Contactless Conductivity Detection Research Articles

Determination of glucosamine in dietary supplement samples by capillary electrophoresis using capacitively coupled contactless conductivity detector (CE-C4D)

The quantification of glucosamine in dietary supplements was investigated by using capillary with capacitively coupled contactless conductivity detection (CE-C4D). Optimal conditions included electrophoresis buffer solution 10 mM Tris/Ace; pH = 5,0; sample injection height 25 cm; sample injection time 30 s; separation voltage 20 kV. The limit of detection (LOD) and the limit of quantification (LOQ) of glucosamine were 8.00 ppm and 26.67 ppm, respectively. Calibration curve was constructed over the range from 20 to 1000 ppm giving a good linear correlation coefficient (R2 = 0.9993). The relative standard deviation (RSD) ranged from 1.28% to 2.33% (n = 3) and the recovery efficiency ranged from 95.2 to 102.3% which indicated good precision and trueness of our method. The method was applied to determine glucosamine in 4 dietary supplement samples randomly collected at the local markets. The CE-C4D results were compared with those obtained by reference HPLC-FLD method, showing good agreement between the two methods (relative error < 15%).

Simultaneous determination of acetyl vardenafil, homosildenafil, hydroxy acetildenafil and mirodenafil by CE-C4D

Dietary supplements are currently popular because of their natural origin, especially male enhancement products. However, some of these products have been found to contain erectile dysfunction drugs that inhibit the enzyme phosphodiesterase-5 (PDE-5), leading to adverse effects on human health. In this study, capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4D) was used to determine four PDE-5 inhibitors (including acetyl vardenafil, homosildenafil, hydroxy acetildenafil and mirodenafil) in dietary supplement samples. The optimum analytical conditions included electrolyte solution of 20 mM arginine (Arg)/acetic acid (Ace)/acetonitrile (ACN) 20% pH 6.5, silica capillary 50 μm inner diameter with effective length of 50 cm, separation voltage of 20kV, siphoning hydrodynamic injection at a height of 25 cm for 30 s. At the optimum analytical conditions, the limits of detection of acetyl vardenafil, homosildenafil, hydroxy acetildenafil, and mirodenafil were 2.1, 1.6, 2.3, and 1.3 mg/L, respectively. The analysis of four PDE-5 inhibitors by CE-C4D also showed good repeatability (< 5 %) and recovery (> 97 %). This method was further applied to simultaneously determine the amount of acetyl vardenafil, homosildenafil, hydroxy acetildenafil, and mirodenafil in three functional products. All four PDE-5 inhibitors were not detected in these samples. To demonstrate the utility of this method, the mixture of four PDE-5 inhibitors standard solutions was added to three samples. The recovery was between 98.8 % and 100.6%, illustrating the potential applicability of the method of determining PDE-5 inhibitors in dietary supplement samples.

A voltage trade study for the design of capillary electrophoresis instruments for spaceflight.

Capillary electrophoresis (CE) systems have undergone extensive development for spaceflight applications. A flight-compatible high voltage power supply and the necessary voltage isolation for other energized components can be large contributors to both the volume and mass of a CE system, especially if typical high voltage levels of 25-30kV are used. Here, we took advantage of our custom CE hardware to perform a trade study for simultaneous optimization of capillary length, high voltage level, and separation time, without sacrificing method performance. A capillary electrophoresis with capacitively coupled contactless conductivity detection (CE-C4 D) method recently developed by our group to target inorganic cations and amino acids relevant to astrobiology was used as a test case. The results indicate that a 50cm long capillary with 15kV applied voltage (half of that used in the original method) can be used to achieve measurement goals while minimizing instrument size.

Concise analysis of γ-hydroxybutyric acid in beverages and urine by capillary electrophoresis with capacitively coupled contactless conductivity detection using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid as background electrolyte

γ-Hydroxybutyric acid (GHB), a neurotransmitter or neuromodulator in the human central nervous system, is often abused in drug-facilitated sexual assaults due to its euphoric and sedative effects. While the analysis of GHB has received continuous attention, its inherent characteristics pose challenges. In the current study, capillary electrophoresis (CE) with capacitively coupled contactless conductivity detection (C4D) was built, and Good's buffers were evaluated as the background electrolytes for CE separation and C4D detection. On this basis, a simple and efficient CE-C4D method was developed for GHB analysis. Through theoretical discussion and experimental optimization, the separation of GHB and related positional isomers α-hydroxybutyric acid (AHB) and β-hydroxybutyric acid (BHB) was achieved within 4 min using 150 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as the running buffer. Under the optimized condition, the relative standard deviations of migration time and peak area were less than 1.1% and 4.5%, indicating good precision. The C4D signal of GHB showed a good linear relationship with GHB concentration in the range of 3-300 µM with a determination coefficient of 0.9997, and the detection limit was calculated to be 0.37 µM based on the signal-to-noise ratio of three. Furthermore, liquid-liquid extraction (LLE) and solid-phase extraction (SPE) were comparatively studied for sample matrix purification. Combined with the optimized SPE procedure, the developed CE-C4D method has been successfully applied for the determination of exogenous GHB in spiked beverages and endogenous GHB in human urine.

