Molecularly imprinted solid phase extraction for rapid screening of metformin

A molecularly imprinted solid phase extraction (MISPE) method has been developed for the rapid screening of metformin. Newly synthesized molecularly imprinted polymer (MIP) particles were slurry-packed into a micro-column for selective solid phase extraction (SPE) of metformin. With CH₃CN flowing at 0.5 ml/min, a total binding capacity of 1600 ng metformin was determined for the 20 mg of MIP particles. A broad range of MISPE conditions was evaluated with respect to sample solvent, pH, and buffer compositions. A 95±2% binding could be achieved from one 20-μl injection of sample solution in acetonitrile plus phosphate buffer, up to 1200 ng of metformin. However, the micro-column interacted indiscriminately with phenformin, a structural analogue, to attain 49±2% binding. Separation of phenformin from metformin was ultimately achieved, using differential pulsed elution (DPE) with 1 M trifluoromethacrylic acid in acetonitrile. Final pulsed elution (FPE) using 3% trifluoroacetic acid in methanol was good for the quantitative elution of metformin. The MISPE–DPE–FPE method, with UV detection at 240 nm, afforded a detection limit of 0.8 μg/ml (or 16 ng) for metformin. Each MISPE–DPE–FPE analysis required less than 5 min to complete.     

Flow Injection Chemiluminescence Determination of Isoniazid Using On-Line Electrogenerated Manganese(III) as Oxidant

A novel flow injection chemiluminescence (CL) system for the determination of isoniazid has been proposed. It is based on the direct CL reaction of isoniazid and Mn(III) in sulfuric acid medium. The unstable Mn(III) was on-line electrogenerated by constant current electrolysis. The CL emission intensity was linear with isoniazid concentration in the range 0.1–10 μg/mL; the detection limit was 3.2 × 10⁻² μg/mL. The whole process could be completed in 1 min with a relative standard deviation of less than 5%. The proposed method is suitable for automatic and continuous analysis and has been applied successfully to the analysis of isoniazid in pharmaceutical preparation.     

A Prussian blue-based flow-through renewable surface optosensor for analysis of ascorbic acid

Ascorbic acid is determined by a simple Bead Injection Spectroscopy–Flow Injection Analysis (BIS–FIA) system with spectrophotometric detection. The sensor is based on the decrease of absorbance obtained (720 nm) when Prussian blue (PB) is reduced by ascorbic acid. Commercial available flow-cell (Hellma 138-OS) is used and an appropriate volume of homogeneous bead suspension of Sephadex QAE A-25 was injected to fill this flow-cell for each measurement. The chromogenic reagent (PB) is injected into the carrier and immobilized on beads. When sample is injected, reaching the bead surface where PB is sorbed, ascorbic acid converts it to Prussian white form, which is transparent, producing the discoloration of the detection zone. At the end of the analysis, beads are discarded by reversing the flow and instantaneously transported out of the system.The calibration graph was linear over the range 5.1×10⁻⁶–6.8×10⁻⁵ M. The detection limit and RSD (%) were 4.5×10⁻⁷ M and 5.0%, respectively, using 800 μl of sample volume. This method is highly selective in the presence of other species that are normally encountered with the analyte. The sensor was applied satisfactorily to the determination ascorbic acid in fruit juices, pharmaceuticals, sweets and conservative liquids.     

Determination of phenols by flow injection and liquid chromatography with on-line quinine-sensitized photo-oxidation and quenched luminol chemiluminescence detection

