Separations using a porous-shell pillar array column on a capillary LC instrument

We investigated the achievable separation performance of a 9-cm-long and 1-mm-wide pillar array channel (volume = 0.6 μL) containing 5 μm diameter Si pillars (spacing 2.5 μm) cladded with a mesoporous silica layer with a thickness of 300 nm, when this channel is directly interfaced to a capillary LC instrument. The chip has a small footprint of only 4 cm × 4 mm and the channel consists of three lanes that are each 3 cm long and that are interconnected using low dispersion turns consisting of a narrow U-turn (10 μm), proceded and preceded by a diverging flow distributor. Measuring the band broadening within a single lane and comparing it to the total channel band broadening, the additional band broadening of the turns can be estimated to be of the order of 0.5 μm around the minimum of the van Deemter curve, and around some 1 μm (nonretained species) and 2 μm (retained species) in the C-term dominated regime. The overall performance (chip + instrument) was evaluated by conducting gradient elution separations of digests of cytochrome c and bovine serum albumin. Peak capacities up to 150 could be demonstrated, nearly completely independent of the flow rate.     

Validation of a high-performance chromatographic method for determination of cefotaxime in biological samples

An analytical method for detecting and quantifying cefotaxime in plasma and several tissues is described. The method was developed and validated using plasma and tissues of rats. The samples were analyzed by reversed phase liquid chromatography (HPLC) with UV detection (254 nm). Calibration graphs showed a linear correlation (r > 0.999) over the concentration ranges of 0.5–200 μg/mL and 1.25–25 μg/g for plasma and tissues, respectively. The recovery of cefotaxime from plasma standards prepared at the concentrations of 25 μg/mL and 100 μg/mL was 98.5 ± 3.5% and 101.8 ± 2.2%, respectively. The recovery of cefotaxime from tissue standards of liver, fat and muscle, prepared at the concentration of 10 μg/g was: 89.8 ± 1.2% (liver), 103.9 ± 6.5% (fat) and 97.8 ± 2.1% (muscle). The detection (LOD) and quantitation (LOQ) limits for plasma samples were established at 0.11 μg/mL and 0.49 μg/mL, respectively. The values of these limits for tissues samples were approximately 2.5 times higher: 0.3 μg/g (LOD) and 1.25 μg/g (LOQ). For plasma samples, the deviation of the observed concentration from the nominal concentration was less than 5% and the coefficient of variation for within-day and between-day assays was less than 6% and 12%, respectively. The method was used in a pharmacokinetic study of cefotaxime in the rat and the mean values of the pharmacokinetic parameters are given.     

Oriented antibody immobilization for atrazine determination by a flow-through fluoroimmunosensor

An atrazine flow-through fluoroimmunosensor was developed, based on an oriented antibody covalently bound to Protein-A (Prot-A) immobilized on Controlled Pore Glass (CPG). Atrazine was detected “in-situ” by placing the immobilized antibody in the optical path of the flow cell. Immobilization of 30 μg of polyclonal anti-atrazine antibody on 0.5 g of Prot-A-CPG provided the highest sensitivity. The effect of several solvents on the covalently immobilized antibodies regeneration was evaluated, the optimum conditions being achieved by pumping 5% acetonitrile (pH = 3) at 0.15 mL/min for 100 s. The detection limit of the immunosensor was 0.7 μg/L and the reproducibility was 2% and 4% for 5 μg/L and 40 μg/L, respectively, in the optimum working concentration range (0.7–50 μg/L). This device allowed 12 samples per hour to be analyzed and had a life-time of 200 assays. Simazine and desisopropylatrazine (DIA) were not cross-reactive, desethylatrazine (DEA) has a cross-reactivity of 8% and propazine and prometryn of 44% and 27%, respectively. The immunosensor was applied to the determination of atrazine in tap and ground water samples spiked at the ¶10 and 30 μg/L concentration level.     

