Electrokinetic Injection Research Articles

Soil Improvement by Electrokinetic Sodium Silicate Injection into a Sand Formation Containing Fine Grains

Soil Improvement by Electrokinetic Sodium Silicate Injection into a Sand Formation Containing Fine Grains

Development and Verification of Electrokinetic Injection Logical Control Mode in Microfluidic Device

Development and Verification of Electrokinetic Injection Logical Control Mode in Microfluidic Device

A novel cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography method for the simultaneous determination of second-generation triazoles in human plasma

In this study, we introduce the first capillary electrophoresis method that combines cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography (CSEI-sweeping-MEKC) for the simultaneous and sensitive quantification of voriconazole (VRC), posaconazole (PSC), and isavuconazole (ISV) levels in human plasma. The sensitivity was remarkably improved, with a 5374-fold enhancement compared to the traditional MEKC method. We systematically optimized key parameters affecting sensitivity, resolution, and efficiency. The separation buffer included 100 mM NaH2PO4, 13 % acetonitrile, and 13 % tetrahydrofuran. This was followed by a high-conductivity buffer zone (1 psi, 50 s) of 100 mM NaH2PO4, an extended electrokinetic sample injection (10 kV, 600 s), and analyte sweeping with 100 mM sodium dodecyl sulfate micelles at −20 kV. Under these optimized conditions, the method allowed for an analytical run in 10 min and was validated for linearity (r ≥ 0.9997), precision (%RSD ≤ 8.8 %), and accuracy (%RE ≤ 2.20 %), delivering satisfactory results. The limit of detection was 2.55 ng/mL for ISV, 0.89 ng/mL for PSC, and 1.04 ng/mL for VRC. This method was successfully applied to analyze plasma samples from patients undergoing triazole antifungal treatment, demonstrating its feasibility for clinical testing.

3D printed cartridge for high-speed capillary electrophoresis with sheath liquid thermostatting and contactless conductivity detection

The high-speed capillary electrophoresis (HSCE) method is a technique that utilizes a high electric field strength applied through a short capillary to reduce the time required for sample separation. However, the increased electric field strength may result in pronounced Joule heating effects. To address this, we describe a 3D-printed cartridge with integrated contactless conductivity detection (C4D) head and a sheath liquid channel. The C4D electrodes and Faraday shield layers are fabricated by casting Wood's metal in chambers inside the cartridge. Effective thermostatting of the short capillary is achieved by flowing Fluorinert liquid, which provides better heat dissipation compared to airflow. A HSCE device is created by using the cartridge and a modified slotted-vial array sample-introduction approach. Analytes are introduced through electrokinetic injection. With the help of sheath liquid thermostatting, background electrolyte concentration can be increased to several hundred mM, resulting in improved sample stacking and peak resolutions. Additionally, the baseline signal is flattened. Typical cations such as NH4+, K+, Na+, Mg2+, Li+, and Ca2+ can be separated within 22 s with an applied field strength of 1200 V/cm. The limit of detection ranges from 2.5 to 4.6 μM with a relative standard deviation of migration times of 1.1–1.2% (n = 17). The method has been applied to detect cations in drinking water and black tea leaching for drink safety testing, and to identify explosive anions in paper swabs. Samples can be directly injected without the need for dilution.

A hydrodynamic injection approach for capillary electrophoresis using rotor-stator valves.

Sample injection is a critical step in a capillary electrophoresis (CE) analysis. Electrokinetic injection is the simplest approach and is often selected for implementation in portable CE instruments. However, in order to minimize the effect of sample matrix upon the results of a CE analysis, hydrodynamic injection is preferred. Although portable CE instruments with hydrodynamic injection have been reported, injection has always been performed at the grounded end of the capillary. This simplifies fluidic handling but limits coupling with electrochemical detectors and electrospray ionization-mass spectrometry (ESI-MS). We demonstrated previously that injection at the high-voltage (HV) end of the capillary could be performed using an HV-compatible rotary injection valve (fixed-volume injection). However, the mismatch between the bore sizes of the channels on the rotor-stator valve and the separation capillary caused peak tailing and undesired mixing, impairing analytical performance. In this work, we present an HV-compatible hydrodynamic injection approach that overcomes the issues associated with the fixed-volume injection approach reported previously. The performance of the CE instrument was demonstrated by analyzing a mixture of 13 amino acids by CE coupled to laser-induced fluorescence, which showed relative standard deviations for peak area and migration time below 5% and 1%, respectively, for triplicate analysis. Additionally, replicate measurements of a mixture of amino acids, peptides, nucleobases, and nucleosides by CE coupled to electrospray ionization-mass spectrometry (CE-ESI-MS) were performed to evaluate peak tailing, and results were similar to those obtained with a commercial CE-ESI-MS setup.

