Determination of imipramine and trimipramine by capillary electrophoresis with electrochemiluminescence detection

The tricyclic antidepressants (TCA) imipramine (Imi) and trimipramine (Tri) were successfully analyzed by capillary electrophoresis (CE) coupling with Tris(2,2-bipyridyl) ruthenium(II)-based (Ru(bpy)₃²⁺) end-column electrochemiluminescence (ECL) detection. The addition of β-CD to the running buffer was found to enable baseline separation of the two analytes and the addition of acetonitrile (ACN) as an organic additive to improve the repeatability and sensitivity of the CE method. Under the optimized separation and detection conditions (50 mM PBS (pH = 7.0) and 2 mM Ru(bpy)₃²⁺ in the ECL detection cell, 20 mM Tris (pH = 2.0), 0.2 mM β-CD and 20% ACN (v/v) as running buffer), wide linear ranges of 0.1-5 μM and 0.1-5 μM were achieved, with the correlation coefficients of 0.9990 (n = 8) and 0.9980 (n = 8) for Imi and Tri, respectively. Detection limits 5 nM and 1 nM (S/N = 3) were obtained for Imi and Tri, respectively. The method was also successfully applied for the determination of Imi in pharmaceutical dosage form.     

Determination of nicotine and its metabolite cotinine in urine and cigarette samples by capillary electrophoresis coupled with electrochemiluminescence

In this paper, CE coupled with electrochemiluminesence (ECL) detection using a 76‐μm Pt disk as working electrode was developed for nicotine (NIC) determination. The major metabolite of NIC is cotinine (COT), which has a similar tertiary amine structure to NIC. However, there is a carbonyl group attached in the structure of COT, which leads to the great decrease in ECL response. In order to improve the ECL response of COT, NaBH₄ was used for carbonyl reduction. After reduction, NIC and COT were separated and detected by CE‐ECL. ECL response plotted with NIC concentration was linear between 5.0×10^?7 and 5.0×10^?5 mol/L (81–8100 μg/L), with LOD of 5.0×10^?8 mol/L (8.1 μg/L). The developed CE‐ECL method was applied for NIC determination in urine and cigarette samples.     

Determination of Reserpine in Urine by Capillary Electrophoresis with Electrochemiluminescence Detection

A fast and sensitive approach to detect reserpine in urine using micellar electrokinetic capillary chromatography with electrochemiluminescence (ECL) of Ru(bpy)₃²⁺ detection is described. Using a 25 μm i.d. capillary as separation column, the ECL detector was coupled to the capillary in the absence of an electric field decoupler. Field‐amplified injection was used to minimize the effect of ionic strength in the sample and to achieve high sensitivity. In this way, the sample was analyzed directly without any pretreatment. The method was validated for reserpine in the urine over the range of 1×10^?6?1×10^?4 mol/L with a correlation coefficient of 0.996. The RSD for reserpine at a level of 5 μmol/L was 4.3%. The LOD (S/N=3) was estimated to be 7.0×10^?8 mol/L. The average recoveries for 10 μmol/L reserpine spiked in human urine were 94%.     

Quantification of 5‐aminolevulinic acid by CE using dynamic pH junction technique

An online dynamic pH junction preconcentration method was developed for quantification of 5‐aminolevulinic acid (ALA) by CE with the separation time less than 6 min. The optimal dynamic pH junction of ALA was carried out between pH 9.3 borate buffer (BGE, 40 mM) and pH 2.5 phosphate buffer (sample matrix, 40 mM) when 4.1 cm of sample plug was hydrodynamically injected into an uncoated fused‐silica capillary (48.5 cm in length, id of 50 μm). If a 24 kV separation voltage was applied, the calibration curve of ALA peak area (200 nm) showed good linearity (R² = 0.9991) ranging from 0.01 to 6.5 mg/mL. The reproducibility of this system was excellent with RSDs (n = 10) of 2.5% for peak area response and 0.6% for migration time at ALA concentration of 0.5 mg/mL. The LOD was evaluated as 1.0 μg/mL (S/N > 3). Compared to conventional CE procedure, the sensitivity was successfully improved over 50‐fold. The analytical results of pharmaceutical formulations show a good agreement with those by HPLC (r = 0.94).     

