Contactless Conductivity Detection Research Articles

Automated Capillary Electrophoresis System Compatible with Multiple Detectors for Potential In Situ Spaceflight Missions.

The in situ search for chemical signatures of life on extraterrestrial worlds requires automated hardware capable of performing detailed compositional analysis during robotic missions of exploration. The use of electrophoretic separations in this search is particularly powerful, enabling analysis of a wide range of soluble organic compounds potentially indicative of life, as well as inorganic compounds that can serve as indicators of habitability. However, to detect this broad range of compounds with a single electrophoresis instrument, a combination of different detection modes is required. For detection of any ionizable species, including organic compounds that do not participate in terrestrial biology (i.e., "unknown unknowns"), mass spectrometry (MS) is essential. Inorganic ions, or any dissolved charged species present, can be analyzed using capacitively coupled contactless conductivity detection (C4D). Additionally, for the trace analysis of compounds of key interest to astrobiology (particularly, amino acids), laser-induced fluorescence (LIF) detection holds unique promise, due to the fact that it has the highest demonstrated sensitivity of any form of detection. Here, we demonstrate a fully automated, portable capillary electrophoresis analyzer that is capable of all these modes of detection. The prototype system developed here addresses the three most significant challenges for doing electrophoretic separations: precise sample injection, HV isolation, and automation of all operational steps. These key challenges were successfully addressed with the use of custom-designed rotor-stator valves with optimized operational sequences incorporating gas purging steps, rinses, and HV application.

Investigation of the Effects of Electrode Geometry on the Performance of C4D Sensor with Radial Configuration.

Electrodes are basic components of C4D (capacitively coupled contactless conductivity detection) sensors, and different electrode structures (the configuration pattern or the electrode geometry) can lead to different measurement results. In this work, the effects of electrode geometry of radial configuration on the measurement performance of C4D sensors are investigated. Two geometrical parameters, the electrode length and the electrode angle, are considered. A FEM (finite element method) model based on the C4D method is developed. With the FEM model, corresponding simulation results of conductivity measurement with different electrode geometry are obtained. Meanwhile, practical experiments of conductivity measurement are also conducted. According to the simulation results and experimental results, the optimal electrode geometry of the C4D sensor with radial configuration is discussed and proposed. The recommended electrode length is 5–10 times of the pipe inner diameter and the recommended electrode angle is 120–160°.

Development of low-cost planar electrodes and microfluidic channels for applications in capacitively coupled contactless conductivity detection (C4 D).

Electrochemical techniques are commonly applied to micro total analysis system (μTAS) devices mainly due to its high sensitivity and miniaturization capacity. Among many electrochemical techniques, capacitively coupled contactless conductivity detection (C4 D) stands out for not requiring direct electrode-solution contact, avoiding several problems such as electrolysis, bubble formation, and metal degradation. Furthermore, the instrumentation required for C4 D measurements is compact, low cost, and easy to use, allowing in situ measurements to be performed even by nonspecialized personal. Contrarily, the production of metallic electrodes and microchannels adequate for C4 D measurements commonly requires specialized facilities and workers, increasing the costs of applying these methods. We propose alternatives to batch manufacture metallic electrodes and polymeric microchannels for C4 D analysis using more straightforward equipment and lower-cost materials. Three devices with different dielectric layer compositions and electrode sizes were tested and compared regarding their analytical performance. The constructed platforms have shown a reduction of more than 64% in cost when compared to traditional techniques and displayed good linearity (R2 ≥ 0.994), reproducibility (RSD ≤ 4.07%, n = 3), and limits of detection (≤0.26mmol/L) when measuring standard NaCl samples. Therefore, the proposed methods were successfully validated and are available for further C4 D applications such as diagnosis of dry-eye syndrome.