Phosphate Determination in Aqueous Samples by Complex Formation with Rhodamine B

Abstract In this study novel approaches for determining trace levels of phosphate in aqueous pore water solutions are described. Phosphate in aqueous samples (freshwater, drinking water, soil pore water) forms an aggregate compound with molybdate and Rhodamine B in 1M HCl. Incident light directed at the Rhodamine B molybdo-phosphate aggregate follows a second order light scattering behaviour. A bench top fluorescence spectrophotometer was used for examining the light scattering properties of the aggregate as well as for examining the feasibility of using this species for quantitative analysis. In this study this approach was adapted for quantitative analysis using capillary electrophoresis equipped with laser induced fluorescence. Quantifying phosphate with CE-LIF could be achieved by detecting the Rhodamine B phospho-molybdate aggregate directly in aqueous samples. Capillary electrophoresis was also used to indirectly detect phosphate by detecting Rhodamine B that dissociated from the aggregate species prior to electrokinetic injection.

Surface sampling capillary electrophoresis-mass spectrometry for a direct chemical characterization of tissue and blood samples.

Capillary electrophoresis (CE) is a powerful separation tool for non-targeted analysis of chemically complex samples, such as blood, urine, and tissue. However, traditionally CE requires samples in solution for analysis, which limits information on analyte distribution and heterogeneity in tissue. The recent development of surface sampling CE-mass spectrometry (SS-CE-MS) brings these advantages of CE to solid samples and enables chemical mapping directly from the tissue surface without laborious sample preparation. Here, we describe developments of SS-CE-MS to increase reproducibility and stability for metabolite, lipid, and protein extraction from tissue sections and dried blood spots. Additionally, we report the first electrokinetic sequential sample injection for high throughput analysis. We foresee that the wide molecular coverage from a distinct tissue region in combination with higher throughput will provide novel information on biological function and dysfunction.

Aptamer Selection Based on Microscale Electrophoretic Filtration Using a Hydrogel-Plugged Capillary Device.

This study reports a novel aptamer selection method based on microscale electrophoretic filtration. Aptamers are versatile materials that recognize specific targets and are attractive for their applications in biosensors, diagnosis, and therapy. However, their practical applications remain scarce due to issues with conventional selection methods, such as complicated operations, low-efficiency separation, and expensive apparatus. To overcome these drawbacks, a selection method based on microscale electrophoretic filtration using a capillary partially filled with hydrogel was developed. The electrophoretic filtration of model target proteins (immunoglobulin E (IgE)) using hydrogel, the electrokinetic injection of DNAs to interact with the trapped proteins, the elimination of DNAs with weak interactions, and the selective acquisition of aptamer candidates with strong interactions were successfully demonstrated, revealing the validity of the proposed concept. Two aptamer candidates for IgE were obtained after three selection cycles, and their affinity for the target was confirmed to be less than 1 nM based on their dissociation constant (KD) values. Therefore, the proposed method allows for the selection of aptamers with simple operations, highly effective separation based on electrophoresis and filtration, and a relatively cheap apparatus with disposable devices.

Nanoemulsion supported microemulsion electrokinetic chromatography coupled with selected preconcentration techniques as an approach for analysis of highly hydrophobic compounds

In this paper, an oil-in-water (O/W) nanoemulsion (NE) prepared by water cold dilution of an O/W microemulsion (ME) was introduced as a sample matrix in microemulsion electrokinetic capillary chromatography (MEEKC) for the highly hydrophobic compounds analysis. Several model compounds with log PO/W values in the 4.1–10.9 range, from different chemical groups, including retinol, α-tocopherol, cholecalciferol, phylloquinone, menaquinone-7, dichlorodiphenyltrichloroethane, ivermectin have been tested. As a proof of the concept of NE formation, a dynamic light scattering technique was employed to determine the size distribution profile of NE particles. Moreover, due to relatively low conductivity of the NE matrix (50–100 times lower in comparison to the separation buffer) and a negative electric charge provided to hydrophobic compounds through NE dispersed phase, NE matrices have been combined with preconcentration techniques based on electrokinetic dosing, namely field amplified sample injection (FASI) and pressure assisted electrokinetic injection (PAEKI). The detection limits for vitamin K1 and K2-MK7 in the NE matrix in combination with FASI (NE–MEEKC–FASI) as well as PAEKI (NE–MEEKC–PAEKI) were up to 42.9 and 12.1 ng mL−1, respectively. In comparison to standard hydrodynamic injection for microemulsion sample matrix NE–MEEKC–PAEKI grant 45-fold improvement in signal sensitivity. The study presents an innovative approach, as it enables the use of preconcentration techniques for highly hydrophobic compounds (log PO/W > 4), which was not previously possible for implementation in the electromigration techniques. Likewise, the use of organic solvents has been reduced by using ME as a solvent for stock solutions and diluting with water prior to the analysis. The application to real samples was investigated using a dietary supplement containing vitamin K2-MK7 obtained from the fermentation product of soybeans.