Dispersive liquid–liquid microextraction based on solidification of floating organic drop combined with field‐amplified sample injection in capillary electrophoresis for the determination of beta(2)‐agonists in bovine urine

Dispersive liquid–liquid microextraction based on solidification of floating organic drop (DLLME–SFO) was for the first time combined with field‐amplified sample injection (FASI) in CE to determine four β₂‐agonists (cimbuterol, clenbuterol, mabuterol, and mapenterol) in bovine urine. Optimum BGE consisted of 20 mM borate buffer and 0.1 mM SDS. Using salting‐out extraction, β₂‐agonists were extracted into ACN that was then used as the disperser solvent in DLLME–SFO. Optimum DLLME–SFO conditions were: 1.0 mL ACN, 50 μL 1‐undecanol (extraction solvent), total extraction time 1.5 min, no salt addition. Back extraction into an aqueous solution (pH 2.0) facilitated direct injection of β₂‐agonists into CE. Compared to conventional CZE, DLLME–SFO–FASI–CE achieved sensitivity enhancement factors of 41–1046 resulting in LODs in the range of 1.80–37.0 μg L^?1. Linear dynamic ranges of 0.15–10.0 mg L^?1 for cimbuterol and 15–1000 μg L^?1 for the other analytes were obtained with coefficients of determination (R²) ≥ 0.9901 and RSD% ≤5.5 (n = 5). Finally, the applicability of the proposed method was successfully confirmed by determination of the four β₂‐agonists in spiked bovine urine samples and accuracy higher than 96.0% was obtained.     

Determination of benzhexol hydrochloride by capillary zone electrophoresis with an end-column electrochemiluminescence detection

A capillary zone electrophoresis with end-column electrochemiluminescence (ECL) detector was described for the determination of benzhexol hydrochloride. The detection was based on the tris(2,2′-bypyridine)ruthenium(II) [Ru(bpy)₃²⁺] ECL reaction with the analyte. Electrophoresis was performed using a 25 μm i.d. uncoated capillary. 10 mM sodium phosphate buffer (pH=8.0) was used as the running buffer. The solution in the detection cell was 80 mM sodium phosphate (pH=8.0) and 5 mM Ru(bpy)₃²⁺. A linear calibration curve of three-orders of magnitude was obtained (with a correlation coefficient of >0.999) from 1.0×10⁻⁸ to 1.0×10⁻⁵ M and the limit of detection was 6.7×10⁻⁹ M (S/N=3). This just provides an easy and sensitive method to determine the active ingredient in pharmaceutical formulations.     

Simultaneous analysis of six cardiovascular drugs by capillary electrophoresis coupled with electrochemical and electrochemiluminescence detection,using a chemometrical optimization approach

CE coupled with dual electrochemical (EC) and electrochemiluminescence (ECL) detection was optimized for simultaneous analysis of six cardiovascular drugs (alprenolol, propafenone, acebutolol, verapamil, atenolol and metoprolol) via central composite design. Following this study, three critical electrophoretic factors governing the CE separation were investigated: Tris‐H₃PO₄ buffer concentration, buffer pH value and separation voltage. A modified chromatographic response was adopted for evaluating CE separation quality. Optimum conditions were achieved using Tris‐H₃PO₄ buffer 35.6 mM (pH 2.3) separated at 13.9 kV, which was employed experimentally and led to the successful simultaneous separation of the above six drugs. The good agreement of the chromatographic response was observed between predicted data and actual experimental results using these optimized conditions (RSD=3.75%). The proposed method was validated for linearity, repeatability and sensitivity, and subsequently successfully applied to determine six basic drugs in urine samples.     

