based on nitrogenous compounds. A high concentration of nitrite can be associated with intoxication processes and metabolic disorders in humans. The present study describes the development of a portable analytical methodology based on microchip electrophoresis coupled with amperometric detection for the determination of nitrite in environmental water samples. Electrophoretic and detection conditions were optimized, and the best separations were achieved within 60 s by employing a mixture of 30 mmol L^-1 lactic acid and 15 mmol L^-1 histidine (pH = 3.8) as a running buffer applying 0.7 V to the working electrode (versus Pt) for amperometric measurements. The developed methodology revealed a satisfactory linear behavior in the concentration range between 20 and 80 μmolL^-1 (R² = 0.999) with a limit of detection of 1.3 μmolL^-1. The nitrite concentration was determined in five water samples and the achieved values ranged from (28.7 ± 1.6) to (67.1 ± 0.5) µmol L^-1. The data showed that using the proposed methodology revealed satisfactory recovery values (83.5-103.8%) and is in good agreement with the reference technique. Due to its low sample consumption, portability potential, high analytical frequency, and instrumental simplicity, the developed methodology may be considered a promising strategy to monitor and quantitatively determine nitrite in environmental samples.

Picomolar detection limits for glyphosate by two-dimensional column-coupled isotachophoresis/capillary zone electrophoresis-mass spectrometry

Capillary electrophoresis-mass spectrometry often lacks sufficient limits of detection for trace substances in the environment due to its low loadability. To overcome this problem, we conducted a feasibility study for column-coupling isotachophoresis to capillary electrophoresis-mass spectrometry. The first dimension isotachophoresis preconcentrated the analytes. The column-coupling of both dimensions was achieved by a hybrid capillary microfluidic chip setup. Reliable analyte transfer by voltage switching was enabled by an in-chip capacitively coupled contactless conductivity detector placed around the channel of the common section between two T-shaped crossings in the chip connecting both dimensions. This eliminated the need to calculate the moment of analyte transfer. A commercial capillary electrophoresis-mass spectrometry instrument with easily installable adaptations operated the setup. Prior to coupling isotachophoresis with capillary zone electrophoresis-mass spectrometry, both dimensions were optimized individually by simulations and verified experimentally. Both dimensions were able to stack/separate all degradation products of glyphosate, the most important herbicide applied worldwide. The first dimension isotachophoresis also removed phosphate, which is a critical matrix component in many environmental samples. Enrichment and separation of glyphosate and its main degradation product aminomethylphosphonic acid by the 2D setup provided an excellent limit of detection of 150 pM (25 ng/L) for glyphosate. This article is protected by copyright. All rights reserved.

Development of Microchip Isotachophoresis Coupled with Ion Mobility Spectrometry and Evaluation of Its Potential for the Analysis of Food, Biological and Pharmaceutical Samples

An online coupling of microchip isotachophoresis (µITP) with ion mobility spectrometry (IMS) using thermal evaporation interface is reported for the first time. This combination integrates preconcentration power of the µITP followed by unambiguous identification of trace compounds in complex samples by IMS. Short-chain carboxylic acids, chosen as model analytes, were first separated by the µITP in a discontinuous electrolyte system at pH 5–6, and subsequently evaporated at 130 °C during their transfer to the IMS analyzer. Various parameters, affecting the transfer of the separated sample components through the evaporation system, were optimized to minimize dispersion and loss of the analytes as well as to improve sensitivity. The following analytical attributes were obtained for carboxylic acids in the standard solutions: 0.1–0.3 mg L⁻¹ detection limits, 0.4–0.9 mg L⁻¹ quantitation limits, linear calibration range from the quantitation limit to 75 mg L⁻¹, 0.2–0.3% RSD of the IMS response and 98–102% accuracy. The analytical potential of the developed µITP-IMS combination was demonstrated on the analysis of various food, pharmaceutical and biological samples, in which the studied acids are naturally present. These include: apple vinegar, wine, fish sauce, saliva and ear drops. In the real samples, 0.3–0.6% RSD of the IMS response and 93–109% accuracy were obtained.

Potential of microchip electrophoresis in pharmaceutical analysis: Development of a universal method for frequently prescribed nonsteroidal anti-inflammatory drugs

A novel microchip electrophoresis method with conductivity detection for the determination of nonsteroidal anti-inflammatory drugs (NSAIDs) in several pharmaceutical formulations was developed. The three frequently used NSAIDs - acetylsalicylic acid, diclofenac and ibuprofen were baseline separated on a poly(methyl methacrylate) microchip with coupled separation channels. Elimination of matrix components such as excipients, was realized through online combination of isotachophoresis (ITP) with zone electrophoresis (ZE). ITP-ZE hyphenation can also facilitate preconcentration of target analytes. ITP was carried out in the first separation channel at pH 6.5, while the second channel of the microchip was used for ZE separation and detection of the analytes at pH 7.0. The developed ITP-ZE method was demonstrated to be applicable for direct and reliable determination of NSAIDs in eleven pharmaceutical formulations. The noticeable advantage of this approach is that only simple sample pretreatment (filtration and dilution) is necessary. The method performance parameters, such as linearity (20-250% of nominal concentration of studied NSAIDs in the test samples), accuracy (98-102%) and precision (less than 2% RSD) were obtained. This universal approach is suitable for the determination of frequently used NSAIDs in a single analysis in less than 15 min. In addition to simple sample pretreatment, low running costs and minimal environmental impact could make this method attractive for the analysis of pharmaceutical preparations.