Two-step micelle-to-solvent stacking of arsenic species from foods in permanently coated tubing for capillary electrophoresis

An effective and sensitive two-step stacking of arsenic species by electrokinetic injection and micelle-to-solvent stacking (MSS) using zwitterionic surfactant 3-(N,N-dimethylpalmitylammonio) propane sulfonate was successfully developed in a co-electro-osmotic flow (co-EOF) capillary zone electrophoresis (CZE). The fused silica capillaries were coated with 1% hexadimethrine bromide solution to satisfy the condition of the same EOF direction between the test anions and capillary. The background solution was 100 mM borax buffer (pH 9.2). The proposed method was performed by a CZE system equipped with a diode array detector, and the detection wavelength was monitored at 192 nm. The separation voltage was −20 kV. The group of micelles and target analytes was stacked by field-enhanced sample injection at −10 kV for 180 s. The second stacking step used 60% MeOH to serve as an organic solvent where the analytes were brought by the micelles to the MSS boundary. The enrichment factors of As(Ⅴ), As(Ⅲ), disodium methyl arsonate hexahydrate (MMA), and dimethylarsinic acid (DMA) were 1230, 840, 3820, and 1450, respectively, compared to typical injections in CZE. The limits of detection (S/N = 3) ranged from 0.382 to 0.911 ng mL−1. The intracapillary repeatabilities (%RSD, n = 3) were 0.5–1.0% for migration time and 0.3–0.7% for peak areas. The technology of two-step stacking by MSS in co-EOF CZE together with a simple extraction procedure for As(Ⅴ), As(Ⅲ), MMA and DMA were demonstrated in kelp and rice samples.

PolyJet-Based 3D Printing against Micromolds to Produce Channel Structures for Microchip Electrophoresis.

In this work, we demonstrate the ability to use micromolds along with a stacked three-dimensional (3D) printing process on a commercially available PolyJet printer to fabricate microchip electrophoresis devices that have a T-intersection, with channel cross sections as small as 48 × 12 μm2 being possible. The fabrication process involves embedding removable materials or molds during the printing process, with various molds being possible (wires, brass molds, PDMS molds, or sacrificial materials). When the molds are delaminated/removed, recessed features complementary to the molds are left in the 3D prints. A thermal lab press is used to bond the microchannel layer that also contains printed reservoirs against another solid 3D-printed part to completely seal the microchannels. The devices exhibited cathodic electroosmotic flow (EOF), and mixtures of fluorescein isothiocyanate isomer I (FITC)-labeled amino acids were successfully separated on these 3D-printed devices using both gated and pinched electrokinetic injections. While this application is focused on microchip electrophoresis, the ability to 3D-print against molds that can subsequently be removed is a general methodology to decrease the channel size for other applications as well as to possibly integrate 3D printing with other production processes.

Stereoselective enrichment and determination of citalopram enantiomers by cation-selective exhaustive injection and sweeping coupled with cyclodextrin modified electrokinetic chromatography

An highly sensitive, rapid and enantioselective method was developed for the enantioseparation and determination of citalopram enantiomers by cation selective exhaustive injection-sweeping-cyclodextrin modified electrokinetic chromatography (CSEI-sweeping-CDEKC). The optimized conditions were: 50 mM pH 3.0 phosphate solution with 25 mg·mL−1 S-β-CD used as background buffer, 50 mbar 300 s hydrodynamical injection of 150 mM pH 3.0 NaH2PO4 buffer followed with 5 s water plug, 10 kV 600 s electrokinetic sample injection, -20 kV CDEKC run. Under the optimized conditions, the resolution was Rs=8.04, the enrichment factor as up to 2163 folds, the LOD values were: 3.6 ng·mL−1 for R-citalopram, 4.1 ng·mL−1 for S-citalopram, and 3 ng·mL−1 for both enantiomers in plasma samples. This new method showed good precision, repeatability and stability, which had been successfully applied to the impurity inspection of escitalopram oxalate and the stereoselective pharmacokinetic study of citalopram enantiomers.