Enzymatic determination of glucose in a flow system by catalytic oxidation of the nicotinamide coenzyme at a modified electrode

A chemically modified electrode for detection of dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) is described. Graphite rods were modified by dipping them into solutions of-dimethylamino-1,2-benzophenoxzinium salt (Meldola blue). The modified electrodes were mounted in a flow-through cell in a flow-injection manifold. Samples (50 μl) of pure nicotinamide coenzymes produced strictly linear calibration graphs from 1 μM to 10 mM with a repeatability of 0.2–0.6% RSD. A packed-bed enzyme reactor (210 μl) containing immobilized glucose dehydrogenase was inserted in the manifold for glucose determinations. Oxidized coenzyme was also added to the carrier electrolyte. Straight calibration graphs were again obtained up to 1mM β-d-glucose. The detection limit was 0.25 μM β-d-glucose for 50-μl samples. The electrode was kept at ?50 to 0 m V vs. SCE which was low enough to avoid interferences from ascorbic acid, uric acid or quinones.     

Systematic comparison of a new generation of columns packed with sub‐2 μm superficially porous particles

The aim of this study was to evaluate the possibilities/limitations of recent RP‐LC columns packed with 1.6 μm superficially porous particles (Waters Cortecs) and to compare its potential to other existing sub‐2 μm core–shell packings. The kinetic performance of Kinetex 1.3 μm, Kinetex 1.7 μm and Cortecs 1.6 μm stationary phases was assessed. It was found that the Kinetex 1.3 μm phase outperforms its counterparts for ultra‐fast separations. Conversely, the Cortecs 1.6 μm packing seemed to be the best stationary phase for assays with longer analysis time in isocratic and gradient modes, considering small molecules and peptides as test probes. This exceptional behaviour was attributed to its favourable permeability and somewhat higher mechanical stability (ΔP_max of 1200 bar). The loading capacity of these three columns was also investigated with basic and neutral drugs analysed under acidic conditions. It appears that the loading capacities of Cortecs 1.6 μm and Kinetex 1.7 μm were very close, while it was reduced by 2–7‐fold on the Kinetex 1.3 μm packing. However, this observation is dependent on the nature of the compound and certainly also on mobile phase conditions.     

A continuous flow system combined with a sensing fluorimetric transductor for the determination of β-naphthol

A single automatic method for continuous flow determination of β-naphthol based on the enhancement of its native fluorescence once the analyte was transitorily retained on-line on a solid support (QAE A-25 resin) is reported. So, a flow-through optosensor was developed using a flow-injection analysis system with solid phase fluorimetric transduction. KCl (0.15 mol l⁻¹) at pH 12.0 was used as carrier solution. To obtain the optimum fluorescence signal the wavelengths chosen were 245 nm (excitation) and 420 nm (emission). The response of the sensor was directly proportional to the sample volume injected in the studied range 40-1500 μl. Approximately one higher order of magnitude is achieved in sensitivity when 1500 μl are used with respect to the use of 40 μl of sample. The sensor was calibrated for three different injection volumes: 40, 600 and 1500 μl, responding linearly in the measuring range of 2-60, 0.5-15 and 0.2-5 μg l⁻¹ with detection limits of 0.5, 0.09 and 0.05 μg l⁻¹, respectively. The relative standard deviation for ten independent determination is 0.6% (40 μl), 0.9% (600 μl) and 2.3% (1500 μl). A recovery study was performed onto three different spiked water samples at concentration levels from 1 to 2.5 μg l⁻¹ and the recovery percentage from the experimental data ranged between 101±2 and 105±5.     

Resolution of a Mononitrophenol Isomers Mixture by Differential Alternative Pulses Voltammetry

The high resolution second order voltammetric technique, Differential Alternative Pulse Voltammetry (DAPV), was applied for the simultaneous quantification of the mononitrophenol (NP) isomers. Complete resolution of the three isomers was achieved at concentration ratios as high as 1 : 5, employing the corresponding anodic and cathodic peaks appeared on the DAPV curve. The working glassy carbon electrode was modified by a nanocomposite of graphite nanopowder and Au nanoparticles to increase the sensitivity. The linear concentration range was found to be extended up to 125 μmol L^?1 for both the m‐NP and p‐NP isomers, while the o‐NP isomer presented calibration plot of two linear sections: up to 100 μmol L^?1 and up to 225 μmol L^?1. The LOD was found to be as low as 1.5 μmol L^?1, 2.5 μmol L^?1, and 0.5 μmol L^?1 for o‐NP, m‐NP, and p‐NP, respectively. The accuracy of the proposed method was evaluated by quantification of spiked tap waters samples. No interference was observed from a range of phenolic compounds such as phenol, 4‐aminophenol, 4‐chlorophenol, 2,4‐dinitrophenol, and resorcinol.     

