Manuel Silva - Academia.edu (original) (raw)

Papers by Manuel Silva

Research paper thumbnail of Foreword

Research paper thumbnail of Thermochemical study of 1-, 3- and 4-piperidinecarboxamide derivatives

Thermochimica Acta, 2007

The standard (p • = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase... more The standard (p • = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, of three piperidinecarboxamide derivatives, namely 1-, 3-and 4-piperidinecarboxamide, were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by Calvet microcalorimenty, respectively.

Research paper thumbnail of Preface

Research paper thumbnail of Tutorial review?Recent strategies in automated reaction rate based determinations

The Analyst, 1993

... The instrumental set-up required is very simple and includes a peristaltic pump, the stop-and... more ... The instrumental set-up required is very simple and includes a peristaltic pump, the stop-and-go sequence of Driving syringes 0 bservati o n Stop ping Observation Air a, cell Time - t - m C DJ v) 0 .- Time - t - m C 0, .- v) 0 Time - Fig. ...

Research paper thumbnail of A Graduate Course on Active, Digital, and Switched Capacitor Filters

IEEE Transactions on Education, 2000

This paper presents a solution to the problem of accommodating the important new subject of switc... more This paper presents a solution to the problem of accommodating the important new subject of switched capacitor networks in already crowded electrical engineering curricula. This is achieved by combining in a one-semester course three different, but related, subjects. This is made possible 1) by placing a strong emphasis on the underlying principles that are common to active, digital, and switched capacitor filters, and 2) by concentrating on the realization of these three types of filters by simulation of passive ladder networks.

Research paper thumbnail of Use of SDS micelles for improving sensitivity, resolution, and speed in the analysis of β-lactam antibiotics in environmental waters by SPE and CE

ELECTROPHORESIS, 2007

This study dealt with the potential of MEKC with LIF detection involving derivatization with sulf... more This study dealt with the potential of MEKC with LIF detection involving derivatization with sulfoindocyanine succinimidyl ester (Cy5) for the separation and determination of beta-lactam antibiotics (ampicillin, amoxicillin, cephradine, and cephalexin) in environmental water samples. Water samples of 50 mL were enriched by SPE by passage through a weak base-cation Amberlite(R) IRA-93 exchange column. SDS micelles play important roles in the whole analytical process by improving the yield (sensitivity) and the kinetics of the labeling reaction, the elution of the retained antibiotics from the SPE preconcentration system and the electrophoretic resolution of their Cy5-derivatives. The optimum procedure includes a derivatization step of the antibiotics at 25 degrees C for 10 min and direct injection for MEKC analysis, which is conducted within about 15 min using 15 mM SDS in the running buffer (35 mM sodium borate at pH 9.3). LODs from 30 to 45 ng/L and RSDs (within-day precision) from 3.5 to 5.9% were obtained for the antibiotics in water samples with average recoveries ranging from 96.4 to 99.4%. These results indicate that the method proposed is a straightforward and sensitive tool for the determination of these antibiotics in environmental water samples providing similar quantitative results to those using more expensive equipment like LC-electrospray MS/MS.

Research paper thumbnail of In-capillary derivatization and analysis of amino acids, amino phosphonic acid-herbicides and biogenic amines by capillary electrophoresis with laser-induced fluorescence detection

ELECTROPHORESIS, 2002

This paper describes a general approach for the in-capillary derivatization of amino compounds an... more This paper describes a general approach for the in-capillary derivatization of amino compounds and the subsequent sensitive determination of the derivatives by micellar electrokinetic chromatography (MEKC) or capillary zone electrophoresis (CZE) with laser-induced fluorescence (LIF) detection. Amino acids, biogenic amines and amino phosphonic acid-herbicides were chosen as model analytes to evaluate the analytical potential of this approach. Fulfilment of the in-capillary reaction of the analytes using LIF detection hinged on the excellent labeling chemistry of 5-(4,6-dichloro-s-triazin-2-ylamino)fluorescein (DTAF) and the good resolution achieved in the separation of derivatized analytes. Careful optimization of the electrophoretic conditions in the mixing step of this protocol allowed the determination of amino acids, biogenic amines and phosphorus-containing amino acid-herbicides with concentration limits of detection at the nug/L level and relative standard deviations from 3.5 to 5.8%. The whole analysis is carried out within 20 min, resulting in a very simple, fast and practical approach for the fully automated analysis of amino acids and related compounds in low-volume and low-concentration samples.