Field-amplified sample injection and graphene quantum dot dual preconcentration in the analysis of melamine and dicyandiamide by capillary electrophoresis

通过热解法制备了硫掺杂的石墨烯量子点(S-GQDs),同石墨烯量子点(GQDs)相比,S原子的引入有效改善了GQDs的表面状态和化学特性、增强其对正电荷的捕获能力,使其更易与阳离子相互作用。以S-GQDs为载体,结合电堆积富集技术,发展了一种基于场放大进样(FASI)和S-GQDs放大的双重富集毛细管电泳(CE)分离检测三聚氰胺和双氰胺的方法。三聚氰胺和双氰胺在酸性介质中带正电荷,电动进样时快速迁移到毛细管入口端进行FASI预富集;同时带负电荷的S-GQDs向阳极端迁移,在样品与缓冲溶液的界面处通过静电作用吸附样品离子,S-GQDs作为载体使检测信号进一步放大。实验考察了缓冲溶液中S-GQDs的体积分数、缓冲溶液的组成及pH、进样时间等因素对富集分离效果的影响。当缓冲溶液的pH为4.6时,进样时间可延长至450 s。同常规电动进样(10 kV×10 s)相比,采用FASI与S-GQDs双重放大技术可使检测灵敏度提高1.6×105倍。该方法对三聚氰胺和双氰胺检测的线性范围是1.0×10-14~1.0×10-8mol/L,相关系数(r2)大于0.999,检出限分别为2.6×10-15和5.7×10-15mol/L。实现了对盐酸二甲双胍中三聚氰胺和双氰胺的高灵敏检测,回收率分别为95.9%~102.4%和92.0%~106.0%,相对标准偏差(RSD)小于5%。该方法操作简单,分离效果好,准确度高,重现性好,适用于分离检测不同盐酸二甲双胍制剂中的三聚氰胺和双氰胺。

Capillary Sodium Dodecyl Sulfate Agarose Gel Electrophoresis of Proteins

Capillary sodium dodecyl sulfate gel electrophoresis has long been used for the analysis of proteins, mostly either with entangled polymer networks or translationally cross-linked gels. In this paper capillary agarose gel electrophoresis is introduced for the separation of low molecular weight immunoglobulin subunits. The light (LC~24 kDa) and heavy (HC~50 kDa) chain fragments of a monoclonal antibody therapeutic drug were used to optimize the sieving matrix composition of the agarose/Tris-borate-EDTA (TBE) systems. The agarose and boric acid contents were systematically varied between 0.2–1.0% and 320–640 mM, respectively. The influence of several physical parameters such as viscosity and electroosmotic flow were also investigated, the latter to shed light on its effect on the electrokinetic injection bias. Three dimensional Ferguson plots were utilized to better understand the sieving performance of the various agarose/TBE ratio gels, especially relying on their slope (retardation coefficient, KR) value differences. The best resolution between the LC and non-glycosylated HC IgG subunits was obtained by utilizing the molecular sieving effect of the 1% agarose/320 mM boric acid composition (ΔKR = 0.035). On the other hand, the 0.8% agarose/640 mM boric acid gel showed the highest separation power between the similar molecular weight, but different surface charge density non-glycosylated HC and HC fragments (ΔKR = 0.005). It is important to note that the agarose-based gel-buffer systems did not require any capillary regeneration steps between runs other than simple replenishment of the sieving matrix, significantly speeding up analysis cycle time.

3-D printed injection system for capillary electrophoresis.

Commercial systems for capillary electrophoresis are designed for the unattended analysis of several samples, and are usually large, complex, and expensive. We report a compact system for manual injection of a single sample in capillary electrophoresis, which is ideal for method development and for student training. The injector consists of two parts that are manufactured by three-dimensional printing (STL and STEP files are included as ESI). One part is immobile and holds an electrode for powering electrophoresis and a gas line for pressurized injection and pumping fluids through the capillary. The second part is removable and is used to hold washing solutions, separation electrolyte, or sample. Conventional machining is used to tap holes to hold the electrode, separation capillary, gas line, and safety interlock. The system is used for either pressure or electrokinetic sample injection, and can be used to pump fluids through the capillary for changing background electrolytes and reconditioning the capillary between runs. We coupled the injection system to our high-dynamic range laser-induced fluorescence detector and evaluated the system by performing capillary zone electrophoresis on solutions of fluorescein. Electrokinetic injection produced a linear response across five orders of magnitude dynamic range (slope of the log-log calibration curve was 1.02), concentration detection limits of 5 pM, and mass detection limits of 1 zmol. Pressure injection produced a linear response across at least four orders of magnitude (slope of the log-log calibration curve was 0.92), concentration detection limits of 2 pM, and mass detection limits of 10 zmol.