Ultra‐rapid capillary zone electrophoresis method for simultaneous determination of arginine and ibuprofen

The combination of arginine and ibuprofen is widely used for pain relief with a faster onset of action than conventional ibuprofen. Therefore, the determination of both compounds in a single run is highly desirable for rapid quality control applications. This paper reports an ultra-fast method (100 injections/h) for simultaneous determination of arginine and ibuprofen using capillary electrophoresis with capacitively coupled contactless conductivity detection. The separation of arginine as cation and ibuprofen as anion was achieved using a background electrolyte composed by an equimolar mixture of 10mmol/L of 2-(cyclohexylamino) ethanesulfonic acid and boric acid with pH adjusted to 8.4 using potassium hydroxide. The limits of detections were 5.3 and 10.0 μmol/L for arginine and ibuprofen, respectively. The proposed method is simple, fast (one analysis every 35 s), environmentally friendly (minimal waste generation) and accurate (recovery values between 95 and 98%).

Tween 20-capped gold nanoparticles for selective extraction of free low-molecular-weight thiols in saliva followed by capillary electrophoresis with contactless conductivity detection

Low-molecular-weight thiols are widely present in human fluids, and are regarded as a kind of potential broad-spectrum evaluation indicators for some clinical diseases. In this work, gold nanoparticles capped with Tween 20 were used for purification and microextraction of the main free thiols (cysteine, homocysteine, glutathione and methionine) in saliva based on Au-S bond formation. Ultrasound further sped up the releasing of the target analytes, and the releasing time needed was only 10 min, and the required sample volume was only 40 µL. The desorption solution could be directly injected for electrophoretic analysis without derivatization, and field-amplified sample stacking of electrophoretic online enrichment technology further improved the detection sensitivity. The synergistic enrichment effect made the enrichment factors of four analytes reach 1119–2067 times. This developed method was applied for the analyses of saliva samples of healthy volunteers. Acceptable sensitivity (LODs: 0.15–1.5 ng mL−1) and recoveries (97.6–116%) were obtained in the saliva sample matrix. This proposed method provides an alternative for the sensitive detection of low-molecular-weight thiols in noninvasive body fluids, which has potential application prospect in the preliminary noninvasive diagnosis of diabetes, cardiovascular diseases, etc.

Advancements in capacitance-to-digital converter-based C4 D technology for detection in capillary electrophoresis using amplified excitation voltages and comparison to classical and open-source C4 Ds.

This work introduces new hardware configurations for a capacitively coupled contactless conductivity detector (C4 D) based on capacitance-to-digital conversion (CDC) technology for CE. The aim was to improve sensitivity, handling, price, and portability of CDC-based C4 D detectors (CDCD) to reach LODs similar to classic C4 Ds with more sophisticated electric circuits. To achieve this, a systematic study on the CDCDs was carried out including a direct comparison to already established C4 D setups. Instrumental setups differing in electrode lengths, measurement modes, and amplification of excitation voltages were investigated to achieve LODs for alkali metal ions of 4 to 12μM, similar to LODs obtained by classic C4 D setups. Lowest LODs were achieved for a setup with two 10mm electrodes at a distance of 0.2mm and an excitation voltage of 24V. The detection head was exceptionally lightweight with only 2.6g and covered only 20mm of the capillary on total. This allowed the use of multiple detectors along the separation path to enable spatial tracking of analytes during separation. The entirely battery-powered detector assembly weighs less than 200g, and the data are transmitted wirelessly for possible portable applications. The freely accessible hardware and software were optimized for fully automated measurements with real time data plotting and allowed handling multidetector setups. The new developments were applied to quantify the potassium salt of glyphosate in its herbicide formulation.

Chip-based separation of organic and inorganic anions and multivariate analysis of wines according to grape varieties

