Fast and reliable BIA/amperometric quantification of acetylcysteine using a nanostructured double hydroxide sensor.

This study reports the preparation and characterization of nickel/lead hydroxide nanoparticles used to construct electrochemical sensors, which were investigated for amperometric quantification of N-acetylcysteine (NAC). The newly synthesised material presents good uniformity, with the lead (II) ions homogenously incorporated into the alpha nickel hydroxide crystal structure, confirmed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy analyses. Films of nanoparticles (3 nm in size) were prepared on conductive fluorine-doped tin oxide-coated glass slides and used connected to a specially built batch injection analysis (BIA) cell with a capacity of only 4 mL and the electrode positioned in the bottom. To attain optimal analytical performance, the main parameters for BIA measurements (volume injected, different velocities of injection and best distance of the pipette from the electrode) were evaluated, as was the working potential, to determine the optimal conditions. Linear responses were obtained for the concentration range from 20 to 220 µmol L-1, and the limits of detection (3σ/slope) and quantification (10σ/slope) were calculated as 0.23 µmol L-1 and 0.70 µmol L-1, respectively. The new NAC sensor does not exhibit a memory effect and has enormous potential utility in the quantitative determination of N-acetylcysteine in drugs. The results of the analysis of NAC obtained using BIA presented good concordance with those obtained by chromatography. The analytical frequency attained using BIA (120 analysis h-1) compares very favourably with the one obtained using chromatography (6 analysis h-1).

A flow-based procedure exploiting the lab-in-syringe approach for the determination of ester content in biodiesel and diesel/biodiesel blends.

The ester content is an important parameter to be monitored in biodiesel for evaluation of the transesterification reaction yield and for assessing the purity of the final product. This is also a relevant quality parameter in diesel/biodiesel blends to avoid frauds, because legislation establishes a minimum amount of biodiesel to be added to diesel. The official method EN14103 requires the addition of an alternative internal standard (methyl nonadecanoate) for analysis of biodiesel from bovine tallow because the methyl heptadecanoate is found in high amounts in this product. In this work, it is proposed a fast, simple, practical, and environmental friendly flow-based spectrophotometric procedure, which exploits the formation of the violet complex between Fe(III) and the hydroxamate generated by the reactions of the alkyl esters with hydroxylamine. All involved steps are carried out inside the syringe pump of a sequential injection analyzer (lab-in-syringe approach). A single phase is attained by using ethanol as mediator solvent between the organic sample and aqueous soluble reagents. Linear responses for biodiesel samples and diesel/biodiesel blends were obtained from 4-99%(v/v) to 2.0-40%(v/v) methyl esters, described by the equations: A = 0.342 + 0.00305C (r = 0.997) and A = 0.174 + 0.00503C (r = 0.999), respectively. The analytical curve can be obtained by in-line dilution of a methyl linoleate stock solution. For biodiesel samples, the coefficient of variation (n = 10), limit of detection (99.7% confidence level), and sampling rate were estimated at 0.8%, 0.36%(v/v), and 15h-1, respectively, whereas the corresponding values for the blend samples were 0.20%, 0.03%(v/v), and 12h-1, respectively. The procedure consumes only 860µg of hydroxylamine, 366µg of Fe2(SO4)3·H2O, and 2.0mL ethanol and generates ca. 3.0mL of residue per determination. The results agreed with those obtained by official methods EN14103/2011 e EN14078, at the 95% confidence level.

Ultrasound-assisted dispersive liquid-liquid microextraction of tetracycline drugs from egg supplements before flow injection analysis coupled to a liquid waveguide capillary cell.

A simple, rapid, and efficient ultrasound-assisted dispersive liquid-liquid microextraction (US-DLLME) method was developed for extraction of tetracycline residues from egg supplement samples, with subsequent determination by flow injection analysis (FIA) coupled to a liquid waveguide capillary cell (LWCC) and a controlled temperature heating bath. Tetracyclines react with diazotized p-sulfanilic acid, in a slightly alkaline medium, to form azo compounds that can be measured at 435 nm. The reaction sensitivity improved substantially (5.12-fold) using an in-line heating temperature of 45 °C. Multivariate methodology was used to optimize the factors affecting the extraction efficiency, considering the volumes of extraction and disperser solvents, sonication time, extraction time, and centrifugation time. Good linearity in the range 30-600 µg L(-1) was obtained for all the tetracyclines, with regression coefficients (r) higher than 0.9974. The limits of detection ranged from 6.4 to 11.1 µg L(-1), and the recoveries were in the range 85.7-96.4 %, with relative standard deviation lower than 9.8 %. Analyte recovery was improved by approximately 6 % when the microextraction was assisted by ultrasound. The results obtained with the proposed US-DLLME-FIA method were confirmed by a reference HPLC method and showed that the egg supplement samples analyzed were suitable for human consumption.