Pressure-assisted electrokinetic injection for the stacking of biogenic amines gives enhancement factor up to 1000 in CE with UV detection.

Pressure-assisted electrokinetic injection (PAEKI) was applied for stacking of positively charged biogenic amines (BAs) to improve the sensitivity of capillary electrophoresis (CE). It is well known that the essential step for PAEKI is finding a stationary state of the running buffer such that the movement of the running buffer due to electroosmotic flow (EOF) is counterbalanced by external pressure in the opposite direction of the EOF under a given electric field. In order to find the balance point systematically and integrally, we studied the velocity of the whole BGE in the capillary by the impetus of opposite direction pressure (-0.1 to -0.6 psi), and the velocity of EOF with different voltages. According to the two sets of linear data, the EOF of CE coupled with PAEKI could be counterbalanced at the opposite direction pressure (-0.1 psi) and voltage (7.8 kV). In this study, the injection time was extended up to 0.35 min for all BAs and 0.70 min for the direct ultraviolet (UV) detection of BAs. Compared with hydrodynamic injection (HDI), the enrichment factors for sample injection times of 0.35 min and 0.70 min were 480-fold and 970-fold, respectively. The limits of detection (LODs) (S/N = 3) of indirect and direct UV detection were respectively 8.7-24.3 nmol L-1 and 0.4-4.5 nmol L-1, which reaches the sensitivity of high-performance liquid chromatography-mass spectrophotometry (HPLC-MS). With appropriate sample dilution, PAEKI can be used in the analysis of BAs in chicken.

Determination of quetiapine and its metabolites in plasma by field-enhanced sample stacking.

Quetiapine is an atypical antipsychotic drug that can be used to treat mental disorders, including schizophrenia, bipolar disorder and Alzheimer's disease. Quetiapine is mainly converted into the active metabolites of norquetiapine and 7-hydroxyquetiapine by the liver enzymes CYP3A4 and CYP2D6. In this study, liquid-liquid extraction (LLE) was used as a sample pretreatment method to eliminate interferences in plasma. tert-Butyl methyl ether was chosen as the extraction solvent. Field-enhanced sample injection (FESS), an online preconcentration technique, was used to analyze quetiapine and its metabolites norquetiapine and 7-hydroxyquetiapine in plasma. The optimal separation condition was 120 mM phosphate (pH 4.0) containing 0.005% (w/v) polyvinyl pyrrolidone and 40% (v/v) methanol. The methanol plug was 0.3 psi for 6 s, the sample was electrokinetic injection by 10 kV for 60 s at positive polarity, and the separation voltage was set at 26 kV. In this experiment, quetiapine, norquetiapine and 7-hydroxyquetiapine were successfully extracted from plasma by the LLE method and stacking and separated by FESS within 15 min. The limits of detection (S/N = 3) of quetiapine, norquetiapine and 7-hydroxyquetiapine were 0.25 ng/mL, 0.50 ng/mL and 1.00 ng/mL, respectively. The linear ranges of quetiapine, norquetiapine and 7-hydroxyquetiapine were 3-120 ng/mL and the correlation coefficients were 0.999. Compared with that of the traditional capillary zone electrophoresis method, the sensitivity enrichment of analytes was 463-835-fold. The optimal experimental conditions were successfully applied to the analysis of plasma samples from patients taking quetiapine for the treatment of schizophrenia.