广东某高校学生膳食硒摄入状况

为了解广东大学生膳食硒的摄入状况,进而了解广州市居民的膳食硒摄入状况,采用单纯随机抽样的方法随机选取广东某高校某班学生10人,男女生各半,用双份饭法收集这10名学生连续7d内每天摄入的全部食物（包括饮水、饮料、零食等）,共70份样品,用荧光分光光度法测定了各样品的硒含量,得到学生在调查期间膳食硒的摄入状况。结果表明,男、女生人均膳食硒摄人量分别为86．51μg／d和69．92μg／d,学生总体人均膳食硒摄入量为78．22μg／d。该校学生膳食硒的摄入量达到了中国营养学会推荐的参考摄入量（50—250μg／d）的要求。     

Determination of vitamins A, D and E in a small volume of human plasma by a high-throughput method based on liquid chromatography/tandem mass spectrometry

We have developed an automated high-throughput assay for the determination of vitamin A (retinol), ergocalciferol (25-OH D2), cholecalciferol (25-OH D3) and vitamin E (α-tocopherol) in a small volume of human plasma. Sample preparation involved mixing 50 μL of plasma with 100 μL of ethanol containing isotope-labelled internal standards, followed by mixing with isooctane/chloroform (3:1, 300 μL). The organic phase was evaporated, and the sample reconstituted in 50 μL methanol. The analysis was performed using reversed-phase liquid chromatography with a gradient mobile phase containing water, methanol and ammonium formate. Chromatographic run-time was 5 min, and positive mode electrospray tandem mass spectrometry (MS/MS) was used for detection. The limits of detection were 0.10 μM for all-trans retinol and 3.3 nM for 25-OH D2 and 25-OH D3. Recoveries were 91.9-105.0%, and within- and between-day coefficients of variance (CVs) 2.4-5.3 and 3.1-8.2, respectively. The assay is presently being used in large-scale studies.     

Liquid-liquid microextraction without phase separation in a multicommuted flow system for diltiazem determination in pharmaceuticals

Liquid-liquid microextraction without phase segmentation was implemented in a multicommuted flow system for determination of the anti-hypertensive diltiazem. The procedure was based on ion pair formation between the drug and the dye bromothymol blue at pH 3.5. The detection was performed without phase separation in a glass tube coupled to a fiber-optics spectrophotometer. The total volume of chloroform was reduced to 50 μL in comparison with 10 mL consumed in batch. A linear response was observed between 9 and 120 μmol L(-1), with a detection limit of 0.9 μmol L(-1) (99.7% confidence level). The coefficient of variation (n=10), sampling rate and extraction efficiency were estimated as 0.6%, 78 determinations per hour and 61%, respectively. About 30 μg of bromothymol blue was consumed and the waste volume was 380 μL per determination. The results for pharmaceutical samples agreed with those obtained by the reference procedure at the 95% confidence level.     

Study of a pulsed glow discharge ion source for time-of-flight mass spectrometry

This paper describes a new type of glow discharge (GD) ion source coupled to a time-of-flight mass spectrometer (TOFMS). The GD is operated in the microsecond pulse (μs-pulse) mode. The operational parameters of the μs-pulse GD were optimized against the ion signals, giving 180 Pa for the discharge pressure, 3 A for the transient discharge current, 1.75 kHz for the discharge frequency and 2 μs for the discharge pulse duration. Experimental results show that the discharge current in the μs-pulse mode can be one order of magnitude higher than that obtained in the d.c. mode. The structure of the interface between the μs-pulse GD and the mass spectrometer was found to be critical, and a Macor disc must be applied in front of the sampling orifice in order to shield the sampling plate from the anode of the GD to achieve both a good vacuum and the best sputtering. A transient sputtering rate of 24.4 μs s⁻¹ mm⁻² was reached in the μs-pulse mode and was significantly higher than that for the d.c.-GD. Typical mass spectra of brass and nickel samples were studied and are discussed. © 1997 Elsevier Science B.V.     