This report describes the use of electrophoresis microchips integrated with contactless conductivity detection for the determination of organic acids and inorganic anions in wine samples and the subsequent classification based on the grape varieties. The best separation was achieved using a buffer composed of 30 mmol L−1 2-(N-morpholino)ethanesulfonic acid, 15 mmol L−1l-histidine and 0.05 mmol L−1 cetyltrimethylammonium bromide (pH 5.8), allowing the determination of chloride, nitrate, sulfate, oxalate, tartrate, maleate, succinate, citrate, acetate, lactate, pyroglutamate and phosphate within ca. 100 s. The relative standard deviations obtained for the migration times were lower than 2%, while the obtained values for peak areas ranged from 2.5 to 8.4%. The limits of detection achieved for all compounds ranged between 3.0 and 12.6 μmol L−1. A total of 18 wines from Brazil and Chile were successfully investigated, including red, white and rosé, and the anionic species were quantified with recovery values between 92 and 117%. A statistical difference has not been observed between the data obtained by using electrophoresis microchips integrated with contactless conductivity detection (ME-C4D) and capillary electrophoresis with ultra-violet detection (CE-UV) and thus the results from newly developed method is validated. Finally, similarities among the anionic profile of wines were investigated by using a multivariate approach, and it was possible to discriminate samples mainly by grapes varieties. Furthermore, the proposed methodology has provided instrumental simplicity and good analytical performance, demonstrating to be useful for routine quality control of wines.

Contactless conductivity sensor as detector for microfluidic paper-based analytical device with application to unique rapid method for quantifying sulfite preservative

Abstract The work presents the latest exploitation of capacitively coupled contactless conductivity detector (C4D) produced from printed-circuit board as durable detector for in-situ headspace microfluidic paper-based analytical devices (μPADs). This is the first reagent-free operating paper-based method for direct determination of sulfite, with storage of acid reagent during μPAD fabrication. In the procedure, the detection zone is first moistened with deionized water, and the μPAD turned over and placed on the C4D sensing area with the moist-zone faced down. Sample is loaded at the top layer of μPAD where it flows into the adjacent covered reaction zone, containing the absorbed acid, forming SO2(g). The gas diffuses through the in-situ headspace in the middle layer and is absorbed into the moist detection zone at the base layer giving a prompt change in the C4D signal due to the change in conductivity from the generated conductive ionic species, viz. H+(aq) and HSO3−(aq). The working range of the C4D-μPAD is 8–100 mg L-1 sulfite, with the limit of detection of 6.61 mg L-1. Compared with other paper-based methods that detect SO2(g), this is the only in-situ method and most rapid method (3.3 min per sample) for fruit juices, white and red wines.

A Rapid and Simple Dual-Channeled Capillary Electrophoresis with Contactless Conductivity Detection Method for the Determination of Adenosine, Cordycepin, and Inosine in Ophiocordyceps sinensis-Based Products

This study aims to propose a tool for food control activities that can be realizable even with a modest infrastructure, using purpose-made capillary electrophoresis (CE) instrumentation with capacitively coupled contactless conductivity detection (C4D). For demonstration of a featured application, we reported herein a simple and inexpensive method for authentication and quality control of Ophiocordyceps sinensis-based products (literally known as “summer grass, winter worm”). This species, originally found on the Tibetan plateau and in the Himalayas, between elevations of 3000–5000 m, is classified as a medicinal mushroom and has been used as a precious component in traditional Chinese and Vietnamese medicine. Due to its high value and rareness, counterfeit products have emerged, leading to the need for quality control activities. For this purpose, a CE-C4D methodology was developed to determine their adenosine, cordycepin, and inosine contents. These compounds were analyzed on two channels of the purpose-made instrument, with the best-achieved detection limits of 11.2–22.0 μg/g using the developed CE-C4D methods. Cross-checking data using the reference method (HPLC-DAD) was provided to prove the reliability of the analytical results obtained with CE-C4D. Good agreement for the two pairs of data was achieved, with the result deviation being less than 12%.

Electrophoretic Determination of Symmetric and Asymmetric Dimethylarginine in Human Blood Plasma with Whole Capillary Sample Injection.