An on-line quinine-sensitized photo-oxidation with quenched chemiluminescence (CL) detection method is developed for phenols using flow injection (FI) and liquid chromatography (LC). This detection method is based on the decrease of light emission from the luminol CL reaction due to the photo-oxidation of phenols that scavenge the photogenerated reactive oxygen species (e.g. singlet oxygen () and superoxide (O₂⁻)). On-line photo-oxidation is achieved using a coil photo-reactor made from fluoroethylene-propylene copolymer tubing ( mm i.d.) coiled around a mercury UV lamp. A buffer of pH 7 and a concentration of 350 μM for quinine sulfate are determined optimum for the sensitized photo-oxidation. Using a carrier system flow rate of 60 μl/min, calibration curves taken by FI for 10 phenolic compounds in aqueous solutions showed this decreasing sensitivity order: 4-chlorophenol, phenol, 4-nitrophenol, 3-hydroxy-l-kynurenine, 2-nitrophenol, salicylate, 3-nitrophenol, catechol, 2,4-dinitrophenol, and 2,4-dichlorophenol. This detection method using two tandem coil photo-reactors is also applied for the LC separation of phenol, 4-nitrophenol and 4-chlorophenol on an octadecyl (C18) silica LC column using acetonitrile-H₂O (40:60, v/v) as a mobile phase. The quenched CL detection limits (about 1 μM or 20 pmol) for phenol and 4-chlorophenol are comparable to those for UV detection at 254 nm. Some selectivity in the quenched CL detection is evident by no interference in the FI phenol response even when benzaldehyde and phenethanol concentrations are 8 and 15 times that of phenol.     

Fluorometric Determination of Fructose, Glucose, and Sucrose Using Zirconyl Chloride

Zirconyl chloride upon hydrolysis in water to form Zr(OH)⁺ has been found to react to form a fluorescent derivative with not only a ketose such as fructose but also a hexose such as glucose and the disaccharide sucrose. When reaction conditions such as a temperature of 99°C and a time of 60 min are used, detection limits below 1 μg/mL are possible. All three zirconyl–sugar derivatives show very similar absorbance and fluorescence spectra, indicating a common mechanism involving formation of an enediol which can be complexed with ZrOH⁺ is likely. Because the reactivity order is glucose < sucrose < fructose, the reaction can be made selective for fructose at a lower reaction temperature and time such as 60°C at 5 min. Because interference from ascorbic acid and caffeine is also avoided, the fluorescent determination of fructose in soft drink samples after simply a dilution step is possible. We have also employed this reaction for flow injection analysis (FIA) using a polystyrene–divinylbenzene-packed HPLC column as a mixing device. Using a 0.01 M HClO₄ with 1% zirconyl chloride carrier, we obtained a linear calibration curve from 2 to 30 μg/mL with a correlation coefficient of 0.994. A detection limit less than 2 μg/mL was possible. A comparison of results for the FIA of soft drinks with the enzymatic method involving fructose-5-dehydrogenase confirmed the FIA method was quite specific for fructose.     

Flow-injection hydride generation atomic absorption spectrometric study of the automated on-line pre-reduction of arsenate, methylarsonate and dimethylarsinate and high-performance liquid chromatographic separation of their -cysteine complexes

An automated on-line pre-reduction of arsenate, monomethylarsonate (MMA) and dimethylarsinate (DMA) using flow injection hydride generation atomic absorption spectrometry (FI-HGAAS) is feasible. The kinetics of pre-reduction and complexation depend strongly on the concentration of -cysteine and on the temperature in the following increasing order: inorganic As(V)<DMA<MMA. Arsenate is pre-reduced/complexed within less than 50 s at 70–100°C compared to 1 h at room temperature, while MMA and DMA require 1.5–2 min at 70–100°C and up to 1–2 h at room temperature. The characteristic masses and concentrations for 100 μl injections are 0.01 ng and 0.1 μg l⁻¹ in integrated absorbance and 0.2 ng and 2 μg l⁻¹ in peak height measurements, and the limits of detection are ca. 0.5 ng and 5 μg l⁻¹, respectively. In a high-performance liquid chromatography (HPLC)–HGAAS system, the -cysteine complexes of inorganic As(III), MMA and DMA are best separated within 7 min by HPLC on a strongly acidic cation exchange column such as Spherisorb S SCX 120×4 mm (5 μm) with a mobile phase containing 12 mmol l⁻¹ phosphate buffer (KH₂PO₄/H₃PO₄)–2.5 mmol l⁻¹ -cysteine, pH 3.3–3.5. Upon dilution to -cysteine levels below 10 mmol l⁻¹, which are compatible with HPLC separations, the DMA–cysteine complex is unstable on storage. No baseline separations are possible with anion exchange and reverse phase C₁₈ HPLC columns. The limits of detection with 50 μl injections in peak area mode are ca. 0.5 ng and 10 μg l⁻¹, respectively.     