Pulsed flows in flow analysis: Potentialities, limitations and applications.

In flow analysis, use of a steady and pulseless flow was considered essential for ensuring a reproducible handling of the flowing sample. To this end, peristaltic and syringe pumps have been the propelling device in the vast majority of the flow analysers. Recently, the number of applications involving pulsed flow has been increasing. Most of them refer to use of solenoid pumps, the essence of the so-called multi-pumping flow systems. This review critically discusses the characteristics, potentialities and limitations of the pulsed flow systems, emphasizing the main advantageous characteristics of the streams involved, such as high radial mass transference and good mixing of the fluids. Diverse contributions ranging from instrumentation development to analytical applications are presented.

Evaluation of sequential injection chromatography for reversed phase separation of triazine herbicides exploiting monolithic and core-shell columns.

This paper describes the development of reversed phase sequential injection chromatography (SIC) methods for separation of simazine (SIM) and atrazine (AT), as well as their metabolites deethylatrazine (DEA), deisopropylatrazine (DIA) and hydroxyatrazine (HAT) exploiting silica based monolithic (50 × 4.6 mm) and core-shell (30 × 4.6 mm, 2.7 µm particles) columns. The separation was made by stepwise elution with two mobile phases: MP1 composed of 15:85 (v/v) acetonitrile: 2.5 mmol L(-1) acetic acid/ammonium acetate buffer (pH 4.2), and MP2, composed of 35:65 (v/v) acetonitrile: 2.5 mmol L(-1) acetic acid/ammonium acetate buffer (pH 4.2).The less hydrophobic compounds (DIA, HAT and DEA) eluted with MP1, whereas SIM and AT eluted with MP2. The method using core-shell column exhibited better chromatographic efficiency compared with monolithic column for separation of SIM and AT, but failed to provide base line separation of DIA and HAT. The proposed composition of mobile phases enabled the monolithic column to separate all the studied compounds with resolution >2.3 at flow rate of 35 µL s(-1) and sampling throughput of 8 analyses per hour, whereas in the core-shell the maximum flow rate allowed in the SIC system was 8 µL s(-1) (sampling throughput of 3 analyses per hour). The limits of detection were between 24 µg L(-1) (AT) and 40 µg L(-1) (DEA) using the monolithic column, and between 20 µg L(-1) (SIM) and 38 µg L(-1) (DEA) with the core-shell. Ultrasound-assisted extraction (80:20 v/v acetonitrile:water) of a soil sample enriched with the five triazines (250, 500 and 1000 µg kg(-1)) resulted recoveries between 51% and 121% of the spiked concentrations.

Automation of 99Tc extraction by LOV prior ICP-MS detection: application to environmental samples.

A new, fast, automated and inexpensive sample pre-treatment method for (99)Tc determination by inductively coupled plasma-mass spectrometry (ICP-MS) detection is presented. The miniaturized approach is based on a lab-on-valve (LOV) system, allowing automatic separation and preconcentration of (99)Tc. Selectivity is provided by the solid phase extraction system used (TEVA resin) which retains selectively pertechnetate ion in diluted nitric acid solution. The proposed system has some advantages such as minimization of sample handling, reduction of reagents volume, improvement of intermediate precision and sample throughput, offering a significant decrease of both time and cost per analysis in comparison to other flow techniques and batch methods. The proposed LOV system has been successfully applied to different samples of environmental interest (water and soil) with satisfactory recoveries, between 94% and 98%. The detection limit (LOD) of the developed method is 0.005 ng. The high durability of the resin and its low amount (32 mg), its good intermediate precision (RSD 3.8%) and repeatability (RSD 2%) and its high extraction frequency (up to 5 h(-1)) makes this method an inexpensive, high precision and fast tool for monitoring (99)Tc in environmental samples.

On-line hyphenation of solid-phase extraction to chromatographic separation of sulfonamides with fused-core columns in sequential injection chromatography.