Research paper thumbnail of Analytical potential of fluorescein analogues for ultrasensitive determinations of phosphorus-containing amino acid herbicides by micellar electrokinetic chromatography with laser-induced fluorescence detection

ELECTROPHORESIS, 2002

The analytical potential of three fluorescein analogues, fluorescein isothiocyanate isomer I (FIT... more The analytical potential of three fluorescein analogues, fluorescein isothiocyanate isomer I (FITC), 5-(4,6-dichlorotriazinylamino) fluorescein (DTAF) and 5(6)-carboxyfluorescein N-succinimidyl ester (CFSE), as labelling reagents for the ultrasensitive determination of phosphorus-containing amino acid herbicides (glufosinate and glyphosate) and aminomethylphosphonic acid (the major metabolite of glyphosate) by nonionic surfactant micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence (LIF) detection was investigated. Practical aspects related to label chemistry and MEKC separation showed that DTAF is the best choice for the determination of these herbicides; in addition, the most important features of these reagents for the derivatization of amino compounds are discussed. The optimum procedure includes a derivatization step of the herbicides at 40 degrees C with DTAF for 1 h and a 2-fold dilution prior to MEKC analysis, which is conducted within about 10 min using Brij-35 in the running buffer. This nonionic surfactant improves the selectivity and therefore the sensitivity of the method at low analyte concentrations by shifting the interfering peaks of the DTAF excess. The lowest detectable analyte concentration ranged from 0.06 to 0.16 microg/L with a precision of 2.1-3.2%. These results indicate that nonionic surfactant MEKC-LIF is useful as a selective, rapid and sensitive tool for the determination of these herbicides showing a great potential for their analysis in environmental samples without previous enrichment steps. The proposed method surpasses other chromatographic alternatives in terms of limit of detection and sample requirements for the analysis.

Research paper thumbnail of Multi-residue analysis ofN-methylcarbamate pesticides and their hydrolytic metabolites in environmental waters by use of solid-phase extraction and micellar electrokinetic chromatography

Electrophoresis, 1999

A method for the simultaneous separation and determination of N-methylcarbamate pesticides and th... more A method for the simultaneous separation and determination of N-methylcarbamate pesticides and their hydrolytic metabolites by micellar electrokinetic chromatography (MEKC) was developed. A mixture of five pesticides (carbaryl, propuxur, carbofuran, aminocarb, and methiocarb) and their corresponding phenols was studied to optimize the separation of its components in terms of various electrophoretic parameters such as buffer type, pH and concentration, sodium dodecyl sulfate concentration, injection conditions, and applied voltage. Excellent separation of all ten analytes was achieved within about 20 min. The optimized method was used for determinations in environmental water samples. Sample volumes of 250 mL were first preconcentrated in the pesticides and metabolites by passage through a LiChrolut EN sorbent column and then further enriched by on-column stacking. Dynamic ranges of 40 ng/L - 6 microg/L, limits of detection at the nanogram-per-liter level, and relative standard deviations from 2.6 to 7.4% were obtained. The proposed method surpasses high-performance liquid chromatography (HPLC) in separation efficiency. In fact, it provides more expeditious separations and allows more flexible adjustment of the selectivity. Also, it enables the quantification for the analytes studied in this work with decreased limits of detection.