Capillary electrophoresis-chemiluminescence determination of norfloxacin and prulifloxacin

A capillary electrophoresis (CE)–chemiluminescence (CL) method for determining norfloxacin (NFLX) and prulifloxacin (PFLX) was developed based on the enhanced CL intensity of the cerium(IV)–sulfite–fluoroquinolone (FQ) reaction sensitized by terbium(III). The separation was conducted in buffer composed of 20 mM sodium citrate, 4 mM citric acid and 10 mM sodium sulfite at pH 6.1. The CL reagent solution consisted of 2 mM cerium(IV), 4 mM terbium(III) and 1.1 mM hydrochloric acid. NFLX and PFLX were baseline separated within 11 min with detection limits (S/N = 3) of 0.057 and 0.084 μg mL⁻¹, respectively. The maximum intra- and inter-day relative standard deviations (R.S.D.s) of migration time of the analytes were less than 4.0% and 4.2%, respectively. The proposed method was applied to detect NFLX and PFLX in fortified urine sample and the results were comparable to high-performance liquid chromatography (HPLC)–UV method. Moreover, the high selectivity of the CL detection and the high-separation efficiency of CE render the method the potential of quick analyzing fluoroquinolones in real complex matrix.     

A study on terbium sensitized chemiluminescence of ciprofloxacin and its application

A novel rapid flow injection method with chemiluminescence (CL) detection was established for the determination of ciprofloxacin (CPLX), which is an antibiotic commonly used. The method is based on CL of Ce(IV)–SO₃²⁻ sensitized by Tb³⁺–CPLX, and showed the intensive bands characteristic of Tb³⁺ (⁵D₄→⁷F₅). The optimum conditions for CL emission were investigated. The linear relationship between the relative CL intensity and the concentration of CPLX is in the range of 9.0×10⁻⁹–1.0×10⁻⁶ mol/l with a detection limit of 3.1×10⁻¹⁰ mol/l. The relative standard deviation is 2.8% (n=11) for a level of 5.0×10⁻⁸ mol/l. The method was applied to the analysis of CPLX in human serum and urine samples with satisfactory results. The possible mechanism for this sensitized CL reaction is also discussed.     

Bulk-modified modified screen-printing carbon electrodes with both lactate oxidase (LOD) and horseradish peroxide (HRP) for the determination of L-lactate in flow injection analysis mode

A screen-printed carbon electrode modified with both HRP and LOD (SPCE–HRP/LOD) has been developed for the determination of l-lactate concentration in real samples. The resulting SPCE–HRP/LOD was prepared in a one-step procedure, and was then optimised as an amperometric biosensor operating at [0, −100] mV versus Ag/AgCl for l-lactate determination in flow injection mode. A significant improvement in the reproducibility (coefficient variation of about 10%) of the preparation of the biosensors was obtained when graphite powder was modified with LOD in the presence of HRP previously oxidised by periodate ion (IO₄⁻). Optimisation studies were performed by examining the effects of LOD loading, periodation step and rate of the binder on analytical performances of SPCE–HRP/LOD. The sensitivity of the optimised SPCE–HRP/LOD to l-lactate was 0.84 nA L μmol⁻¹ in a detection range between 10 and 180 μMol. The possibility of using the developed biosensor to determine l-lactate concentrations in various dairy products was also evaluated.     

Microdetermination of Proteins with the 1,10-Phenanthroline–H2O2–Cetyltrimethylammonium Bromide–Cu(II) Chemiluminescence System

Proteins can enhance the chemiluminescence (CL) intensity of the 1,10-phenanthroline–H₂O₂–cetyltrimethylammonium bromide (CTMAB)–Cu(II) system because unsaturated complexes of Cu(II) with protein have a much stronger catalytic effect on the CL reaction than does Cu(II). On this basis, a new flow injection analysis method for detection of some proteins was established. The method gives linear responses over two orders of magnitude and detection limits at the 0.02–0.05 μg ml⁻¹level for bovine serum albumin, human serum albumin, γ-globulin, and egg albumin. The method was used for determination of proteins in human serum with satisfactory results.     