Highly Sensitive and Selective Detection of Pb(II) by NH2−SiO2/Ru(bpy)32+−UiO66 based Solid‐state ECL Sensor

In this study, a novel electrochemiluminescence (ECL) sensor for highly sensitive and selective detection of Pb(II) was developed based on Ru(bpy)₃²⁺ encapsulated UiO66 metal‐organic‐framework (Ru(bpy)₃²⁺?UiO66 MOF) and ?NH₂ group functionalized silica (NH₂?SiO₂). The NH₂?SiO₂ with large surface area provided an excellent platform for the ECL sensor. As numerous exposed carboxyl groups were present on UiO66 backbone, the Ru(bpy)₃²⁺?UiO66 could be steadily immobilized to NH₂?SiO₂ by forming amide bonds. Meanwhile, the introduced UiO66 MOF which used for the encapsulation of Ru(bpy)₃²⁺, significantly enhanced the ECL efficiency of the proposed sensor, as it possessed a large specific surface area and porosity for the loading of Ru(bpy)₃²⁺. Moreover, a high quenching effect on ECL intensity was obtained in the presence of Pb(II) in the electrolyte. Under the optimal conditions, the quenched ECL intensity showed a good linear relationship within Pb(II) concentration in the range from 1.0×10^?6 to 1.0×10² μM, with a detection limit of 1.0×10^?7 μM (S/N=3). The proposed sensor for Pb(II) detection was simple in operation, rapid in testing, stable in signal, and showed a good anti‐interference ability to some other metal ions. Besides, its application for detecting Pb(II) in a real sample was also investigated here. This work provides a potential platform for metal ions detection in environmental monitoring field.     

Efficient capillary electrophoresis separation and determination of free amino acids in beer samples

Simultaneous detection of various o‐phthalaldehyde (OPA)‐labeled amino acids (AAs) in food samples was reported based on CE separation. Ionic liquid was used for the first time for CE analysis of AAs with in‐capillary derivatization. Several other additives, including SDS, α/β‐CD, and ACN, as well as key parameters for CE separation (buffer pH value, separation voltage), were also investigated. Our results show that the multiple additive strategy exhibits good stable and repeatable character for CE analysis of OPA‐labeled AAs, for either in‐capillary derivatization or CE separation, and allows simultaneous quantification of different OPA‐labeled AAs in a large concentration range of 50 μM to 3.0 mM with LOD down to 10 μM. Seventeen OPA‐labeled AAs, except for two pairs of AAs (His/Gln and Phe/Leu), which were separated with resolutions of 1.1 and 1.2, respectively, were baseline separated and identified within 23 min using the present multiple additive strategy. The method was successfully applied for simultaneous analysis of AAs in seven beer samples and as many as eleven trace‐amount AAs were detected and quantified, indicating the valuable potential application of the present method for food analysis.     

Pyrolyzed Photoresist Carbon Electrodes for Microchip Electrophoresis with Dual‐Electrode Amperometric Detection

The fabrication and evaluation of pyrolyzed photoresist films (PPF) for microchip capillary electrophoresis (CE) with dual‐electrode electrochemical (EC) detection is described. The sensitivity, linearity, and reproducibility were evaluated using catecholamines and related compounds, including dopamine (DA), 5‐hydroxyindole‐3‐acetic acid (5‐HIAA), ascorbic acid (AA), and catechol. Initial studies with DA show the response of the PPF electrodes to be linear between 25 and 500 μM (r²=0.999) with a limit of detection (LOD) of 5 μM (S/N=3) and sensitivity of 5.8 pA/μM. Selectivity was further enhanced by employing dual‐electrode detection in the series configuration for detection of species exhibiting chemically reversible redox reactions.     

Electrochemiluminescence from tris(2,2′-bipyridyl)ruthenium(II)-graphene-Nafion modified electrode

An electrochemiluminescence (ECL) sensor based on Ru(bpy)₃²⁺-graphene-Nafion composite film was developed. The graphene sheet was produced by chemical conversion of graphite, and was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and Raman spectroscopy. The introduction of conductive graphene into Nafion not only greatly facilitates the electron transfer of Ru(bpy)₃²⁺, but also dramatically improves the long-term stability of the sensor by inhibiting the migration of Ru(bpy)₃²⁺ into the electrochemically inactive hydrophobic region of Nafion. The ECL sensor gives a good linear range over 1 × 10⁻⁷ to 1 × 10⁻⁴ M with a detection limit of 50 nM towards the determination of tripropylamine (TPA), comparable to that obtained by Nafion-CNT. The ECL sensor keeps over 80% and 85% activity towards 0.1 mM TPA after being stored in air and in 0.1 M pH 7.5 phosphate buffer solution (PBS) for a month, respectively. The long-term stability of the modified electrode is better than electrodes modified with Nafion, Nafion-silica, Nafion-titania, or sol-gel films containing Ru(bpy)₃²⁺. Furthermore, the ECL sensor was successfully applied to the selective and sensitive determination of oxalate in urine samples.     