Research paper thumbnail of Rapid determination of fungicides in fruit juices by micellar electrokinetic chromatography: Use of organic modifiers to enhance selectivity and on-column high-salt stacking to improve sensitivity

Electrophoresis, 2000

A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micel... more A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micellar electrokinetic chromatography (MEKC) was developed. Excellent separation of the eight fungicides (carbendazim, metalaxyl, captan, procymidone, folpet, captafol, vinclozolin and iprodione) is achieved within about 10 min by using optimized electrophoretic conditions that include the addition of a mixture of organic modifiers to the running buffer for improved resolution. The sensitivity of the method is enhanced by using an enrichment step that involves on-column high-salt stacking. Limits of detection in the microgram-per-liter region and relative standard deviations from 2.1 to 5.9% are thus obtained for the fungicides without detracting from peak resolution. These results reveal that the high-salt stacking method provides highly improved sensitivity and enables highly flexible adjustment of the selectivity of the separation method. Also, the method surpasses other stacking alternatives used in MEKC and affords routine analyses of fruit juice containing fungicides at trace levels following a straightforward sample treatment. The robustness of the high-salt stacking method as demonstrated in this work makes MEKC methods involving stacking procedures an attractive choice for routine analyses.

Research paper thumbnail of MEKC: An update focusing on practical aspects

ELECTROPHORESIS, 2007

This paper reviews recent methodological and instrumental advances in MEKC. Improvements in sensi... more This paper reviews recent methodological and instrumental advances in MEKC. Improvements in sensitivity arising from the use of on-line sample concentration (sweeping, stacking, and combination of both protocols) and derivatization (in-capillary reactions and coupling with flow-injection systems) and improvements in resolution obtained by changing the composition of the BGE (e.g., with organic modifiers, ionic liquids, nonionic and zwitterionic surfactants, mixed micelles, and vesicles) or using coated capillaries are discussed in detail. In addition, MS and LIF spectroscopy are examined in relation to their advantages and restrictions as applied to MEKC analysis. Some thoughts on potential future directions are also expressed.

Research paper thumbnail of Rapid determination of fungicides in fruit juices by micellar electrokinetic chromatography: Use of organic modifiers to enhance selectivity and on-column high-salt stacking to improve sensitivity

Electrophoresis, 2000

A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micel... more A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micellar electrokinetic chromatography (MEKC) was developed. Excellent separation of the eight fungicides (carbendazim, metalaxyl, captan, procymidone, folpet, captafol, vinclozolin and iprodione) is achieved within about 10 min by using optimized electrophoretic conditions that include the addition of a mixture of organic modifiers to the running buffer for improved resolution. The sensitivity of the method is enhanced by using an enrichment step that involves on-column high-salt stacking. Limits of detection in the microgram-per-liter region and relative standard deviations from 2.1 to 5.9% are thus obtained for the fungicides without detracting from peak resolution. These results reveal that the high-salt stacking method provides highly improved sensitivity and enables highly flexible adjustment of the selectivity of the separation method. Also, the method surpasses other stacking alternatives used in MEKC and affords routine analyses of fruit juice containing fungicides at trace levels following a straightforward sample treatment. The robustness of the high-salt stacking method as demonstrated in this work makes MEKC methods involving stacking procedures an attractive choice for routine analyses.

Research paper thumbnail of Micellar electrokinetic chromatography: Methodological and instrumental advances focused on practical aspects

Research paper thumbnail of Simultaneous determination of phosphorus-containing amino acid-herbicides by nonionic surfactant micellar electrokinetic chromatography with laser-induced fluorescence detection