Sensitive monitoring of 3-hydroxybutyrate as an indicator of human fasting by capillary electrophoresis in a PAMAMPS coated capillary

Sensitive electrophoretic determination of 3-hydroxybutyrate (3HB) as an indicator of human ketogenesis is performed in fused silica capillary covalently coated by an anionic copolymer of poly(acrylamide-co-sodium-2-acrylamido-2-methylpropanesulphonate) (PAMAMPS). Baseline separation of 3HB from other components of human serum is achieved in a 20 μm capillary with an effective length of 17 cm covered by 4% PAMAMPS, which generates a cathodic EOF with a mobility of 8.30 ± 0.00 · 10−9 m2/V.s in 80 mM MES/His as background electrolyte. 3HB migrates in counter-current electrophoretic mode against EOF, that effectively improving electrophoretic resolution. Sample pre-treatment is based on adding of 45 μL acetonitrile to 15 μL serum and, after shaking, a 28 mm long zone of supernatant is injected into the capillary, and sharpened after turning on a separation voltage of 20 kV using the technique of large volume sample stacking, where the EOF forces the residual acetonitrile from the capillary. When combined with universal contactless conductivity detection, the achieved LOD and LOQ are 0.43 μM and 1.44 μM, respectively, that are sufficiently low for monitoring the physiological 3HB level. The performed clinical study subsequently showed that serum 3HB increases from a concentration of 71 μM, corresponding to normal food, to level of 1924 μM after 60 h of fasting and returns to the normal physiological concentration 48 h after commencing consumption of high-saccharide food.

Quantum Dots and Double Surfactant-Co-modified Electromembrane Extraction of Polar Aliphatic Bioamines in Water Samples Followed by Capillary Electrophoresis with Contactless Conductivity Detection

Quantum Dots and Double Surfactant-Co-modified Electromembrane Extraction of Polar Aliphatic Bioamines in Water Samples Followed by Capillary Electrophoresis with Contactless Conductivity Detection

Using multi-material fused deposition modeling (FDM) for one-step 3D printing of microfluidic capillary electrophoresis with integrated electrodes for capacitively coupled contactless conductivity detection

For the first time, the fused deposition modeling (FDM) 3D printing method was used to fabricate a microchip capillary electrophoresis (MCE) with channel dimensions (58 × 65 µm) below 100 µm. Moreover, electrodes to conduct capacitively coupled contactless conductivity detection (C4D) were integrated into the MCE using three conductive thermoplastic filaments containing graphene, carbon black, and copper. After optimizing the parameters for multi-material printing, the MCE-C4D device was fabricated in a single (one step) 3D printing process using an FDM 3D printer equipped with two extrusion nozzles. An optimization design method allowed finding that the distance (gap) between the electrodes of around 1 mm provided the maximum response of the C4D. The optimized frequency of the excitation signal applied in the C4D electrodes depended on the composition of the conductive filament used to print the electrodes. The optimized frequencies for the electrodes printed with thermoplastic polymers containing copper and carbon black were 268 and 384 kHz, respectively. The MCE-C4D successfully separated and detected a mixture of K+, Na+, and Li+ with good resolution, efficiency, precision, linearity, and limits of detections (LOD) of 10.2, 11.6, and 14.1 µmol L-1, respectively.

Monitoring of amoxicilline and ceftazidime in the microdialysate of diabetic foot and serum by capillary electrophoresis with contactless conductivity detection.