Electroanalytical Investigation and Simultaneous Determination of Etodolac and Thiocolchicoside at a Non-modified Glassy Carbon Electrode in Anionic Surfactant Media

In this study, a differential pulse voltammetry method was designed for the simultaneous determination of etodolac and thiocolchicoside in a tablet dosage form. This method is a sensitive, costless and reproducible one initially designed for the simultaneous determination of etodolac and thiocolchicoside using bare/unmodified glassy carbon electrode in the presence of sodium dodecyl sulfate. At optimized conditions, the method showed linear responses with etodolac and thiocolchicoside concentration in the range of 1 μM to 80 μM. The limits of detection and quantification were calculated as 0.11 μM and 0.38 μM for etodolac and 0.20 μM and 0.67 μM for thiocolchicoside, respectively.     

Determination of Amphotericin-B in Human Plasma by Reversed-Phase High Performance Liquid Chromatography Using a Short Octyl Column

Abstract

Amphotericin-B is a polyene antifungal antibiotic used for the treatment of severe systemic fungal infections. For effective treatment of urinary fungaria and the prevention of significant adverse-effects, monitoring the concentration of Amphotericin-B in biological samples of humans (ingesting the drug) is required. In this experiment, Amphotericin-B was isolated from plasma endogenous substances by adding 200 μL of acetonitrile in 800 μL of plasma. This mixture was vortex mixed, 20 mg of zinc sulfate and 10 mg of monobasic potassium phosphate was added to the mixture. This mixture was again vortex mixed and followed by centrifugation. The supernatant was filtered through a 0.45 μm membrane and a 100 μL aliquot of this solution was injected onto the chromatographic system. A short column of 60 mm × 4.6 mm packed with 3 μm octyl particles was used with an isocratic elution of 50/50, acetonitrile/0.01M KH₂PO₄ (v/v). The pH of the mobile phase mixture was adjusted to 3.5 with H₃PO₄. The intact drug molecule (parent drug) was monitored by a W-visible detector at 410 nm and 0.10-0.005 A.U.F.S. The limits of detection of the method were 0.03 μg/mL for 100 μl injection volume at signal-to-noise ratio of 3.     

Determination of vanadium(V) and chromium(VI) in a single sample on a fibrous ion exchanger disk

The determination of vanadium(V) and chromium(VI) in a single sample on a support disk was studied. Polyacrylonitrile fiber filled with an AV-17 anion exchanger was used as a solid phase. After adsorption, the elements were successively detected in the solid phase by color complexation reactions of vanadium with 8-hydroxyquinoline-5-sulfonic acid and chromium with 1.5-diphenylcarbazide. A procedure was developed for the adsorption-spectroscopic determination of 0.04–0.4 μg/mL vanadium(V) and 0.01–0.05 μm/mL chromium(VI) at V: Cr ratios of 1: 1 to 10: 1 in the dynamic mode. The limits of detections for vanadium and chromium were 0.04 and 0.005 μg/mL, respectively. It was shown that the background salt content of potable water had no appreciable effect; no preliminary sample preparation was required.     

Development of a New Voltammetric Methodology for the Determination of Ciprofloxacin in Beef Samples Using a Carbon Paste Electrode Modified with Nafion and Fullerenes

A sensitive, selective, and low cost electrochemical new methodology was developed for the quantification of ciprofloxacin (Cip) in beef samples by cyclic voltammetry and differential pulse voltammetry, using a CPE electrode modified with Nafion and Fullerenes (N−F/CPE). The optimum parameters for the composition of the N−F/CPE electrode are 0.19 g mineral oil, 0.01 g Nafion, 50 μL fullerene, and graphite powder 0.3 g. The electrochemical characterization was carried out by obtaining maximum anodic peak current associated with the oxidation of ciprofloxacin at 1.1 V, where the electrochemical process resulted to be irreversible and diffusion-controlled. The analytical characterization of the proposed methodology was carried out resulting in a LOD of 1.0 μmol L⁻¹, a LOQ of 3.0 μmol L⁻¹, a sensitivity of 0.37±0.006 μA/μmolL⁻¹, and repeatability of 5.38 %.     