ELECTROPHORESIS, 2001

The potential of micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence (L... more The potential of micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence (LIF) detection for the separation and determination of phosphorus-containing amino acid-herbicides (glufosinate and glyphosate), and aminomethylphosphonic acid (the major metabolite of glyphosate), involving derivatization with fluorescein isothiocyanate (FITC) isomer I, was investigated. Different variables that affect derivatization (pH, FITC concentration, time and temperature) and separation (pH and concentration of the buffer, kind and concentration of surfactants and applied voltage) were studied. The analysis was conducted within about 8 min and the use of the nonionic surfactant Triton X-100 improved the selectivity, thus indirectly enhancing sensitivity by shifting of the interfering peaks of the FITC excess. Dynamic ranges of 2.0-3,000 microg/L, limits of detection at microgram or submicrogram-per-liter level, and relative standard deviations from 4.7 to 6.4% were obtained. The ensuing method--nonionic surfactant MEKC-- is a useful choice for the determination of these herbicides as it provides limits of detection similar or lower than those reported by existing chromatographic alternatives without the use of an additional preconcentration technique such as solid-phase extraction. The separation of a mixture of nine FITC-derivatized amino acids, selected as target compounds, was also carried out to assess the discrimination power of the nonionic surfactant MEKC method for the analysis of closely related anionic analytes.

Research paper thumbnail of Steady-state performance evaluation of continuous mono-T-semiflow Petri nets

Automatica, 2005

The number of states in discrete event systems can increase exponentially with respect to the siz... more The number of states in discrete event systems can increase exponentially with respect to the size of the system. A way to face this state explosion problem consists of relaxing the system model, for example by converting it to a continuous one. In the scope of Petri nets, the firing of a transition in a continuous Petri net system is done in a real amount. Hence, the marking (state) of the net system becomes a vector of non-negative real numbers. The main contribution of the paper lies in the computation of throughput bounds for continuous Petri net systems with a single T-semiflow. For that purpose, a branch and bound algorithm is designed. Moreover, it can be relaxed and converted into a linear programming problem. Some conditions, under which the system always reaches the computed bounds, are extracted. The results related to the computation of the bounds can be directly applied to a larger class of nets called mono T-semiflow reducible.

Research paper thumbnail of Kinetic Determination of Nitrazepam in Tablets

Analytical Letters, 1992

A kinetic method for the determination of nitrazepam by the continuous addition of reagent (CAR) ... more A kinetic method for the determination of nitrazepam by the continuous addition of reagent (CAR) technique is reported. The method involves the formation of an azo dye between 1-naphthol and a diazonium salt that is in turn obtained by reaction between 2-amino-7-nitrobenzophenone (the hydrolysis product of the drug) and sodium nitrite. The reaction is monitored by measuring the rate of

Research paper thumbnail of Processing analytical data obtained from second-order reactions by using continuous reagent addition

Analytical Chemistry, 1992

Thk paper reporb a new approach to secondorder klnetlc analyses based on the contlnwm addHlon of ... more Thk paper reporb a new approach to secondorder klnetlc analyses based on the contlnwm addHlon of the reagent to the analyte rolutlon at a constant rate. A mathematkal model was dev.loped to study the klneth of the process. Methods for the determlnatlon of the analyte and the -der rate constant were developed and tested experbnentally. The Fe( I I)/SCN-system In the presence of hydrogen poroxlde as analyte was used as the model reactlon. The method$ for the determlnatlon of hydrogen peroxlde thus developed are compared by urlng a Ieast-squares ffl procedure; those used to obtain the wcondorder rate constant perform qulte well wtth relatlve standard devlatlona between 2 4 % .

Research paper thumbnail of Stopped-flow determination of paracetamol and oxyphenbutazone alone and in mixtures by a differential reaction rate method

Fresenius' Zeitschrift f�r Analytische Chemie, 1989

Research paper thumbnail of Chemiluminescence measurement of tertiary amines using the continuous-addition-of-reagent technique. Determination of trimethylamine in fish samples

Analytica Chimica Acta, 1997

Research paper thumbnail of A Simple and Fast Algorithm to Obtain All Invariants of a Generalized Petri Net

Application and Theory of Petri Nets, 1981

Google, Inc. (search). ...

