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PubMedDate not supplied

Comparison of solid-phase extraction and micro-solid-phase extraction for liquid chromatography/mass spectrometry analysis of pesticides in water samples.

Our recent on-line solid-phase extraction (SPE) device for micro-liquid chromatography, known as micro-solid-phase extraction (microSPE), was compared with traditional SPE for the analysis, from aqueous samples, of 4 pesticides belonging to different classes. Two different kinds of adsorbents, C18 and graphitized carbon black, were tested. A 2-stage ion trap mass spectrometer, equipped with homemade microflow electrospray ion (ESI) source, was used. Detection limits with a signal-to-noise ratio of 3:1 for both extraction methods were in the range of 0.1 microg/L for all compounds. However, better recoveries were obtained when microSPE traps were used.

Adsorption↗
PubMed1993

Enhanced robotic dissolution system with concurrent off-line analysis.

A Zymate II robotic dissolution system was modified by addition of a liquid chromatography (LC) system equipped with an Isco ISIS autosampler. Using the Concurrent EasyLab programming language, operation of this LC system was integrated into the main robotic dissolution procedure. This enabled simultaneous dissolution sampling and LC analysis of standard and pre-pulled sample solutions. The resulting dual-tasking system effects considerable efficiency of operation and results in significant time savings. Comparison of the robotic and non-robotic data indicate good correlation and statistically insignificant differences at the 95% confidence level. Validation studies confirm linearity and precision of the quantitative LC method, accuracy and precision of volume and temperature measurements and negligible vessel-to-vessel carryover.

Capsules↗
PubMed2003

General methods for analysis of sequential "n-step" kinetic mechanisms: application to single turnover kinetics of helicase-catalyzed DNA unwinding.

Helicase-catalyzed DNA unwinding is often studied using "all or none" assays that detect only the final product of fully unwound DNA. Even using these assays, quantitative analysis of DNA unwinding time courses for DNA duplexes of different lengths, L, using "n-step" sequential mechanisms, can reveal information about the number of intermediates in the unwinding reaction and the "kinetic step size", m, defined as the average number of basepairs unwound between two successive rate limiting steps in the unwinding cycle. Simultaneous nonlinear least-squares analysis using "n-step" sequential mechanisms has previously been limited by an inability to float the number of "unwinding steps", n, and m, in the fitting algorithm. Here we discuss the behavior of single turnover DNA unwinding time courses and describe novel methods for nonlinear least-squares analysis that overcome these problems. Analytic expressions for the time courses, f(ss)(t), when obtainable, can be written using gamma and incomplete gamma functions. When analytic expressions are not obtainable, the numerical solution of the inverse Laplace transform can be used to obtain f(ss)(t). Both methods allow n and m to be continuous fitting parameters. These approaches are generally applicable to enzymes that translocate along a lattice or require repetition of a series of steps before product formation.

Algorithms↗
PubMed1991

Time window analysis and sorting.

Flow cytometric hardware and procedures were developed to continuously analyse and to sort a particular time window in a kinetic response. The technique uses balanced air pressure to drive a stimulus-bead mixture from a vial to a t-junction where it mixes passively with cells. The t-junction is distanced from the flow cell and air pressure regulated so that the stimulation occurs at a fixed and adjustable time before the cells are interrogated by the laser beam. Practical applications of the device demonstrate utility with cells whose responses are seen in seconds or minutes. The device is easily implemented on any sorting flow cytometer.

Animals↗
PubMedDate not supplied

Development of batch injection analysis for electrochemical measurements of trace metal ions in ecotoxicological test media.

Batch injection analysis with square-wave anodic stripping voltammetry has been developed as a powerful and rapid technique for obtaining data on the concentration of the fraction of labile metal ions present in media used in ecotoxicological tests. Microlitre samples of solution, without pretreatment, are injected directly over a detector electrode, the surface of which is protected by a thin Nafion polymer coating against irreversible adsorption by organic components. Examples are given showing the effect of adding vitamins and organic extract, singly and together, to the ASTM medium employed for tests using Daphnia magna and with lead and cadmium test ions. Such a methodology can be extended to other electroactive species present in these and similar media.

Animals↗
PubMed2005

[Spectral study and determination of metoclopramide and procaine hydrochloride by sequential injection analysis].