On-line sample pretreatment (clean-up and analyte preconcentration) is for the first time coupled to sequential injection chromatography. The approach combines anion-exchange solid-phase extraction and the highly effective pentafluorophenylpropyl (F5) fused-core particle column for separation of eight sulfonamide antibiotics with similar structures (sulfathiazole, sulfanilamide, sulfacetamide, sulfadiazine, sulfamerazine, sulfadimidine, sulfamethoxazole and sulfadimethoxine). The stationary phase was selected after a critical comparison of the performance achieved by three fused-core reversed phase columns (Ascentis(®) Express RP-Amide, Phenyl-Hexyl, and F5) and two monolithic columns (Chromolith(®) High Resolution RP-18 and CN). Acetonitrile and acetate buffer pH 5.0 at 0.60 mL min(-1) were used as mobile phase to perform the separations before spectrophotometric detection. The first mobile phase was successfully used as eluent from SPE column ensuring transfer of a narrow zone to the chromatographic column. Enrichment factors up to 39.2 were achieved with a 500 µL sample volume. The developed procedure showed analysis time <10.5 min, resolutions >1.83 with peak symmetry ≤1.52, LODs between 4.9 and 27 µg L(-1), linear response ranges from 30.0 to 1000.0 µg L(-1) (r(2)>0.996) and RSDs of peak heights <2.9% (n=6) at a 100 µg L(-1) level and enabled the screening control of freshwater samples contaminated at the 100 µg L(-1) level. The proposed approach expanded the analytical potentiality of SIC and avoided the time-consuming batch sample pretreatment step, thus minimizing risks of sample contamination and analyte losses.

Development of a flow-injection analysis system with fluorescence detection for gatifloxacin determination in organized medium.

This work reports the development and optimization of a flow injection analysis system with fluorescence detection (FIA-FLUO) for gatifloxacin (GFX) determination in organized medium. The analytical system was based on the enhanced fluorescence of gatifloxacin in micellar medium containing sodium dodecyl sulfate (SDS) at pH 6.0. The influence of physical (carrier flow rate, sample volume and volume of reaction coil) and chemical (pH, concentration of buffer and concentration of SDS) parameters that could affect the performance of the FIA system was evaluated in order to reach optimum conditions in terms of sensitivity and analytical throughput. Under optimized conditions, the FIA-FLUO system allowed the injection of 40 samples per hour with a limit of quantification of 72 µg/L and a RSD of 3.5% at 0.20 mg/L. Real samples of commercial pharmaceutical formulations containing GFX were analyzed, and no statistical difference was observed between the results obtained using the developed system and those obtained using the reference method based on high-performance liquid chromatography with UV detection.

Sensitive flow-injection spectrophotometric analysis of bromopride.

A flow injection spectrophotometric procedure employing merging zones is proposed for direct bromopride determination in pharmaceutical formulations and biological fluids. The proposed method is based on the reaction between bromopride and p-dimethylaminocinnamaldehyde (p-DAC) in acid medium, in the presence of sodium dodecyl sulfate (SDS), resulting in formation of a violet product (λmax=565nm). Experimental design methodologies were used to optimize the experimental conditions. The Beer-Lambert law was obeyed in a bromopride concentration range of 3.63×10(-7) to 2.90×10(-5)molL(-1), with a correlation coefficient (r) of 0.9999. The limits of detection and quantification were 1.07×10(-7) and 3.57×10(-7)molL(-1), respectively. The proposed method was successfully applied to the determination of bromopride in pharmaceuticals and human urine, and recoveries of the drug from these media were in the ranges 99.6-101.2% and 98.6-102.1%, respectively. This new flow injection procedure does not require any sample pretreatment steps.

A spectrophotometric flow injection system for streptomycin determination in veterinary samples.

In this work a spectrophotometric flow injection analysis system for streptomycin determination in veterinary samples, is being proposed. The method is based on streptomycin alkaline hydrolysis that forms guanidine, followed by the reaction with Fe(II). The colored product has absorption peak at 520 nm. To evaluate and optimize the system parameters, chemometrics tools, such as factorial design, Pareto chart and Doelhert design, were used. The veterinary samples are diluted in water and introduced in the FIA system, therefore no sample preparation is required. The optimized system presented: linear range of 60 up to 1000 mg L(-1), limit of detection of 18 mg L(-1) and sampling rate of 36 readings per hour. The precision was checked and the CV for veterinary sample readings were always less than 6.5%. The accuracy was studied by comparison with chromatographic method, thus, five samples of pharmaceutical veterinary were determined by HPLC and by the proposed method, and the results are in agreement (t-test, p=0.05).