Research paper thumbnail of Foreword

Research paper thumbnail of Thermochemical study of 1-, 3- and 4-piperidinecarboxamide derivatives

Thermochimica Acta, 2007

The standard (p • = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase... more The standard (p • = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, of three piperidinecarboxamide derivatives, namely 1-, 3-and 4-piperidinecarboxamide, were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by Calvet microcalorimenty, respectively.

Research paper thumbnail of Preface

Research paper thumbnail of Tutorial review?Recent strategies in automated reaction rate based determinations

The Analyst, 1993

... The instrumental set-up required is very simple and includes a peristaltic pump, the stop-and... more ... The instrumental set-up required is very simple and includes a peristaltic pump, the stop-and-go sequence of Driving syringes 0 bservati o n Stop ping Observation Air a, cell Time - t - m C DJ v) 0 .- Time - t - m C 0, .- v) 0 Time - Fig. ...

Research paper thumbnail of A Graduate Course on Active, Digital, and Switched Capacitor Filters

IEEE Transactions on Education, 2000

This paper presents a solution to the problem of accommodating the important new subject of switc... more This paper presents a solution to the problem of accommodating the important new subject of switched capacitor networks in already crowded electrical engineering curricula. This is achieved by combining in a one-semester course three different, but related, subjects. This is made possible 1) by placing a strong emphasis on the underlying principles that are common to active, digital, and switched capacitor filters, and 2) by concentrating on the realization of these three types of filters by simulation of passive ladder networks.

Research paper thumbnail of Use of SDS micelles for improving sensitivity, resolution, and speed in the analysis of β-lactam antibiotics in environmental waters by SPE and CE

ELECTROPHORESIS, 2007

This study dealt with the potential of MEKC with LIF detection involving derivatization with sulf... more This study dealt with the potential of MEKC with LIF detection involving derivatization with sulfoindocyanine succinimidyl ester (Cy5) for the separation and determination of beta-lactam antibiotics (ampicillin, amoxicillin, cephradine, and cephalexin) in environmental water samples. Water samples of 50 mL were enriched by SPE by passage through a weak base-cation Amberlite(R) IRA-93 exchange column. SDS micelles play important roles in the whole analytical process by improving the yield (sensitivity) and the kinetics of the labeling reaction, the elution of the retained antibiotics from the SPE preconcentration system and the electrophoretic resolution of their Cy5-derivatives. The optimum procedure includes a derivatization step of the antibiotics at 25 degrees C for 10 min and direct injection for MEKC analysis, which is conducted within about 15 min using 15 mM SDS in the running buffer (35 mM sodium borate at pH 9.3). LODs from 30 to 45 ng/L and RSDs (within-day precision) from 3.5 to 5.9% were obtained for the antibiotics in water samples with average recoveries ranging from 96.4 to 99.4%. These results indicate that the method proposed is a straightforward and sensitive tool for the determination of these antibiotics in environmental water samples providing similar quantitative results to those using more expensive equipment like LC-electrospray MS/MS.

Research paper thumbnail of In-capillary derivatization and analysis of amino acids, amino phosphonic acid-herbicides and biogenic amines by capillary electrophoresis with laser-induced fluorescence detection

ELECTROPHORESIS, 2002

This paper describes a general approach for the in-capillary derivatization of amino compounds an... more This paper describes a general approach for the in-capillary derivatization of amino compounds and the subsequent sensitive determination of the derivatives by micellar electrokinetic chromatography (MEKC) or capillary zone electrophoresis (CZE) with laser-induced fluorescence (LIF) detection. Amino acids, biogenic amines and amino phosphonic acid-herbicides were chosen as model analytes to evaluate the analytical potential of this approach. Fulfilment of the in-capillary reaction of the analytes using LIF detection hinged on the excellent labeling chemistry of 5-(4,6-dichloro-s-triazin-2-ylamino)fluorescein (DTAF) and the good resolution achieved in the separation of derivatized analytes. Careful optimization of the electrophoretic conditions in the mixing step of this protocol allowed the determination of amino acids, biogenic amines and phosphorus-containing amino acid-herbicides with concentration limits of detection at the nug/L level and relative standard deviations from 3.5 to 5.8%. The whole analysis is carried out within 20 min, resulting in a very simple, fast and practical approach for the fully automated analysis of amino acids and related compounds in low-volume and low-concentration samples.