Flow injection analysis with inductively coupled plasma-atomic emission spectroscopy: critical comparison of conventional pneumatic, ultrasonic and direct injection nebulization

The analytical figures of merit observed under flow injection analysis (FIA) conditions of a direct injection nebulizer (DIN) interfaced to an inductively coupled plasma-atomic emission spectroscopy (ICP-AES) facility were found to be comparable to or better than conventional pneumatic nebulization in terms of limits of detection, reproducibility and interelement effects. The DIN offered clog-free operation and part per billion limits of detection for 30μl sample injection volumes and carrier stream consumption rates in the range of 100–200μl min ⁻¹. The relative detection limits observed were generally comparable to those obtained for: (a) FIA introduction of 200μl or continuous sample introduction into a conventional cross flow nebulizer; and (b) FIA introduction of 500μl or continuous sample introduction into an ultrasonic nebulizer. Absolute and relative detection limits were comparable to or within the range of values reported for electrothermal vaporization-ICP-AES and comparable or superior to those reported for the graphite cup, direct insertion-ICP-AES. The reported absolute detection limits for the graphite-rod direct insertion approach ranged from comparable values to superior by a factor of 30. At the normal compromise observation height (20 mm), the interelement effects, to the extent they were observable, were comparable in magnitude for both the DIN and conventional cross-flow pneumatic nebulizer.     

Some observations on the use of a new matrix modification electrothermal atomization-atomic absorption spectrophotometric method for the determination of micro- and sub-microgram amounts of molybdenum in human teeth

A very sensitive and selective electrothermal atomization-atomic absorption Spectrophotometric (ETA-AAS) matrix-modified method for the determination of micro- and submicrogram amounts (0–25 μ g⁻¹ molybdenum) in whole human one-rooted teeth has been developed. Hydrazine sulfate, (NH₂)₂ · H₂SO₄, which has been used as a matrix modifying reagent (MMR) is found to be very influential in removing matrix interference effects such as calcium which is present in hydroxy apatite (the main mineral constituent of tooth), at pH 2.0–2.2. Beer's law is obeyed over the range 0–1.5 ng molybdenum 5/μl injected solution. The absolute sensitivity and detection limit of the method are respectively 6.46 and 1.32 pg molybdenum/5 μl injected sample solution. The recovery percentage and RSD% are also determined. Compared to the neutron activation method, the proposed method is rapid, more available, and less expensive and requires no grinding of the tooth with metals, mortar, or mills. It is more sensitive and simpler than flame techniques. Background correction is not necessary. No separation or preconcentration of molybdenum is required. The method has been applied for the determination of molybdenum in teeth taken from representative districts of Baghdad.     

Highly-sensitive simultaneous detection of lanthanide(III) ions as kinetically stable aromatic polyaminocarboxylato complexes via capillary electrophoresis using resolution enhancement with carbonate ion

An aromatic polyaminocarboxylate ligand, 1-(4-aminobenzyl)ethylenediamine-N,N,N,N-tetraacetate (ABEDTA), is proposed as a complexing reagent in the pre-capillary mode so as to form kinetically inert Ln(III) complexes, meaning that no added ligand is necessary in alkaline carrier buffer solutions. In addition, highly-sensitive detection is possible through a light-absorbing moiety of an aminobenzyl group in the ligand. The fine-tuning of the electrophoretic mobilities of the Ln-abedta complexes is successfully achieved by adding an auxiliary carbonate ion ligand which alters the charge-to-size ratio of the complexes through fast exchange equilibria in a carrier buffer. In fact, all of the complexes are detectable with very similar analytical sensitivity and acceptable resolution (except for Ln=Sm, Eu, Gd) by using NaOH-borate carrier buffer solution at pH 12.35 with 20 mM of Na₂CO₃. A typical detection limit for Tb(III) ion (to 3) is as low as 0.94 M, which translates to an absolute amount of 9.4 fmol in a 1.0×10^–8 dm^-3 (10 nL) injection.     