Asymmetric and symmetric dimethylarginines are toxic non-coded amino acids. They are formed by post-translational modifications and play multifunctional roles in some human diseases. Their determination in human blood plasma is performed using capillary electrophoresis with contactless conductivity detection. The separations are performed in a capillary covered with covalently bonded PAMAPTAC polymer, which generates anionic electroosmotic flow and the separation takes place in the counter-current regime. The background electrolyte is a 750 mM aqueous solution of acetic acid with pH 2.45. The plasma samples for analysis are treated by the addition of acetonitrile and injected into the capillary in a large volume, reaching 94.5% of the total volume of the capillary, and subsequently subjected to electrophoretic stacking. The attained LODs are 16 nm for ADMA and 22 nM for SDMA. The electrophoretic resolution of both isomers has a value of 5.3. The developed method is sufficiently sensitive for the determination of plasmatic levels of ADMA and SDMA. The determination does not require derivatization and the individual steps in the electrophoretic stacking are fully automated. The determined plasmatic levels for healthy individuals vary in the range 0.36–0.62 µM for ADMA and 0.32–0.70 µM for SDMA.

Development of a Mobile Analytical Chemistry Workstation Using a Silicon Electrochromatography Microchip and Capacitively Coupled Contactless Conductivity Detector.

Capillary electrochromatography (CEC) is a separation technique that hybridizes liquid chromatography (LC) and capillary electrophoresis (CE). The selectivity offered by LC stationary phase results in rapid separations, high efficiency, high selectivity, minimal analyte and buffer consumption. Chip-based CE and CEC separation techniques are also gaining interest, as the microchip can provide precise on-chip control over the experiment. Capacitively coupled contactless conductivity detection (C4D) offers the contactless electrode configuration, and thus is not in contact with the solutions under investigation. This prevents contamination, so it can be easy to use as well as maintain. This study investigated a chip-based CE/CEC with C4D technique, including silicon-based microfluidic device fabrication processes with packaging, design and optimization. It also examined the compatibility of the silicon-based CEC microchip interfaced with C4D. In this paper, the authors demonstrated a nanofabrication technique for a novel microchip electrochromatography (MEC) device, whose capability is to be used as a mobile analytical equipment. This research investigated using samples of potassium ions, sodium ions and aspirin (acetylsalicylic acid).

High-resolution Arduino-based data acquisition devices for microscale separation systems

In this work, we have designed, constructed, and evaluated simple, inexpensive open-source data acquisition systems based on various analog-to-digital converter modules (ADS 1115, MCP 3424, LTC 2400, with resolution from 16 to 24-bit) and a miniature Arduino Nano ™ microcontroller. The constructed data acquisition systems provide excellent performance and are comparable to a commercial, 24-bit device. We provide full schematics and corresponding source codes so that analytical chemists can easily construct any of the developed systems without extensive electronic or programming knowledge. The 24-bit LTC 2400 based device provided the best and comparable performance to a commercial, high-end 24-bit sigma to delta converter (ORCA 2800) at a fraction of cost (less than 50 USD compared to 870 USD for the commercial counterpart). The excellent performance was verified using a capillary electrophoresis system with contactless conductivity detection and separation of inorganic ions in clinical skin wipe and tap water samples.

Back Cover: Separation and determination of alkyl sulfate surfactants in wastewater by capillary electrophoresis coupled with contactless conductivity detection after preconcentration by simultaneous adsorption using alumina beads

DOI: 10.1002/elps.202000086 The cover picture shows the determination of four alkyl sulfate (AS) surfactants (C14, C12, C10, and C8) by capillary electrophoresis (CE) after preconcentration by the simultaneous adsorption of four AS surfactants on Al2O3 surface. The EOF electroosmotic flow (EOF) at high pH pushes the AS surfactants move to the contactless conductivity detector (C4D). The electropherograms show the separation of the four AS surfactants in the order of C14, C12, C10, and C8.

3-(4-Carboxybenzoyl)quinoline-2-carboxaldehyde labeling for direct analysis of amino acids in plasma is not suitable for simultaneous quantification of tryptophan, tyrosine, valine, and isoleucine by CE/fluorescence.