Research paper thumbnail of Analytical potential of fluorescein analogues for ultrasensitive determinations of phosphorus-containing amino acid herbicides by micellar electrokinetic chromatography with laser-induced fluorescence detection

ELECTROPHORESIS, 2002

The analytical potential of three fluorescein analogues, fluorescein isothiocyanate isomer I (FIT... more The analytical potential of three fluorescein analogues, fluorescein isothiocyanate isomer I (FITC), 5-(4,6-dichlorotriazinylamino) fluorescein (DTAF) and 5(6)-carboxyfluorescein N-succinimidyl ester (CFSE), as labelling reagents for the ultrasensitive determination of phosphorus-containing amino acid herbicides (glufosinate and glyphosate) and aminomethylphosphonic acid (the major metabolite of glyphosate) by nonionic surfactant micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence (LIF) detection was investigated. Practical aspects related to label chemistry and MEKC separation showed that DTAF is the best choice for the determination of these herbicides; in addition, the most important features of these reagents for the derivatization of amino compounds are discussed. The optimum procedure includes a derivatization step of the herbicides at 40 degrees C with DTAF for 1 h and a 2-fold dilution prior to MEKC analysis, which is conducted within about 10 min using Brij-35 in the running buffer. This nonionic surfactant improves the selectivity and therefore the sensitivity of the method at low analyte concentrations by shifting the interfering peaks of the DTAF excess. The lowest detectable analyte concentration ranged from 0.06 to 0.16 microg/L with a precision of 2.1-3.2%. These results indicate that nonionic surfactant MEKC-LIF is useful as a selective, rapid and sensitive tool for the determination of these herbicides showing a great potential for their analysis in environmental samples without previous enrichment steps. The proposed method surpasses other chromatographic alternatives in terms of limit of detection and sample requirements for the analysis.

Research paper thumbnail of Multi-residue analysis ofN-methylcarbamate pesticides and their hydrolytic metabolites in environmental waters by use of solid-phase extraction and micellar electrokinetic chromatography

Electrophoresis, 1999

A method for the simultaneous separation and determination of N-methylcarbamate pesticides and th... more A method for the simultaneous separation and determination of N-methylcarbamate pesticides and their hydrolytic metabolites by micellar electrokinetic chromatography (MEKC) was developed. A mixture of five pesticides (carbaryl, propuxur, carbofuran, aminocarb, and methiocarb) and their corresponding phenols was studied to optimize the separation of its components in terms of various electrophoretic parameters such as buffer type, pH and concentration, sodium dodecyl sulfate concentration, injection conditions, and applied voltage. Excellent separation of all ten analytes was achieved within about 20 min. The optimized method was used for determinations in environmental water samples. Sample volumes of 250 mL were first preconcentrated in the pesticides and metabolites by passage through a LiChrolut EN sorbent column and then further enriched by on-column stacking. Dynamic ranges of 40 ng/L - 6 microg/L, limits of detection at the nanogram-per-liter level, and relative standard deviations from 2.6 to 7.4% were obtained. The proposed method surpasses high-performance liquid chromatography (HPLC) in separation efficiency. In fact, it provides more expeditious separations and allows more flexible adjustment of the selectivity. Also, it enables the quantification for the analytes studied in this work with decreased limits of detection.