Surface-Bonding-Enhanced Self-Co-Reactant Electrogenerated Chemiluminescence for Sensitive and Selective Detection of Thioglycolic Acid in Cosmetics

Thioglycolic acid (TGA) is an organic compound widely used in cosmetics that cause a variety of health problems when overexposed to it. So far many attempts have been made to develop methods for TGA detection, but most of them need sophisticated instrumentations and are a little bit complicated. Therefore, a simple, cheap and sensitive detection method of TGA is highly desired. Herein, we demonstrated for the first time an Au−S bonding amplified, highly sensitive electrochemiluminescence (ECL) sensing method for TGA detection using tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) as a luminophore and TGA as a self-co-reactant, via an anodic reaction at the Au electrode surface. Due to different molecular coordination environments of the TGA at the electrode surface, the ECL signal intensity of the developed ECL system gives much higher ECL signal in borate buffer than phosphate buffer of the same pH. Under the optimized experimental conditions, the ECL intensity has a direct relationship with the concentration of TGA in the range of 0.03 μM to 300 μM and a limit of detection of 0.013 μM (3σ/m). The reported ECL system has further been applied for the detection of TGA in cosmetics with acceptable recoveries.     

Capillary electrophoresis coupled with electrochemiluminescence detection for the separation and determination of thyreostatic drugs in animal feed

A rapid, simple, and practical method for the determination of four of the most used thyreostatic drugs (methimazole, 2‐thiouracil, 6‐methyl‐2‐thiouracil, and 6‐propyl‐2‐thiouracil) using CE coupled to electrochemiluminescence detection has been established, based on the electrochemiluminescence enhancement of tris(2,2‐bipyridyl)ruthenium(II) with these analytes. Parameters that affect separation and detection were optimized. Under the optimum experimental conditions, the four analytes could be well separated within 11 min at the separation voltage of 16 kV in a running solution containing 20 mM phosphate buffer (pH 9.0) and 1.0 × 10^?4 M Ru(bpy)₃²⁺, with a solution of 20 mM phosphate buffer (pH 12.0) containing 1.0 × 10^?4 M Ru(bpy)₃²⁺ in the electrochemiluminescence detection cell. The detection limits for methimazole, 6‐methyl‐2‐thiouracil, 6‐propyl‐2‐thiouracil, and 2‐thiouracil were 0.1, 0.05, 0.05, and 0.01 μM, respectively. The proposed method was applied to analyze these drugs in spiked animal feed samples. The recoveries were 88.2～99.0 and 86.4～98.7% for the intraday and interday analyses, respectively. The RSDs were 2.7～4.8 and 1.8～5.0% for the intraday and interday analyses, respectively. The results demonstrate that the proposed method has promising applications in the detection of thyreostatic drugs in animal feeds.     

Development of a fast and efficient CE enzyme assay for the characterization and inhibition studies of α‐glucosidase inhibitors

The inhibition of the α‐glucosidase enzyme plays an important role in the treatment of diabetes mellitus. We have established a highly sensitive, fast, and convenient CE method for the characterization of the enzyme and inhibition studies of α‐glucosidase inhibitors. The separation conditions were optimized; the pH value and concentration of the borate‐based separation buffer were optimized in order to achieve baseline separation of p‐nitrophenyl‐α‐d ‐glucopyranoside and p‐nitrophenolate. The optimized method using 25 mM tetraborate buffer, pH 9.5, was evaluated in terms of repeatability, LOD, LOQ, and linearity. The LOD and LOQ were 0.32 and 1.32 μM for p‐nitrophenyl‐α‐d ‐glucopyranoside and 0.83 and 3.42 μM for p‐nitrophenolate, respectively. The value of the Michaelis–Menten constant (K_m) determined for the enzyme is 0.61 mM, which is in good agreement with the reported data. The RSDs (n = 6) for the migration time was 0.67 and 1.83% for substrate and product, respectively. In the newly established CE method, the separation of the reaction analytes was completed in <4 min. The developed CE method is rapid and simple for measuring enzyme kinetics and for assaying inhibitors.     