Exploiting flow injection and sequential injection anodic stripping voltammetric systems for simultaneous determination of some metals

Flow injection (FI) and sequential injection (SI) systems with anodic stripping voltammetric detection have been exploited for simultaneous determination of some metals. A pre-plated mercury film on a glassy carbon disc electrode was used as a working electrode in both systems. The same film can be repeatedly applied for at least 50 analysis cycles, thus reducing the mercury consumption and waste. A single line FI voltammetric system using an acetate buffer as a carrier and an electrolyte solution was employed. An injected standard/sample zone was mixed with the buffer in a mixing coil before entering a flow cell. Metal ions were deposited on the working electrode by applying a potential of −1.1 V vs Ag/AgCl reference electrode. The stripping was performed by anodically scanning potential of working electrode to +0.25 V, resulting a voltammogram. Effects of acetate buffer concentration, flow rate and sample volume were investigated. Under the selected condition, detection limits of 1 μg l⁻¹ for Cd(II), 18 μg l⁻¹ for Cu(II), 2 μg l⁻¹ for Pb(II) and 17 μg l⁻¹ for Zn(II) with precisions of 2–5% (n=11) were obtained. The SI voltammetric system was similar to the FI system and using an acetate buffer as a carrier solution. The SI system was operated by a PC via in-house written software and employing an autotitrator as a syringe pump. Standard/sample was aspirated and the zone was then sent to a flow cell for measurement. Detection limits for Cd(II), Cu(II), Pb(II) and Zn(II) were 6, 3, 10 and 470 μg l⁻¹, respectively. Applications to water samples were demonstrated. A homemade UV-digester was used for removing organic matters in the wastewater samples prior to analysis by the proposed voltammetric systems.     

Direct electrochemistry of glucose oxidase based on its direct immobilization on carbon ionic liquid electrode and glucose sensing

Direct electrochemistry of glucose oxidase (GOx) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE) with a conductive hydrophobic ionic liquid, 1-butyl pyridinium hexafluophosphate ([BuPy][PF₆]) as binder for the first time. A pair of reversible peaks is exhibited on GOx/CILE by cyclic voltammetry. The peak-to-peak potential separation (ΔE_P) of immobilized GOx is 0.056 V in 0.067 M phosphate buffer solution (pH 6.98) with scan rate of 0.1 V/s. The average surface coverage and the apparent Michaelis–Menten constant are 6.69 × 10⁻¹¹ mol·cm⁻² and 2.47 μM. GOx/CILE shows excellent electrocatalytic activity towards glucose determination in the range of 0.1–800 μM with detection limit of 0.03 μM (S/N = 3). The biosensor has been successfully applied to the determination of glucose in human plasma with the average recoveries between 95.0% and 102.5% for three times determination. The direct electrochemistry of GOx on CILE is achieved without the help of any supporting film or any electron mediator. GOx/CILE is inexpensive, stable, repeatable and easy to be fabricated.     

Analytical evaluation of electrothermal vaporization/low-pressure inductively coupled plasma atomic emission spectrometry for trace elemental analysis in microliter samples

A novel method for the determination of trace elements in microliter samples using the tantalum filament electrothermal vaporization/low-pressure inductively coupled plasma (ETV/LP-ICP) atomic emission spectrometry has been developed. An improved tantalum filament ETV was directly coupled with LP-ICP system for efficient vaporization of microliter samples and further quantitative analysis. The experimental parameters including ETV current, rf power and mass flow rate of argon carrier gas were optimized using the copper emission signal produced by 5 μl of standard solution (5 μg/ml). Under the optimized condition, the analytical performances including linearity, precision and detection limit for the developed system were investigated. Absolute detection limits in the range of 22–391 pg for selected eight elements (Fe, Cu, Cr, Mn, Pb, K, Zn and Mg) were obtained with satisfactory precision (<8.9% RSD). The feasibility of the developed system has been demonstrated by analyzing wheat gluten NIST standard sample.     