Research paper thumbnail of Rapid determination of fungicides in fruit juices by micellar electrokinetic chromatography: Use of organic modifiers to enhance selectivity and on-column high-salt stacking to improve sensitivity

Electrophoresis, 2000

A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micel... more A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micellar electrokinetic chromatography (MEKC) was developed. Excellent separation of the eight fungicides (carbendazim, metalaxyl, captan, procymidone, folpet, captafol, vinclozolin and iprodione) is achieved within about 10 min by using optimized electrophoretic conditions that include the addition of a mixture of organic modifiers to the running buffer for improved resolution. The sensitivity of the method is enhanced by using an enrichment step that involves on-column high-salt stacking. Limits of detection in the microgram-per-liter region and relative standard deviations from 2.1 to 5.9% are thus obtained for the fungicides without detracting from peak resolution. These results reveal that the high-salt stacking method provides highly improved sensitivity and enables highly flexible adjustment of the selectivity of the separation method. Also, the method surpasses other stacking alternatives used in MEKC and affords routine analyses of fruit juice containing fungicides at trace levels following a straightforward sample treatment. The robustness of the high-salt stacking method as demonstrated in this work makes MEKC methods involving stacking procedures an attractive choice for routine analyses.

Research paper thumbnail of MEKC: An update focusing on practical aspects

ELECTROPHORESIS, 2007

This paper reviews recent methodological and instrumental advances in MEKC. Improvements in sensi... more This paper reviews recent methodological and instrumental advances in MEKC. Improvements in sensitivity arising from the use of on-line sample concentration (sweeping, stacking, and combination of both protocols) and derivatization (in-capillary reactions and coupling with flow-injection systems) and improvements in resolution obtained by changing the composition of the BGE (e.g., with organic modifiers, ionic liquids, nonionic and zwitterionic surfactants, mixed micelles, and vesicles) or using coated capillaries are discussed in detail. In addition, MS and LIF spectroscopy are examined in relation to their advantages and restrictions as applied to MEKC analysis. Some thoughts on potential future directions are also expressed.

Research paper thumbnail of Rapid determination of fungicides in fruit juices by micellar electrokinetic chromatography: Use of organic modifiers to enhance selectivity and on-column high-salt stacking to improve sensitivity

Electrophoresis, 2000

A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micel... more A rapid, reliable method for the multiresidue analysis of eight commonly used fungicides by micellar electrokinetic chromatography (MEKC) was developed. Excellent separation of the eight fungicides (carbendazim, metalaxyl, captan, procymidone, folpet, captafol, vinclozolin and iprodione) is achieved within about 10 min by using optimized electrophoretic conditions that include the addition of a mixture of organic modifiers to the running buffer for improved resolution. The sensitivity of the method is enhanced by using an enrichment step that involves on-column high-salt stacking. Limits of detection in the microgram-per-liter region and relative standard deviations from 2.1 to 5.9% are thus obtained for the fungicides without detracting from peak resolution. These results reveal that the high-salt stacking method provides highly improved sensitivity and enables highly flexible adjustment of the selectivity of the separation method. Also, the method surpasses other stacking alternatives used in MEKC and affords routine analyses of fruit juice containing fungicides at trace levels following a straightforward sample treatment. The robustness of the high-salt stacking method as demonstrated in this work makes MEKC methods involving stacking procedures an attractive choice for routine analyses.

Research paper thumbnail of Micellar electrokinetic chromatography: Methodological and instrumental advances focused on practical aspects

Research paper thumbnail of Simultaneous determination of phosphorus-containing amino acid-herbicides by nonionic surfactant micellar electrokinetic chromatography with laser-induced fluorescence detection