Determination of the broad-spectrum antibiotics amoxicilline (AMX) and ceftazidime (CTZ) in blood serum and microdialysates of the subcutaneous tissue of the lower limbs is performed using CE withcontactless conductivity detection (C4 D). Baseline separation of AMX is achieved in 0.5 M acetic acid as the background electrolyte and separation of CTZ in3.2 M acetic acid with addition of 13% v/v methanol. The CE-C4 D determination is performed in a 25µm capillary with suppression of the EOF using INST-coating on an effective length of 18cm and the attained migration time is 4.2min for AMX and 4.4min for CTZ. The analysis was performed using 20µl of serum and 15µl of microdialysate, treated by the addition of acetonitrile in a ratio of 1/3 v/v and the sample is injected into the capillary using the large volume sample stacking technique. The LOQ attained in the microdialysate is 148ng/ml for AMX and 339ng/ml for CTZ, and in serum 143ng/ml for AMX and 318ng/ml for CTZ. The CE-C4 D method is employed for monitoring the passage of AMX and CTZ from the blood circulatory system into the subcutaneous tissue at the sites of diabetic ulceration in patients suffering from diabetic foot syndrome and also for measuring the pharmacokinetics following intravenous application of bolus antibiotic doses.

A Study on Double Inputs Direct Contact and Single Output Capacitively Coupled Conductivity Detector.

In this paper, an improved double inputs direct contact and single output capacitively coupled conductivity detector (DISODCD) based on traditional contactless capacitively coupled conductivity detector (C4D) is developed. The sensor uses double inputs of the contact electrode and capacitively coupled output of the contactless electrode and a lock-in amplifier to reduce interfering noise signals and amplify gain. Parallel circuit counteracts the part of the adverse capacitance reactance introduced by electrode polarization and reduces the effect of the impedance caused by the coupled wall capacitance to measure the resistance of solution. The sensor reduces limit of detection (LOD) of analyte and improves the sensitivity of the device. The LOD of the potassium chloride solution is 1 nM, and the detection range is 0.01 μM to 10 mM in actual testing for a single sample. The ratio of the response of potassium chloride solution to background ultrapure water at low concentrations is better than that of double input capacitively coupled contactless conductivity detector (DIC4D) and direct contact conductivity detection (DCD) under the same condition. In the case that the test cell is contaminated with impurities, pollution of impurities has little effect on the response of DISODCD. In practical application, it has a good service life.

Multi-channel contactless conductivity detection device for online detection of free-flow electrophoresis separation

现有自由流电泳(FFE)装置因不具备在线检测功能,其实用性仍然存在明显不足。针对这一问题,该工作发展了一种多通道电容耦合式非接触电导检测(MC-C4D)装置并开发了自动测量软件。MC-C4D装置采用了并行分时的非接触电导检测技术,即由多个同样的非接触电导检测模块并行排列,而单个电导检测模块又由多个非接触电导检测池组成,采用模拟开关切换这些检测池,能够分时检测流经相应检测池溶液的电导率。多个电导检测模块的检测池总数等于FFE的组分数,它们分别串行接入到FFE各流路中,这样MC-C4D装置就可在线并行分时在线测量各组分溶液的电导率。为验证所设计MC-C4D装置的检测性能,采用配制的氯化钾标准溶液作为检测对象对MC-C4D装置进行了标定和测试。实验数据表明,MC-C4D装置电导率检测范围为0.015~2.5 mS/cm,检出限(LOD)为0.002 mS/cm,日内相对标准偏差(RSD, n=3)为2.31%,测量相对误差(RE)为3.03%和通道间测量相对偏差为1.60%,这些参数表明该装置检测范围较大,LOD低,重复性好,准确性高,通道间测量相对偏差小。另外,将MC-C 4D装置应用于往复式自由流等电聚焦电泳(RFFIEF)在蛋白质聚焦过程中对各组分溶液电导率进行实时在线检测,结果表明,所开发的MC-C4D装置不仅可实现对FFE各组分溶液电导率的实时在线检测,而且还可在RFFIEF实验中辅助掌握分离的实验进度,提高FFE装置的实用性。

Determination of Glyphosate, Glufosinate, and Their Major Metabolites in Tea Infusions by Dual-Channel Capillary Electrophoresis following Solid-Phase Extraction.