Application of a Thiadiazole-derivative in a Tyrosinase-based Amperometric Biosensor for Epinephrine Detection

Enzyme-based amperometric biosensors are proving to be important analytical tools in several fields such as food, environmental and, in recent years, the biomedical one. This work describes the use of 4,7-bis(5-(pyridin-2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TBT) in the development of a tyrosinase-based biosensor for epinephrine detection. The modifying agent was obtained as a film by electrochemical oxidation of TBT on a gold disk electrode. Electrochemical characterization and scanning electrode microscopy (SEM) images suggest the formation of a conducting film on the electrode surface. Tyrosinase from mushroom was then immobilized by a mixed technique of adsorption and cross-linking. Glutaraldehyde was used as a coupling agent. The obtained device shows a very good linear response (0.1–50 μM) with a LoD value of 0.06 μM and a LoQ of 0.09 μM. Moreover, good selectivity towards some typical interferents (namely, ascorbic acid, tryptophan, uric acid and L-cysteine) and satisfactory recoveries have been observed.     

An improved determination of ammonia in kjeldahl digests and acidic solutions with a buffered berthelot reaction

The addition of trisodium citrate to acidic samples and sodium carbonate to an alkaline hypochlorite reagent ensures that up to 100 μmol of hydrogen ion in the sample does not significantly affect the absorbance. Calibration was linear from 0.05–3 μg of ammonia-N. For aliquots (2.5 μg N) of a Kjeldahl digest from gorse leaves, the relative standard deviation was 0.38% (n = 10).     

A Simple Isocratic High-Performance Liquid Chromatographic (HPLC) Determination of Naproxen in Human Plasma using a Microbore Column Technique

Abstract

A simple and sensitive HPLC method was developed for the determination of naproxen in human plasma. The assay employs a microbore column packed with a C18 reversed-phase material (5 μm ODS Hypersil) with an isocratic mixture of acetonitrile and 10 mM phosphate buffer, pH 2.5 (40:60, v/v) as the mobile phase. The mobile phase was pumped at a flow rate of 0.5 ml/min. For sample analysis 200 μl of acetonitrile containing internal standard (flurbiprofen) was added to 100 μl of plasma. After centrifugation 10 mM phosphate buffer, pH 7.4 (200 μl) was added to the tube, then vortexed and centrifuged. The supernatant (20 μl) was injected onto the HPLC column. The chromatographic separation was monitored by a fluorescence detector at an emission wavelength of 350 nm with an excitation wavelength of 225 nm. The direct precipitation of plasma protein using acetonitrile gave a good recovery for both naproxen and the internal standard. The detection limit was 0.1 μg/ml for naproxen. The intra- and inter-assay coefficients of variation at different concentrations evaluated were less than 10%.     

Determination of uranium in water by neutron activation and a low energy photon spectrometer

Ground and drinking water samples were analyzed for uranium by neutron activation analysis and a low energy photon spectrometer. Two methods were used: direct irradiation of water and pre-concentration by evaporation. The concentrations varied from 0.1 to 21 μg/l. The lowest detection limit obtained by pre-concentration was 0.01 μg/l while for the direct irradiation of water it was 0.04 μg/l. With these methods, we were able to determine the U concentration in all samples which were submitted for analysis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Using a co-culture microsystem for cell migration under fluid shear stress

We have successfully developed a microsystem to co-cultivate two types of cells with a minimum defined gap of 50 μm, and to quantitatively study the impact of fluid shear stress on the mutual influence of cell migration velocity and distance. We used the hydrostatic pressure to seed two different cells, endothelial cells (ECs) and smooth muscle cells (SMCs), on opposite sides of various gap sizes (500 μm, 200 μm, 100 μm, and 50 μm). After cultivating the cells for 12 h and peeling the co-culture microchip from the culture dish, we studied the impacts of gap size on the migration of either cell type in the absence or presence of fluid shear stress (7 dyne cm(-2) and 12 dyne cm(-2)) influence. We found that both gap size and shear stress have profound influence on cell migration. Smaller gap sizes (100 μm and 50 μm) significantly enhanced cell migration, suggesting a requirement of an effective concentration of released factor(s) by either cell type in the gap region. Flow-induced shear stress delayed the migration onset of either cell type in a dose-dependent manner regardless of the gap size. Moreover, shear stress-induced decrease of cell migration becomes evident when the gap size was 500 μm. We have developed a co-culture microsystem for two kinds of cells and overcome the conventional difficulties in observation and mixed culture, and it would have more application for bio-manipulation and tissue repair engineering.