ELECTROPHORESIS, 2001

The potential of micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence (L... more The potential of micellar electrokinetic chromatography (MEKC) with laser-induced fluorescence (LIF) detection for the separation and determination of phosphorus-containing amino acid-herbicides (glufosinate and glyphosate), and aminomethylphosphonic acid (the major metabolite of glyphosate), involving derivatization with fluorescein isothiocyanate (FITC) isomer I, was investigated. Different variables that affect derivatization (pH, FITC concentration, time and temperature) and separation (pH and concentration of the buffer, kind and concentration of surfactants and applied voltage) were studied. The analysis was conducted within about 8 min and the use of the nonionic surfactant Triton X-100 improved the selectivity, thus indirectly enhancing sensitivity by shifting of the interfering peaks of the FITC excess. Dynamic ranges of 2.0-3,000 microg/L, limits of detection at microgram or submicrogram-per-liter level, and relative standard deviations from 4.7 to 6.4% were obtained. The ensuing method--nonionic surfactant MEKC-- is a useful choice for the determination of these herbicides as it provides limits of detection similar or lower than those reported by existing chromatographic alternatives without the use of an additional preconcentration technique such as solid-phase extraction. The separation of a mixture of nine FITC-derivatized amino acids, selected as target compounds, was also carried out to assess the discrimination power of the nonionic surfactant MEKC method for the analysis of closely related anionic analytes.

Research paper thumbnail of Steady-state performance evaluation of continuous mono-T-semiflow Petri nets

Automatica, 2005

The number of states in discrete event systems can increase exponentially with respect to the siz... more The number of states in discrete event systems can increase exponentially with respect to the size of the system. A way to face this state explosion problem consists of relaxing the system model, for example by converting it to a continuous one. In the scope of Petri nets, the firing of a transition in a continuous Petri net system is done in a real amount. Hence, the marking (state) of the net system becomes a vector of non-negative real numbers. The main contribution of the paper lies in the computation of throughput bounds for continuous Petri net systems with a single T-semiflow. For that purpose, a branch and bound algorithm is designed. Moreover, it can be relaxed and converted into a linear programming problem. Some conditions, under which the system always reaches the computed bounds, are extracted. The results related to the computation of the bounds can be directly applied to a larger class of nets called mono T-semiflow reducible.

Research paper thumbnail of Kinetic Determination of Nitrazepam in Tablets

Analytical Letters, 1992

A kinetic method for the determination of nitrazepam by the continuous addition of reagent (CAR) ... more A kinetic method for the determination of nitrazepam by the continuous addition of reagent (CAR) technique is reported. The method involves the formation of an azo dye between 1-naphthol and a diazonium salt that is in turn obtained by reaction between 2-amino-7-nitrobenzophenone (the hydrolysis product of the drug) and sodium nitrite. The reaction is monitored by measuring the rate of

Research paper thumbnail of Processing analytical data obtained from second-order reactions by using continuous reagent addition

Analytical Chemistry, 1992

Thk paper reporb a new approach to secondorder klnetlc analyses based on the contlnwm addHlon of ... more Thk paper reporb a new approach to secondorder klnetlc analyses based on the contlnwm addHlon of the reagent to the analyte rolutlon at a constant rate. A mathematkal model was dev.loped to study the klneth of the process. Methods for the determlnatlon of the analyte and the -der rate constant were developed and tested experbnentally. The Fe( I I)/SCN-system In the presence of hydrogen poroxlde as analyte was used as the model reactlon. The method$ for the determlnatlon of hydrogen peroxlde thus developed are compared by urlng a Ieast-squares ffl procedure; those used to obtain the wcondorder rate constant perform qulte well wtth relatlve standard devlatlona between 2 4 % .

Research paper thumbnail of Stopped-flow determination of paracetamol and oxyphenbutazone alone and in mixtures by a differential reaction rate method

Fresenius' Zeitschrift f�r Analytische Chemie, 1989

Research paper thumbnail of Chemiluminescence measurement of tertiary amines using the continuous-addition-of-reagent technique. Determination of trimethylamine in fish samples

Analytica Chimica Acta, 1997

Research paper thumbnail of A Simple and Fast Algorithm to Obtain All Invariants of a Generalized Petri Net

Application and Theory of Petri Nets, 1981

Google, Inc. (search). ...