In this study, two analytical procedures were developed and validated using dual-channel capillary electrophoresis-coupled contactless conductivity detection (CE-C4D) followed by solid-phase extraction (SPE) for simultaneous determination of glyphosate (GLYP), glufosinate (GLUF), and their two major metabolites, aminomethylphosphonic acid (AMPA) and 3-(methylphosphinico) propionic acid (MPPA), respectively, in a popular beverage such as tea infusions. GLYP, GLUF, and AMPA were analyzed in the first channel using background electrolyte (BGE) of 1 mM histidine (His) adjusted to pH 2.75 by acetic acid (Ace). In contrast, MPPA was quantified in the second channel with a BGE of 30 mM His adjusted to pH 6.7 by 3-(N-morpholino) propanesulfonic acid (MOPS) and 10 µM of cetyltrimethylammonium bromide (CTAB). In addition, the samples of tea infusions were treated using SPE with 10 mL of 0.5 mM HCl in methanol as eluent. At the optimized conditions, the method detection limit (MDL) of GLYP, GLUF, AMPA, and MPPA is 0.80, 1.56, 0.56, and 0.54 μg/l, respectively. The methods were then applied to analyze four target compounds in 16 samples of tea infusions. GLYP was found in two infusion samples of oolong tea with concentrations ranging from 5.34 to 10.74 µg/L, and GLUF was recognized in three samples of green tea infusion in the range of 45.1–53.9 µg/L.

A Novel Planar Grounded Capacitively Coupled Contactless Conductivity Detector for Microchip Electrophoresis.

In the microchip electrophoresis with capacitively coupled contactless conductivity detection, the stray capacitance of the detector causes high background noise, which seriously affects the sensitivity and stability of the detection system. To reduce the effect, a novel design of planar grounded capacitively coupled contactless conductivity detector (PG-C4D) based on printed circuit board (PCB) is proposed. The entire circuit plane except the sensing electrodes is covered by the ground electrode, greatly reducing the stray capacitance. The efficacy of the design has been verified by the electrical field simulation and the electrophoresis detection experiments of inorganic ions. The baseline intensity of the PG-C4D was less than 1/6 of that of the traditional C4D. The PG-C4D with the new design also demonstrated a good repeatability of migration time, peak area, and peak height (n = 5, relative standard deviation, RSD ≤ 0.3%, 3%, and 4%, respectively), and good linear coefficients within the range of 0.05–0.75 mM (R2 ≥ 0.986). The detection sensitivity of K+, Na+, and Li+ reached 0.05, 0.1, and 0.1 mM respectively. Those results prove that the new design is an effective and economical approach which can improve sensitivity and repeatability of a PCB based PG-C4D, which indicate a great application potential in agricultural and environmental monitoring.

Automated online monitoring of lactate and pyruvate in tamoxifen resistant MCF-7 cells using sequential-injection capillary electrophoresis with contactless conductivity detection (SI-CE-C4D) and correlation with MCT1 and MCT4 genes expression

Breast cancer is among the most common cancer types worldwide. The first and second line treatment protocols for various stages of breast cancer in females rely on tamoxifen. Until now, the development of tamoxifen resistance is not entirely understood. In this study, an automated sequential injection capillary electrophoresis with capacitively coupled contactless conductivity detector was developed for online levels monitoring of both lactate and pyruvate from the supernatant media of MCF-7 cells developing tamoxifen resistance. Changes in concentration of the two metabolites were simultaneously monitored from three model cell cultures and two control untreated cells. The electrophoretic separation was performed under reversed electroosmotic flow conditions. The system delivers high sample throughput at low sample consumption of 20 µL per analysis. The method is robust and achieved high sensitivity and resolution. Limits of detection were 8.0 and 17.0 nM and linear ranges were 0.15–5 and 0.01–1 mM with a correlation coefficient of 0.9951 and 0.9963 for lactate and pyruvate, respectively. Inter-run and intra-run accuracy were in the range of 95.37–107.02% with precision (RSD, %) of ≤9.84%. The method was completely validated and the data obtained were in agreement with results achieved using the lactate and pyruvate assay kits. The highly informative generated data revealed a significant increase in lactate and pyruvate production in the three tamoxifen resistant MCF-7 cells models compared to the two control cells starting from 5.8 hours and 6.8 hours culturing times, respectively. The increase in concentrations of both lactate and pyruvate were correlated with an increase in MCT1 and MCT4 genes expression.

Development of On-Site Rapid Detection Device for Soil Macronutrients Based on Capillary Electrophoresis and Capacitively Coupled Contactless Conductivity Detection (C4D) Method

The acquisition of nutrient data on a precise scale has played a vital role in nutrient management processes for soils. However, the lack of rapid precise and multi-index detection techniques for soil macronutrient contents hinders both rational fertilization and cost reduction. In this paper, a rapid detection method and device were devised, combining capillary electrophoresis (CE) and capacitively coupled contactless conductivity detection (C4D), and presented to detect macronutrient contents of soil. The device consisted of a capillary channel, C4D detector, high-voltage system, etc. It separated macronutrient ions using capillary electrophoresis and then measured the ion concentration based on the C4D principle. Lime concretion black soil samples from a complete field were collected and detected. NO3−, NH4+, H2PO4− and K+ in sample solutions could be detected in 5 min with relative standard deviations (RSDs) from 1.0 to 7.51%. The injection voltage was set to 10 kV for 5 s, and the separation voltage was set to 14 kV. This demonstrated the excellent performance of the C4D device on the detection of soil macronutrients, which could help to guide fertilization operations more effectively.