A sequential injection spectrophotometric method is proposed for the determination of metoclopramide or procaine hydrochloride, based on the reactions of metoclopramide and procaine hydrochloride with cerium(IV) in sulfuric acid medium. Red intermediate products were observed and were found unstable so that their analytical use is only possible by sequential injection technology. For metoclopramide, the detection limit is 6.5 microg x mL(-1), and the linear range of determination is 9.7-116.6 microg x mL(-1) with a sampling frequency of 45 h(-1). For procaine hydrochloride, the detection limit is 7.4 microg x mL(-1), and the linear range of the determination is 10.0-130.0 microg x mL(-1) with a sampling frequency of 45 h(-1). The method has been applied to the determination of metoclopramide and procaine hydrochloride in tablets and injections with satisfactory results as compared with standard method.

Cerium↗
PubMed2006

Development of a multicommuted flow-through optosensor for the determination of a ternary pharmaceutical mixture.

The combination of multicommutation and flow-through multioptosensing is presented in this work as a powerful strategy for the routine analysis of active principles in pharmaceuticals. By coupling methodologies, the selectivity and sensitivity of optosensors is maintained, while the use of the multicommutation approach provides additional advantages, such as low reagent consumption, low waste generation and reduced human supervision. The potential of this integration is enhanced when implemented with multiwavelength detection mode. An UV sensor is here developed for the simultaneous determination of three widely used active principles: salicylamide, caffeine and propyphenazone. The measuring wavelengths were 276 nm for caffeine and propyphenazone, and 302 nm for salicylamide. The five three-way solenoid valves used in the system are controlled by Java-written home-made software. The sensor is based on the on-line selective retention of two of the three analytes on a precolumn placed just before the sensing zone and filled with the same solid support than the flow-through cell (C(18) silica gel). This approach allows the sequential arrival of the analytes to the sensing zone, so allowing their determination with only one sample injection. So, the use of C(18) placed, in both the precolumn and the flow-cell combines the advantages of the increase of sensitivity and selectivity in the detection solid zone with the additional increase of the selectivity in the precolumn. The sensor was applied to the determination of the analytes in several pharmaceutical preparation of the Spanish Pharmacopoeia, obtaining satisfactory results.

Antipyrine↗
PubMedDate not supplied

Monosegmented flow-analysis of serum cholesterol.

A monosegmented flow system is designed for enzymatic spectrophotometric determination of cholesterol in blood serum. The sample (4.5 microliters), enzymatic reagent (150 microliters) and an air plug (100 microliters) are simultaneously inserted into a carrier stream buffered to pH 7.4 (potassium dihydrogenphosphate). In order to avoid the step of air removal, a relocating detector was used. This system handles about 42 samples per hour, yielding precise results (R.S.D. usually < 3.0%). Sensitivity is 46 mAU 1/mmol (mAU stands for milliabsorbance units), being the method linear up to 10.3 mmol/l cholesterol. Accuracy was assessed by running 30 samples already analysed by a conventional procedure: no statistical difference between methods was found at the 95% confidence level.

Buffers↗
PubMed2003

A universal temperature controlled membrane interface for the analysis of volatile and semi-volatile organic compounds.

A universal temperature controlled membrane interface (TCMI) has been constructed for hollow-fibre membranes. The membrane temperature is controllable in the range -70 to 250 degrees C using an electric heater and a flow of cooled nitrogen or helium gas. Volatile and semi-volatile organic compounds may be detected either by continuous diffusion across the membrane or by in-membrane pre-concentration followed by thermal desorption into the detector. The TCMI interface is demonstrated in combination with mass spectrometry and GC-MS, for the determination of VOCs and SVOCs in aqueous and air samples and for the on-line monitoring of a bioreactor.

Flow Injection Analysis↗
PubMed2006

Studying drug-plasma protein interactions by two-injector microchip electrophoresis frontal analysis.

We developed a simple, rapid, and sensitive two-injector microchip electrophoresis frontal analysis (MCE-FA) method for studying drug-plasma protein interactions. In this method, large volumes of a reference sample and drug-plasma protein mixture were simultaneously introduced into the respective sections of the microchannel through the separated injectors and then electrophoresed. Since the reference sample did not meet with the interacting species during migration, it could be used as an external standard. The interaction between heparin and HSA was quantitatively characterized as a model system. The binding constant was found to be (1.53 +/- 0.01) x 10(4) M(-1).

Anticoagulants↗
PubMed2005

Solid-phase ultraviolet sensing system for determination of methylxanthines.