Capillary electrophoresis coupled to LED-induced fluorescence detection is a robust and sensitive technique used for amino acids (AA) analysis in biological media, after labeling with 3-(4-carboxybenzoyl)quinoline-2-carboxaldehyde (CBQCA). We wanted to quantitate in plasma tryptophan (Trp), tyrosine (Tyr), valine (Val), and isoleucine (Ile). Among the different labeled AA-CBQCA, Trp has the lowest fluorescence yield, which makes its detection and quantification very difficult in biological samples such as plasma. We tried to improve Trp analysis by CE/LED-induced fluorescence detection to its maximal sensitivity by using large volume sample stacking as a preconcentration step in our analytical protocol. At pH 9.5, this step caused a drop in resolution during the separation of the four AAs and it was therefore necessary to work at pH 10. We have found that Tyr, Val, Ile, and Trp are detected and well separated from the other AAs, but Trp cannot be quantified in plasma samples, mainly because of the low fluorescence yield of the Trp-CBQCA derivative. The recorded LOD is 0.18μM for Trp-CBQCA in standard solution with a resolution between Trp and Tyr of 1.2, while the LOD is 6μM in plasma with the same resolution. Trp, Tyr, Val, and Ile are, however, efficiently quantified when using a 3 M acetic acid electrolyte and CE associated with capacitively coupled contactless conductivity detection, which also has the advantage of not requiring derivatization or large volume sample stacking. This article demonstrates, for the CE user, that quantitative analysis of these four AA in mouse plasma can be performed by CE-fluorescence after CBQCA labeling, with the exception of Trp. It can be advantageously replaced by CE/capacitively coupled contactless conductivity detection, the only efficient one for Trp, Tyr, Val, and Ile quantification. In this case, the LOD for Trp is 2μM. The four AAs are separated with resolution with neighbors above 1.5.

Advances of capillary electrophoresis enantioseparations in pharmaceutical analysis (2017-2020).

Capillary electrophoresis is a powerful technique for the analysis of polar chiral compounds and has been widely accepted for analytical enantioseparations of drug compounds in pharmaceuticals and biological media. In addition, many mechanistic studies have been conducted in an attempt to rationalize enantioseparations in combination with spectroscopic and computational techniques. The present review will focus on recent examples of mechanistic aspects and summarize recent applications of stereoselective pharmaceutical and biomedical analysis published between January 2017 and November 2020. Various separation modes including electrokinetic chromatography in combination with several detection modes including laser-induced fluorescence, mass spectrometry and contactless conductivity detection will be discussed. A general trend also observed in other analytical techniques is the application of quality by design principles in method development and optimization.

Capillary electrophoresis study on evolution of phase of mixed micelles

Capillary electrophoresis (CE) coupled with the contactless conductivity detector (C4D) was applied to characterize the transformation (TX-100, SDS) → TX-100/SDS. Three states of the TX-100/SDS micelle were established due to transformation, i.e. the cluster state, highly mobile homogeneous micelles obtained from the phase (TX-100 (2.5–3.5%)/SDS (0.04–0.1 M)); the state of polydispersity, i.e. polydisperse micelles with the lower mobility obtained from the phase (TX-100 (7%)/SDS (0–0.1 M); and the state of metastability at high content of SDS surfactant in TX-100/SDS phase. It was established that these states cover two different regions in the phase evolution according to descriptors used to characterize the phase of mixed micelles. The detected cluster state was wider characterized, and for this, the electrophoretic data were converted into micelle characteristics. By higher viscosity of the background electrolyte (BGE), new situations in the evolution of the TX-100/SDS phase were visualized, especially the transformation of a state of the metastability of a mixed micelle. Based on the obtained results, the two segment/segment systems, based on mixed micelles, were proposed. It was found that only one of the systems was able to exchange micelles between two micellar segments during the electrophoretic run and the issue was explained taking into account the obtained in the work results.

Single-chip based contactless conductivity detection system for multi-channel separations.

In this work, the design and characterization of a multi-cell capacitively coupled contactless conductivity detection system are described. The operation and simultaneous acquisition from 3 detector cells are demonstrated, however, the system is capable of supplying 8 detection cells and can be easily upgraded to maintain 64 capacitively coupled contactless conductivity detection cells. On performing flow-injection analysis, the system recorded as low as 0.01 mM of acetic acid, phosphoric acid, NaH2PO4, and Na2B4O7 solutions in water. The instrument was also capable of recording and distinguishing different mixtures of organic solvents: (a) methanol-acetonitrile, (b) hexane-acetone. The designed detection system is expected to be used coupled with multi-channel separation devices for monitoring simultaneous processes.