Determination of Nanoplastics Using a Novel Contactless Conductivity Detector with Controllable Geometric Parameters.

Plastic waste in the environment is continuously degraded to form nanoplastic particles, and its harm has attracted widespread attention. At present, the identification and quantification of nanoplastics are performed by visual observation and using some spectroscopy methods, which are time-consuming and lack accuracy. Therefore, this study proposes a contactless conductivity detector (C4D) based on a glass microfluidic chip with controllable geometric parameters to quantify nanoplastics. We found that when the insulating layer thickness was 15 μm, the electrode spacing was 1 mm, and the shielding method was on-chip shielding, the detector displayed the best performance. The detector possesses a simple structure with high sensitivity and outstanding reproducibility, that is, the limit of detection of KCl solutions can reach the micromolar level, and the intraday RSD is 0.2% (n = 5). This work uses a microfluidic chip C4D to study nanoplastics for the first time, and the limit of detection is 0.25 μg/mL and the quantitative limit is 0.8 μg/mL. In addition, plant experiments have verified that terrestrial plants can absorb nanoplastics in water, expanding the application of contactless conductivity detectors and providing a new method for the quantitative analysis of nanoplastics.

Capillary electrophoresis applied for the determination of acidity constants and limiting electrophoretic mobilities of ionizable herbicides including glyphosate and its metabolites and for their simultaneous separation.

Thermodynamic acidity constants and limiting ionic mobilities were determined for polyprotic non-chromophore analytes using capillary electrophoresis with capacitively coupled contactless conductivity detection. It was not necessary to work with buffers of identical ionic strength as ionic strength effects on effective electrophoretic mobilities were corrected by modeling during data evaluation (software AnglerFish). The mobility data from capillary electrophoresis coupled to conductivity detection were determined in the pH range from 1.25 to 12.02 with a high resolution (36pH steps). With this strategy, thermodynamic acidity constants and limiting ionic mobilities for various acidic herbicides were determined, sometimes for the first time. The model analytes included glyphosate, its metabolites, and its acetylated derivates (aminomethyl phosphonic acid, glyoxylic acid, sarcosine, glycine, N-acetyl glyphosate, N-acetyl aminomethyl phosphonic acid, hydroxymethyl phosphonic acid). The obtained data were used in simulations to optimize separations by capillary electrophoresis. Simulations correlated very well to experimental results. With the new method, the separation of glyphosate from interfering components like phosphate in beer samples was possible.

Fast determination of paracetamol and its hydrolytic degradation product p-aminophenol by capillary and microchip electrophoresis with contactless conductivity detection.

Paracetamol (PAC) is one of the most extensively used analgesics and antipyretic drugs to treat mild and moderate pain. P-aminophenol (PAP), the main hydrolytic degradation product of PAC, can be found in environmental water. Recently, CE has been developed for the detection of a wide variety of chemical substances. The purpose of this study is to develop a simple and fast method for the detection and separation of PAC and its main hydrolysis product PAP using CE and microchip electrophoresis with capacitively coupled contactless conductivity detection. The determination of these compounds using microchip electrophoresis with capacitively coupled contactless conductivity detection is being reported for the first time. The separation was run for all analytes using a BGE (20mM β-alanine, pH 11) containing 14% (v/v) methanol. The RSDs obtained for migration time were less than 4%, and RSDs obtained for peak area were less than 7%. The detection limits (S/N = 3) that were achieved ranged from 0.3 to 0.6mg/L without sample preconcentration. The presented method showed rapid analysis time (less than 1min), high efficiency and precision, low cost, and a significant decrease in the consumption of reagents. The microchip system has proved to be an excellent analytical technique for fast and reliable environmental applications.

Simultaneous separation of 12 different classes of antibiotics under the condition of complete protonation by capillary electrophoresis-coupled contactless conductivity detection.