In this study the use of a single continuous-flow solid-phase UV spectrophotometric sensing system for determination of methylxanthines was evaluated. Two methods were developed to determine caffeine (CF) and theophylline (TP) in pharmaceuticals and CF and theobromine (TB) in food and beverages. The sensor is based on transient and sequential retention of the analytes on a hydrophobic sensing solid zone (octadecyl silane C18 gel) and detection of their intrinsic UV absorbance. Temporary sequencing of the arrival of the analytes at the sensing zone is achieved by on-line separation of one of the analytes using a pre-column of the same particulate material, placed just before the flow cell. After TB or TP had been carried toward the sensing zone (by the appropriate carrier solution), produced its transitory signal, and been eluted by the carrier, an appropriate eluting solution (25% MeOH) was used to elute CF, which was strongly retained on the minicolumn, so that its transient signal could be recorded. The sensing zone was completely regenerated with this eluting solution, and so was ready for analysis of another sample. After selecting the most suitable conditions, the sensing system was calibrated in the range 1-16 and 1-12 mg L(-1) for CF and TP-TB, respectively, giving detection limits below 0.1 mg L(-1) with RSD values less than 3%. The usefulness of this approach has been evaluated by applying it to the determination of caffeine, theobromine, and theophylline in different samples of food, beverages, and pharmaceutical formulations. The results were in satisfactory agreement with those obtained by use of an HPLC reference method.

Chromatography, High Pressure Liquid↗
PubMed1999

Liquid chromatography-tandem mass spectrometry of some anabolic steroids.

In this paper a procedure is described for the analysis of 36 anabolic steroids, regularly found in kidney fat matrixes. After preparative HPLC is carried out, six fractions, containing the different steroids, were obtained. These fractions were then separated and on-line mass-analyzed, using APcI LC/MS. When used in tandem mass spectrometry (MRM) mode, it is possible to obtain detection limits below 1 ppb for all steroids and below 0.1 ppb for most of them. As such, it is possible to meet the regulations for detection of these anabolic steroids. To verify the influence of matrixes, real-world samples were analyzed in the same manner. This resulted in almost analog detection limits.

Adipose Tissue↗
PubMed2000

Ring opening is not rate-limiting in the GTP cyclohydrolase I reaction.

GTP cyclohydrolase I catalyzes a mechanistically complex ring expansion affording dihydroneopterin triphosphate and formate from GTP. Single turnover quenched flow experiments were performed with the recombinant enzyme from Escherichia coli. The consumption of GTP and the formation of 5-formylamino-6-ribosylamino-2-amino-4(3H)-pyrimidinone triphosphate, formate, and dihydroneopterin triphosphate were determined by high pressure liquid chromatography analysis. A kinetic model comprising three consecutive unimolecular steps was used for interpretations where the first intermediate, 5-formylamino-6-ribosylamino-2-amino-4(3H)-pyrimidinone 5'-triphosphate, was formed in a reversible reaction. The rate constant k(1) for the reversible opening of the imidazole ring of GTP was 0.9 s(-1), the rate constant k(3) for the release of formate from 5-formylamino-6-ribosylamino-2-amino-4(3H)-pyrimidinone triphosphate was 2.0 s(-1), and the rate constant k(4) for the formation of dihydroneopterin triphosphate was 0.03 s(-1). Thus, the hydrolytic opening of the imidazole ring of GTP is rapid by comparison with the overall reaction.

Aldose-Ketose Isomerases↗
PubMedDate not supplied

Chemiluminescence microflow injection analysis system on a chip for the determination of uric acid without enzyme.

A new microflow injection analysis (microFIA) system on a chip coupled with chemiluminescence (CL) for the non-enzymatic determination of uric acid is described. The microFIA system produced by using two transparent poly(methylmethacrylate) (PMMA) chips measured 50 x 40 x 5 mm, the microchannels, etched by CO2 laser, were 200 microm wide and 100 microm deep, and the volume of the reaction area (RA) was about 1.2 microL. The injection pump, with accurate time control, monitored all reagents, including the sample. The uric acid was sensed by the chemiluminescence reaction between luminol and ferricyanide. The linear range of the uric acid concentration was 0.8-30 mg/L and the detection limit was 0.5 mg/L (S/N = 3). The relative standard deviation was 4.42% for 5 mg/L uric acid (n = 8). The proposed method has been successfully applied to the non-separation determination of uric acid in human serum and urine.