Separation and determination of acesulfame-K in soy sauce by capillary electrophoresis with contactless conductivity detection using liquid-liquid extraction and field-amplified sample injection

A new method was established for the sensitive determination of artificial synthetic sweetener acesulfame-K in soy sauce by capillary electrophoresis (CE) with field-amplified sample injection (FASI)and capacitively coupled contactless conductivity detection (C4D). Liquid-liquid extraction (LLE) technology was employed to eliminate the complex matrix interference co-existing in soy sauce. Ethyl acetate was used as the extraction solution. The pH of the sample solution was adjusted to 1.7, and both inorganic salts and organic compounds causing interferences were effectively removed by LLE. The type and volume of the extraction solvents, pH of the sample solutions, extraction method and extraction time were investigated in detail. Under the optimized experimental conditions, acesulfame-K in soy sauce was well separated and sensitively detected. The limit of detection and limit of quantification were 0.15 mg/kg and 0.48 mg/kg, respectively. The accuracy was tested by spiking acesulfame-K into soy sauce samples, and the recoveries ranged from 92.3% to 108.1%. The relative standard deviations were below 8.0%. The proposed method can meet the requirements for the fast screening and sensitive detection of acesulfame-K in soy sauce samples.

An electrophoretic ion analyzer for on-site autonomous water monitoring

An on-site ion analyzer based on capillary electrophoresis with pressure-driven flow through injection and capacitively coupled contactless conductivity detection has been developed for field monitoring of cations and anions in environmental waters. Automated time-pressure based hydrodynamic injection provides stable pL-nL scale injection (RSD = 1.96%, n = 30). A mixture of 400 mM Bis-Tris, 400 mM MOPS and 2 mM 18-crown-6 is used as the background electrolyte to provide repeatable separations. A proprietary hydrophilic coated 25 μm id capillary is used to suppress the electroosmotic flow. Separations of anions (Cl-, NO3-, NO2-, SO42-, F- and PO43-) and cations (NH4+, K+, Na+, Ca2+ and Mg2+) are achieved by switching the polarity of the high voltage power supply in two individual runs. Signal fluctuations caused by the temperature or viscosity changes in on-site monitoring are corrected by on-line introduction of internal standards. RSDs of the migration time and the corrected peak height over ~35 h and 350 analysis cycles are <4.06%. The LODs of inorganic ions are in the range of 2.1 μM (K+) to 6.8 μM (PO43-). The feasibility for on-site water monitoring with this system has been validated by a standard Ion chromatography method with comparable results obtained.

3-(Methacryloxy)propyl]trimethoxysilane-based monolithic stationary phases for capillary-scale chromatography and their characterization by contactless conductivity detection and solid-state NMR spectroscopy

This work describes the preparation, characterization and application of monolithic stationary phases for capillary-scale chromatography. The columns were prepared by polymerization of [3-(methacryloxy)propyl]trimethoxysilane (MPTMS) monomer in two consecutive steps: i) sol–gel process and ii) photoinitiated radical addition. The photopolymerization was carried out in 100 μm i.d. UV-transparent coating fused silica capillaries. The columns were applied in the qualitative separation of five polycyclic aromatic hydrocarbons (PAH) by both capillary liquid chromatography (nano-LC) and capillary electrochromatography (CEC) in a capillary electrophoresis equipment. In an attempt to improve the separation of the PAHs, the stationary phase was modified by incorporating benzyl methacrylate (BM) as a comonomer in the MPTMS-based polymer structure, but no improvement was observed. The physical homogeneity of the columns was evaluated by using an optical microscope and a capacitively coupled contactless conductivity detector (C4D) as a non-invasive scanner moving longitudinally along the capillary. The chemical characterization of the polymer was done by both 13C and 29Si solid-state nuclear magnetic resonance spectroscopy (SSNMR). It was possible to propose a carbon chain structure and the substitution degree of the silyl group indicating the cross-linking of the polymer chains through the formation of dimers and trimers by condensation of the alkoxysilane groups.