Online preconcentration methodology that realizes over 2000-fold enhancement by integrating the free liquid membrane into electrokinetic supercharging in capillary electrophoresis for the determination of trace anionic analytes in complex samples

The analysis of anionic compounds via the sensitive FLM-EKS method remains challenging due to their inverse migration behaviors to EOF in the capillary. A highly sensitive ionic liquid coating-based online preconcentration methodology via electrokinetic supercharging (EKS) coupled to a free liquid membrane (FLM) was first proposed and successfully used to determine vanillic acid (VA), salvianic acid A sodium (SAAS) and ferulic acid (FA) in a complex sample matrix. EKS was recognized as the first step preconcentration, and its separation conditions were optimized. Then, the second preconcentration step, namely FLM, which was performed by simple injection of a short water-immiscible solvent plug as a membrane interface prior to electrokinetic injection (EKI) of the sample, was further investigated to increase the stacking efficiency. The influences of multifarious parameters on FLM-EKS performance were examined, and the optimal FLM-EKS conditions were identified: a buffer electrolyte of 20 mM Tris-HOAc (pH = 6) with 5% (v/v) methanol; hydrodynamic injection (HI) of a leading electrolyte (50 mM NH4Cl, 30 s, 0.5 psi) followed by the HI of 2-nitrophenyl octyl ether (NPOE) as the FLM (5 s, 0.5 psi); EKI of the sample (120 s, −10 kV) and there with the HI of terminator (20 mM N-cyclohexyltaurine, CHES, 40 s, 0.5 psi). The newly developed method yielded analytes with 2006–2862-fold enhancement compared to the normal injection, there by producing detection limits of 1.5–5.0 ng mL−1 with satisfactory repeatability below 2.92%, which provides 1.5–2.0 times higher stacking efficiency than that obtained by EKS only. Feasibility in real sample analysis was demonstrated by using Naoxintong capsules and rat feces, which suggests that the presented methodology has a high potential for the reliable and sensitive determination of trace anionic components in complex sample matrices.

Determination of multiple drugs of abuse in human urine using dispersive liquid–liquid microextraction and capillary electrophoresis with PDA detection

A new method was developed for pre-concentration and determination of multiple drugs of abuse in human urine using dispersive liquid–liquid microextraction (DLLME) and capillary electrophoresis (CE) with photodiode array detection. The method was based on the formation of tiny droplets of an organic extractant in the prepared sample solution using water-immiscible organic solvent (chloroform) dissolved in water-miscible organic dispersive solvent (isopropyl alcohol). The organic phase, which extracted eight drugs of abuse from the prepared urine solution, was separated by centrifugation. The sedimented phase was transferred into a small volume CE auto-sampler vial with 10 µL of 1% HCl methanol solution and evaporated to dryness. The residue was reconstituted in lidocaine hydrochloride (internal standard) aqueous solution and introduced by electrokinetic injection into CE. Under the optimum conditions, acceptable linear relationship was observed in the range of 3.0–500 ng/mL with the correlation coefficient ( r ) of 0.9982–0.9994 for spiked urine samples. The limit of detection (LOD) (S/N = 3) was estimated to be 1.0 ng/mL. A recovery of 75.7%–90.6% was obtained for spiked samples. The mean relative error (MRE) was within ±7.0% and the relative standard deviation (RSD) was less than 6.9%. The proposed DLLME-CE procedure offers an alternative analytical approach for the sensitive detection of drugs of abuse in real urine samples. Key points: The dispersive liquid-liquid microextraction (DLLME) was involved for the determination of drugs in urine with capillary electrophoresis with photodiode array detection (CE-PDA). Good linearity, sensitivity, recovery and precision were achieved. The proposed method was eco-friendly with microliter scale solvent consumption.

Fine-Tuning Electrokinetic Injections Considering Nonlinear Electrokinetic Effects in Insulator-Based Devices.

The manner of sample injection is critical in microscale electrokinetic (EK) separations, as the resolution of a separation greatly depends on sample quality and how the sample is introduced into the system. There is a significant wealth of knowledge on the development of EK injection methodologies that range from simple and straightforward approaches to sophisticated schemes. The present study focused on the development of optimized EK sample injection schemes for direct current insulator-based EK (DC-iEK) systems. These are microchannels that contain arrays of insulating structures; the presence of these structures creates a nonuniform electric field distribution when a potential is applied, resulting in enhanced nonlinear EK effects. Recently, it was reported that the nonlinear EK effect of electrophoresis of the second kind plays a major role in particle migration in DC-iEK systems. This study presents a methodology for designing EK sample injection schemes that consider the nonlinear EK effects exerted on the particles being injected. Mathematical modeling with COMSOL Multiphysics was employed to identify proper voltages to be used during the EK injection process. Then, a T-microchannel with insulating posts was employed to experimentally perform EK injection and separate a sample containing two types of similar polystyrene particles. The quality of the EK injections was assessed by comparing the resolution (Rs) and number of plates (N) of the experimental particle separations. The findings of this study establish the importance of considering nonlinear EK effects when planning for successful EK injection schemes.