A novel capillary electrophoresis - capacitively coupled contactless conductivity detection (CE-C4D) method for the separation of 12 antibiotics, including four types of aminoglycosides, three types of fluoroquinolones, two types of tetracyclines, and three types of macrolides, was developed. Half of these antibiotics were not determined by ultraviolet (UV) because of their lack of UV-absorbing groups. Formic acid (FA) (pH 2.50) with low conductivity was employed as the background electrolyte (BGE) in comparison with three BGE systems (i.e., HAc, HCl and H3PO4), which not only allowed complete protonation and electrophoresis separation but provided more cost-effectiveness and shorter analysis time. Under these conditions, a UV detector was employed as an additional detection mode to evaluate the qualitative analysis of 6 antibiotics possessing UV absorbing groups. Moreover, it was found that the sensitivities of the C4D and UV detectors were similar. Albeit a slightly reduced sensitivity of C4D in the analysis of norfloxacin, enrofloxacin and tylosin compared to UV, enough points were achieved to detect all analytes by C4D. The repeatability with respect to peak areas and migration times was better than 4.69% and 2.48% (n = 5), respectively. Mixed liquid pharmaceutical formulations of tobramycin eye drops having non-UV absorbing groups and ofloxacin eye drops possessing UV absorbing groups have been separated and detected in a single run by this technique. The studied recoveries of the two were 100% and 103%, respectively.

An improved dual-channel capacitively coupled contactless conductivity detector with high detection performance.

Conductivity detectors are widely used electrochemical sensors. It has long been a goal of researchers to improve detection performance. In this contribution, we propose a multi-input capacitively coupled contactless conductivity detector (MIC4D) with high sensitivity, and we carry out a detailed theoretical investigation of the detector. In order to overcome the problem of a rising baseline level as a result of sensitivity improvements when using the multi-input detection method, we innovatively combine MIC4D with differential detection to propose a further-improved detector (DFMIC4D). The detector is composed of two channels, one for the reference and the other for the analyte. The signal output from differential amplification can effectively reduce the high baseline level and detection interference. In KCl solution with a concentration range of 10-4 to 10-5 M, the response to the solution is a linear function of the logarithm of the concentration, and this detector has a high slope. The slope of DFMIC4D is 1.393, higher than a traditional single-input capacitively coupled contactless conductivity detector (C4D: 0.905) and a double-input capacitively coupled contactless conductivity detector (DIC4D: 1.314). For 10-3 M KCl solution, the response-to-baseline ratio is 1.776 for C4D, 1.779 for DIC4D, and 12.06 for DFMIC4D, with a ratio increase of nearly 6-fold shown by our new detector. At a S/N (signal-to-noise) ratio of 3, the limit of detection (LOD) of DFMIC4D is low, reaching 0.7 nM. In addition, DFMIC4D can be applied to the detection of low-conductivity solutions and total dissolved solids (TDS) analysis. Compared with a standard conductivity meter, our detector shows better detection performance.

A facile online multi-gear capacitively coupled contactless conductivity detector for an automatic and wide range monitoring of high salt in HPLC.

Sensing the electrolyte solution or aqueous-organic mixture has attracted much interest in chemical separation, pharmaceutical engineering, bioprocess, and biochemical experiments. However, reports on online contactless sensor with automatic and wide range sensing of high content electrolyte have been rarely presented. Herein, a facile model and theory of online multi-gear capacitively coupled contactless conductivity detection (M-C4D) sensor was proposed using one excitation electrode and multiple detection electrodes. Further, the relevant digital computation based on the M-C4D theory was developed for parameter optimization: the electrode gap of 5-150 mm, inner radius of 0.25-0.75 mm, electrode length of 10-60 mm, and frequency of 40-250 kHz using MATLAB. To demonstrate the model, theory, and digital computation, liquid chromatography (LC) was chosen as the model of bioprocess, and the sensor was designed and used as an online sensing device for the automatic monitoring of high salt elution in LC. The experiments showed that (i) the detection results were in agreement with the digital data, validating the digital computation, theory, and model of M-C4D and (ii) the monitoring data of M-C4D were in agreement with those via the traditional meter, further validating the model and theory. Finally, the developed sensor was applied to the automated detection of high salt gradient in LC. In contrast to the currently used meters and C4D, the developed M-C4D sensor had the following merits: (i) facile and automatic online detection avoiding cumbersome manual switching of detector heads, (ii) fair linear range of 0.015-20 mS cm-1 (equivalently 0.1-159 mM KCl) that does not fit the range of traditional C4D, and (iii) fair accuracy of less than 1.50% relative error. All these results indicate that the developed model, theory, and sensor have potential for the process monitoring of high content electrolytes transfer in biochemical engineering and clinic pre-warning.