Ferricyanides↗
PubMed2006

Oxygen consumption of cell suspension in a poly(dimethylsiloxane) (PDMS) microchannel estimated by scanning electrochemical microscopy.

A quantitative analysis of the oxygen concentration profile near a poly(dimethylsiloxane) (PDMS) microfluidic device was performed using scanning electrochemical microscopy (SECM). A microchannel filled with sodium sulfite (Na(2)SO(3)) aqueous solution was imaged by SECM, showing that the oxygen diffusion layer of the PDMS microchannel was observed to be hemicylindrical. Based on a theoretical analysis of the hemicylindrical diffusion layer of the microchannel, the total oxygen mass transfer rates of oxygen to the PDMS microchannel filled with the Na(2)SO(3) solution was calculated to be (4.01 +/- 0.30) x 10(-12) mol s(-1). This is the maximum value of the oxygen transfer rate for this PDMS microchannel device. The oxygen consumption rate increased almost linearly with the logarithm of the concentration of E. coli cells (10(6) approximately 10(8) cells). The respiratory activity for a single E. coli cell was estimated to be approximately 4.31 x 10(-20) mol s(-1) cell(-1).

Cells↗
PubMed2005

Epoxidized soy bean oil migrating from the gaskets of lids into food packed in glass jars. Analysis by on-line liquid chromatography-gas chromatography.

The migration of epoxidized soy bean oil (ESBO) from the gasket in the lids of glass jars into foods, particularly those rich in edible oil, often far exceeds the legal limit (60 mg/kg). ESBO was determined through a methyl ester isomer of diepoxy linoleic acid. Transesterification occurred directly in the homogenized food. From the extracted methyl esters, the diepoxy components were isolated by normal-phase LC and transferred on-line to gas chromatography with flame ionization detection using the on-column interface in the concurrent solvent evaporation mode. The method involves verification elements to ensure the reliability of the results for every sample analyzed. The detection limit is 2-5 mg/kg, depending on the food. Uncertainty of the procedure is below 10%.

Chromatography, Liquid↗
PubMed2001

Nucleotide release and associated conformational changes regulate function in the COOH-terminal Src kinase, Csk.

The COOH-terminal Src kinase (Csk) regulates a broad array of cellular processes via the specific phosphorylation and downregulation of Src family protein kinases. While Csk has been a topic for steady-state kinetic studies, the individual steps associated with substrate phosphorylation have not been investigated. To understand active-site phenomena, pre-steady-state and transient-state kinetic methods were applied to develop a catalytic pathway for substrate processing. Rapid quench flow techniques show that the phosphorylation of a substrate peptide, generated from a random library, occurs in two kinetic phases: a rapid, exponential "burst" phase followed by a slow, linear phase. The amplitude of the burst phase increases as a function of enzyme concentration, indicating that the biphasic kinetics are not the result of product inhibition. Analysis of the burst rate as a function of substrate concentration indicates that the phosphoryl transfer step is fast (k3 > or = 140 s(-1) and highly favorable (k3/k-3 > or = 6). The apparent dissociation rate constant for ADP (0.6 s(-1), measured using stopped-flow kinetic methods and a fluorescent trapping agent, mant-ATP, is close to kcat. Since the substrate dissociation constant is high, the release of phosphopeptide is not likely to limit turnover. These findings indicate that Csk rapidly delivers the gamma-phosphate of ATP to the substrate and rapidly releases the phosphoproduct. Overall rate limitation in the steady state is then attributed to the slow, net dissociation of ADP. Viscosometric studies suggest that this final event in the catalytic cycle is coupled with slow conformational changes.

Adenine Nucleotides↗
PubMed2004

[Speciation analysis of arsenic in traditional Chinese medicines by hydride generation-atomic fluorescence spectrometry with ion exchange resin separation].

A method for the determination of arsenic species (arsenide, arsenate) in traditional Chinese medicines using hydride generation atomic fluorescence spectrometry (HG-AFS) with ion exchange resin separation has been studied. The ion exchange resin was used to separate the arsenide and arsenate, and the separation conditions were investigated. The experimental conditions that influence the fluorescence intensity and the reduction of arsenic(V) were investigated and optimized, and the influence of foreign ions and its elimination were studied. The proposed method was applied to the determination of arsenic species in water leachate of traditional Chinese medicines. A detection limit of 89.1 ng x mL(-1) and a recovery range of 91.1%-109.3% were obtained.

Arsenates↗