Novel Designing of Chemically Modified Electrode (CME) of the Bio-MOF-1 for the Detection of Dopamine Based on Inhibition of [Ru(bpy)3]2+/DBAE System

Metal organic frameworks (MOFs) have attracted extensive attention in electrochemical research fields due to their high surface area and controlled porosity. Current study is design to investigate the ECL performance of the chemically modified electrode (CME) based on the bio-MOF-1, a porous zinc-adenine framework, which loaded ruthenium complex and employed for the detection of dopamine (DA). The composite material [Ru(bpy)₃]²⁺@bio-MOF-1 (Ru-bMOF) modified carbon glassy electrode (Ru-bMOF/GCE) exhibited an excellent ECL performance having a linear co-efficient response (R²=0.9968) for 2-(dibutyl amino) ethanol (DBAE), a classical ECL co-reactant was obtained over a concentration range of 1.0×10⁻⁹ M to 1.0×10⁻⁴ M in 0.10 M pH=6.0 phosphate buffer solution (PBS). Furthermore, DA was detected based on its inhibition effect on [Ru(bpy)₃]²⁺/DBAE system. Compared to traditional analytical methods, this method has various advantages such as simple electrode preparation, quick response, high reproducibility (RSD<2.0 %), low limit of detection (LOD=1.0×10⁻¹⁰ mol/L). This chemical investigated modified electrode had exploited potential for detection of DA.     

Electrochemiluminescence of SDBS‐Ru(bpy)32+‐CPM System and Its Application

When the concentration of dodecyl benzene sulfonic acid sodium salt (SDBS) is 0.7 mmol·L^?1, the electrochemical and electrochemiluminescence (ECL) intensity of Ru(bpy)₃²⁺‐chlorpheniramine maleate (CPM) system at the Au electrode were studied. The results showed that compared with the absence of SDBS, enhancement of the ECL intensity was 14‐fold at Au electrode. Base on this, an ECL method was established for efficient and simple determination of CPM at Au electrode. Under the optimum experimental condition, the enhanced ECL intensities had good linear relationship with the concentration of CPM in the range of 1.0×10^?4–1.0×10^?7 mol·L^?1, and a linear regression equation was obtained as follows: I (counts)=48.805×10⁶c+394.03 (r=0.9975), the detection limit for CPM was 1.4×10^?8 mol·L^?1. The RSD for 5 times determinations of 1.0×10^?5 mol·L^?1 CPM was 3.2%. The results of recovery test were between 96.3%–102.5%, and the RSD of recovery test (n=5) was 2.7%. In addition, eleven kinds of tertiary amines‐Ru(bpy)₃²⁺ systems were investigated in the absence and presence of SDBS. The results showed that the enhancement of SDBS on ECL intensity of tertiary amines‐Ru(bpy)₃²⁺ systems was universal.     

Generation of gold nanostructures at the surface of platinum electrode by electrodeposition for ECL detection for CE

In this paper, we report a sensitive method for ECL detection for CE based on generation of gold nanostructures at the surface of Pt electrode by electrodeposition. Difenidol hydrochloride was used as a model analyte. With the increase of electrodeposition amount, the morphology of gold nanostructures changed from discrete nanoflowers to dense nanoparticle array. Interestingly, the variation of deposition amount also greatly affected the ECL intensity of difenidol. The ECL intensity increased remarkably with deposition amount and reached the maximum value at the deposition amount of 7.0×10^?8C; further increasing the deposition amount, however, caused the ECL intensity to decrease. Other conditions, including applied potential, injection time and voltage, buffer pH, were also optimized in detail. Under the optimized conditions, the linear response range of difenidol is from 1.0×10^?8 to 5.0×10^?5 M, and the detection limit was 4.0×10^?9 M (S/N=3). The RSDs of ECL intensity and migration time were 2.0 and 1.6%, respectively (n=5, at 7.5 μM difenidol). Compared with using bare electrode, the detection sensitivity was significantly improved by ca. two orders of magnitude. Notably, the nanogold was prepared at the surface of electrode and no nanogold was added to the electrophoretic buffer or detection cell, thus causing no interference to the separation. Finally, the proposed method was successfully applied to the analysis of difenidol in tablets and urine samples. With high sensitivity and good reproducibility, this method provides a promising platform for the determination of pharmaceuticals that have a tertiary amine group such as difenidol.     