Flow injection flame atomic absorption analysis of Fe and Mn in cement samples

Summary A new procedure has been developed for the determination of Fe and Mn in cements. It consists in dispersing 50 mg of the solid sample in 25 ml of 0.15 mol/l HNO₃ and 0.12 mol/l HCl solution. Acid slurries are heated at 50°C for 10 min and then different volumes of the slurry are injected into a water carrier stream. This previous acid treatment leaches the elements to be determined and permits the use of acid solutions as standards. For the Mn determination, the use of a single line flow injection manifold provides a limit of detection of 0.03 mgl^–1 and a dynamic range up to 6 mgl^–1. For the determination of Fe, the on-line dispersion of samples, using a well stirred mixing chamber, increases the dynamic range up to a concentration of 125 mgl^–1 and provides a limit of detection of 1 mgl^–1. The procedure has been applied to the analysis of real cement samples and a certified reference material. Results were in agreement with those obtained by a reference procedure involving the alkaline fusion of samples and batch analysis by flame atomic absorption spectrometry.     

Determination of hydrogen peroxide in a flow system with microperoxidase as catalyst for the luminol chemiluminescence reaction

Hydrogen peroxide is determined by a chemiluminescence method with a reagent containing 100 μM luminol and 3 μM microperoxidase at pH 10 (carbonate buffer). Microperoxidase is superior to hematin as a catalyst. The method uses an automated flow injection system with a throughput of 2 samples per minute. The log—log calibration plot is linear (slope 1.3) from the detection limit, 3 × 10^-9 M up to 10^-5 M H₂O₂. The background emission is low. Impurities in the carrier stream and from some plastics may cause elevated background unless precautions are taken.     

Solid Phase Extraction and Spectrophotometric Determination of Silver with 2-(2-Quinolylazo)-5-Dimethylaminophenol

A new chromogenic reagent, 2-(2-quinolylazo)-5-Dimethylaminophenol (QADMAP) was synthesized, and a sensitive, selective, and rapid method was developed for the determination of the μg/L level of silver ions. The method is based on the rapid reaction of silver(I) with QADMAP and the solid phase extraction of the colored chelate using a C₁₈ cartridge. The QADMAP reacts with Ag(I) in the presence of a citric acid-sodium hydroxide buffer solution (pH 5.0) and a sodium dodecyl sulfonate (SDS) medium to form a violet chelate of molar ratio 1 : 2 (silver to QADMAP). This chelate was enriched by solid phase extraction with C₁₈ cartridge, and the retained chelate was eluted from the cartridge using ethanol (with 1% acetic acid). In the ethanol medium (with 1% acetic acid), the molar absorptivity of the chelate was 1.25 × 10⁵ L mol⁻¹ cm⁻¹ at 584 nm. Beer’s law was obeyed in the range 0.01–0.6 μg/mL. The relative standard deviation for eleven replicate samples of 0.01 μg/mL was 1.86%. The detection limit is 0.02 μg/L in the original samples. The method was applied to the determination of μg/L levels of silver ions in water with good results.__________From Zhurnal Analiticheskoi Khimii, Vol. 60, No. 6, 2005, pp. 566–570.Original English Text Copyright © 2005 by Huang, Yang, Hu, Yin.This article was submitted by the authors in English.     

Determination of Proteins in a Mesofluidic Lab-on-Valve System

The application of a mesofluidic lab-on-valve system to the spectrophotometric determination of protein was investigated. Protein species in the sample solution reacts rapidly with Congo red at pH 4.1, which forms a complex with a maximum absorption at 496 nm. A univariant approach was used for the optimization of the experimental parameters. A sample volume of 20 μl was used along with 4.0 μl of Congo red solution of 0.9 g l⁻¹, and a flow rate for the detection process of 20 μl s⁻¹ was used. Under optimal conditions, a linear calibration curve was obtained in the range of 12.5–200 μg ml⁻¹ of bovine serum album, along with a detection limit (3σ) of 5.6 μg ml⁻¹ and a sampling frequency of 60 per hour. Protein concentrations in human serums, urine, milk, and yoghourt were determined using this procedure, and satisfactory agreements were obtained with that achieved using the Coomassie brilliant blue method.