A Simple and Rapid Spectrophotometric Method for Nitrite Detection in Small Sample Volumes

A simple, rapid, and environmentally-friendly spectrophotometric method for nitrite detection was developed. Detection was based on a redox reaction with iodide ions in an acidic condition. The reaction was evaluated by detecting the increase in absorbance of the colored product of iodine at 362 nm wavelength. To obtain a good spectrophotometric performance, the iodide ions concentration, hydrochloric acid concentration, and reaction time were optimized. In the optimal condition, the developed spectrophotometric method provided a linear range of 0.0625 to 4.00 mg L−1 (r = 0.9985), reaction time for 10 min, a limit of detection of 25 µg L−1, and a limit of quantitation of 85 µg L−1. This method showed good repeatability (RSD < 9.21%), high sample throughput (9 samples min−1), and good accuracy (recovery = 88 ± 2 to 99.5 ± 0.4%). The method has the potential to be used in crime scene investigations as a rapid screening test for gunshot residue detection via nitrite detection.

Capillary electrophoresis of small ions and molecules in less conventional human body fluid samples: A review

In recent years, advances in sensitive analytical techniques have encouraged the analysis of various compounds in biological fluids. While blood serum, blood plasma and urine still remain the golden standards in clinical, toxicological and forensic science, analyses of other body fluids, such as breast milk, exhaled breath condensate, sweat, saliva, amniotic fluid, cerebrospinal fluid, or capillary blood in form of dried blood spots are becoming more popular. This review article focuses on capillary electrophoresis and microchip electrophoresis of small ions and molecules (e.g. inorganic cations/anions, basic/acidic drugs, small acids/bases, amino acids, peptides and other low molecular weight analytes) in various less conventional human body fluids and hopes to stimulate further interest in the field.

Microchip electrophoresis with amperometric detection method for profiling cellular nitrosative stress markers

The overproduction of nitric oxide (NO) in cells results in nitrosative stress due to the generation of highly reactive species such as peroxynitrite and N2O3. These species disrupt the cellular redox processes through the oxidation, nitration, and nitrosylation of important biomolecules. Microchip electrophoresis (ME) is a fast separation method that can be used to profile cellular nitrosative stress through the separation of NO and nitrite from other redox-active intracellular components such as cellular antioxidants. This paper describes a ME method with electrochemical detection (ME-EC) for the separation of intracellular nitrosative stress markers in macrophage cells. The separation of nitrite, azide (interference), iodide (internal standard), tyrosine, glutathione, and hydrogen peroxide (neutral marker) was achieved in under 40 s using a run buffer consisting of 7.5 to 10 mM NaCl, 10 mM boric acid, and 2 mM TTAC at pH 10.3 to 10.7. Initially, NO production was monitored by the detection of nitrite (NO2(-)) in cell lysates. There was a 2.5- to 4-fold increase in NO2(-) production in lipopolysaccharide (LPS)-stimulated cells. The concentration of NO2(-) inside a single unstimulated macrophage cell was estimated to be 1.41 mM using the method of standard additions. ME-EC was then used for the direct detection of NO and glutathione in stimulated and native macrophage cell lysates. NO was identified in these studies based on its migration time and rapid degradation kinetics. The intracellular levels of glutathione in native and stimulated macrophages were also compared, and no significant difference was observed between the two conditions.

Microchip capillary electrophoresis of nitrite and nitrate in cerebrospinal fluid

Microchip capillary electrophoresis (MCE) is a relatively new analytical method requiring only small sample amounts, which is very favorable for the analysis of volume-limited biofluids. The practical use of MCE in bioanalysis is still restricted in terms of requirements for simplifying and/or concentrating sample pretreatment techniques. Here, we describe an MCE method for trace analysis of nitrite and nitrate, indicators of various neurological diseases, in cerebrospinal fluid (CSF). The complex CSF samples were simplified by solid-phase microextraction prior to an online combination of isotachophoresis with capillary zone electrophoresis performed on a microchip with coupled channels and a high-volume sample injection channel (9.9 μL). The method is suitable for rapid (total analysis time lasted 20 min), reproducible (0.6-2.4 % RSD for migration time), and sensitive (3-9 nM limits of detection) determinations of nitrite and nitrate in 15-50 times diluted CSF samples.

Determination of metabolic organic acids in cerebrospinal fluid by microchip electrophoresis

A new MCE method for the determination of oxalic, citric, glycolic, lactic, and 2- and 3-hydroxybutyric acids, indicators of some metabolic and neurological diseases, in cerebrospinal fluid (CSF) was developed. MCE separations were performed on a PMMA microchip with coupled channels at lower pH (5.5) to prevent proteins interference. A double charged counter-ion, BIS-TRIS propane, was very effective in resolving the studied organic acids. The limits of detection (S/N = 3) ranging from 0.1 to 1.6 μM were obtained with the aid of contact conductivity detector implemented directly on the microchip. RSDs for migration time and peak area of organic acids in artificial and CSF samples were <0.8 and <9.7%, respectively. Recoveries of organic acids in untreated CSF samples on the microchip varied from 91 to 104%. Elimination of chloride interference, a major anionic constituent of CSF, has been reached by two approaches: (i) the use of coupled channels microchip in a column switching mode when approximately 97-99% of chloride was removed electrophoretically in the first separation channel and (ii) the implementation of micro-SPE with silver-form resin prior to the MCE analysis, which selectively removed chloride from undeproteinized CSF samples.