Analysis of glyphosate and aminomethylphosphonic acid by capillary electrophoresis with electrochemiluminescence detection

A capillary electrophoresis (CE) method coupled with electrochemiluminescence (ECL) detection for the analysis of glyphosate (GLY) and its major metabolite aminomethylphosphonic acid (AMPA) is presented. Complete separation of GLY and AMPA was achieved in 8 min using a background electrolyte of 20 mM sodium phosphate (pH 9.0) and a separation voltage of 21 kV. ECL detection was performed with an indium tin oxide (ITO) working electrode bias at 1.6 V (vs. a Pt-wire reference) in a 30 0mM sodium phosphate buffer (pH 8.0) containing 3.5mM Ru(bpy)3 2+ (where bpy=2.2'-bipyridyl). Linear correlation (r>or=0.997) between ECL intensity and analyte concentration was obtained in the ranges 0.169-16.9 and 5.55-111 microg ml(-1) for GLY and AMPA, respectively. The limits of detection (LODs) for GLY and AMPA in water were 0.06 microg ml(-1) and 4.04 microg ml(-1), respectively. The developed method was applied to the analysis of GLY in soybeans. The LOD of GLY in soybean was 0.6 microg g(-1). Total analysis time including sample pretreatment was less than 1h.     

Determination of the spectrum of low molecular mass organic acids in urine by capillary electrophoresis with contactless conductivity and ultraviolet photometric detection--an efficient tool for monitoring of inborn metabolic disorders

A mixture of 29 organic acids (OAs) occurring in urine was analyzed by capillary electrophoresis (CE) with capacitively coupled contactless conductivity detection (C⁴D) and UV photometric detection. The optimized analytical system involved a 100 cm long polyacrylamide-coated capillary (50 μm i.d.) and the background electrolyte of 20 mM 2-morpholinoethanesulfonic acid (MES)/NaOH + 10% (v/v) methanol, pH 6.0 (pH is related to the 20 mM MES/NaOH buffer in water). The LOD values obtained by C⁴D for the OAs which do not absorb UV radiation range from 0.6 μM (oxalic acid) to 6.8 μM (pyruvic acid); those obtained by UV photometry for the remaining OAs range from 2.9 μM (5-hydroxy-3-indoleacetic acid) to 10.2 μM (uric acid). The repeatability of the procedure developed is characterized by the coefficients of variation, which vary between 0.3% (tartaric acid) and 0.6% (5-hydroxy-3-indoleacetic acid) for the migration time and between 1.3% (tartaric acid) and 3.5% (lactic acid) for the peak area. The procedure permitted quantitation of 20 OAs in a real urine sample and was applied to monitoring of the occurrence of the inborn metabolic fault of methylmalonic aciduria.     

Electrogenerated Chemiluminescence Ethanol Biosensor Based on Carbon Nanotube‐Titania‐Nafion Composite Film

Mesoporous titania‐Nafion composite doped with carbon nanotube (CNT) has been used for the immobilization of tris(2,2′‐bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) and alcohol dehydrogenase on an electrode surface to yield a highly sensitive and stable electrogenerated chemiluminescence (ECL) ethanol biosensor. The presence of CNT in the composite film increases not only the sensitivity of the ECL biosensor but also the long‐term stability of the biosensor. The present biosensor responds linearly to ethanol in the wide concentration ranges from 1.0×10^?5 M to 1.0×10^?1 M with a detection limit of 5.0×10^?6 M (S/N=3). The present ECL ethanol biosensor exhibited higher ECL response compared to that obtained with the ECL biosensor based on the corresponding composite without CNT. The present CNT‐based ECL biosensor showed good long‐term stability with 75% of its initial activity retained after 2 weeks of storage in 50 mM phosphate buffer at pH 7.0.