On-line solid-phase extraction using a C18 minicolumn coupled to a flow injection system for determination of caffeine in green and roasted coffee beans.

A novel and fast method based on the solid phase extraction (SPE) coupled to a flow injection system for the determination of caffeine in coffee beans was developed. The caffeine extraction of coffee beans was carried out with hot water. A C18 reverse-phase mini-column was coupled to a continuous flow manifold to carry out the on-line SPE and the quantification of caffeine from aqueous extracts. Column length, retention time, elution volume, extracting solution and injection volume were evaluated. The retention time was of 90s and the elution was carried out with 400 µL of a methanol:water mixture (25:75). The proposed on-line SPE was compared against a chloroform extraction from aqueous extracts. With the proposed method the sample preparation was minimised and the sample throughput was increased (10 determination/h) because no dilution was required. Green coffee beans and beans with different roasting degree were analyzed.

On-line monitoring of the photocatalytic degradation of 2,4-D and dicamba using a solid-phase extraction-multisyringe flow injection system.

A fully automated on-line system for monitoring the photocatalytic degradation of herbicides was developed using multisyringe flow injection analysis (MSFIA) coupled to a solid phase extraction (SPE) unit with UV detection. The calibration curves were linear in the concentration range of 100-1000 µg L(-1) for 3,6-dichloro-2-methoxybenzoic acid (dicamba) and 500-3000 µg L(-1) for 2,4-dichlorophenoxyacetic acid (2,4-D), while the detection limits were 30 and 135 µg L(-1) for dicamba and 2,4-D, respectively. The monitoring of the photocatalytic degradation (TiO2 anatase/UV 254 nm) of these two herbicides was performed by MSFIA-SPE system using a small sample volume (2 mL) in a fully automated approach. The degradation was assessed in ultrapure and drinking water with initial concentrations of 1000 and 2000 µg L(-1) for dicamba and 2,4-D, respectively. Degradation percentages of approximately 85% were obtained for both herbicides in ultrapure water after 45 min of photocatalytic treatment. A similar degradation efficiency in drinking water was observed for 2,4-D, whereas dicamba exhibited a lower degradation percentage (75%), which could be attributed to the presence of inorganic species in this kind of water.

A monosegmented flow-batch system for slow reaction kinetics: spectrophotometric determination of boron in plants.

This work introduces the monosegmented flow-batch (MSFB) analysis concept. This system combines favourable characteristics of both flow-batch and the monosegmented analysers, allowing use of the flow-batch system for slow reaction kinetics without impairing sensitivity or sampling throughput. The MSFB was evaluated during spectrophotometric determination of boron in plant extracts, which is a method that involves a slow reaction between boron and azomethine-H. All calibration solutions were prepared in-line, and all analytical processes completed by simply changing the operational parameters in the MSFB control software. The limit of detection was estimated at 0.008 mg L(-1). The measurements could be performed at a rate of 120 samples per hour with satisfactory precision. The proposed MSFB was successfully applied to analyse 10 plant samples and the results are in agreement with the reference method at a 95% level of confidence.

Fast batch injection analysis system for on-site determination of ethanol in gasohol and fuel ethanol.

A simple, accurate and fast (180 injections h(-1)) batch injection analysis (BIA) system with multiple-pulse amperometric detection has been developed for selective determination of ethanol in gasohol and fuel ethanol. A sample aliquot (100 µL) was directly injected onto a gold electrode immersed in 0.5 mol L(-1) NaOH solution (unique reagent). The proposed BIA method requires minimal sample manipulation and can be easily used for on-site analysis. The results obtained with the BIA method were compared to those obtained by gas-chromatography and similar results were obtained (at 95% of confidence level).

Green chemistry and the evolution of flow analysis. A review.

Flow analysis has achieved its majority as a well-established tool to solve analytical problems. Evolution of flow-based approaches has been analyzed by diverse points of view, including historical aspects, the commutation concept and the impact on analytical methodologies. In this overview, the evolution of flow analysis towards green analytical chemistry is demonstrated by comparing classical procedures implemented with different flow approaches. The potential to minimize reagent consumption and waste generation and the ability to implement processes unreliable in batch to replace toxic chemicals are also emphasized. Successful applications of greener approaches in flow analysis are also discussed, focusing on the last 10 years.

Determination of glyphosate in water samples by multi-pumping flow system coupled to a liquid waveguide capillary cell.

A simple screening method was developed for the determination of glyphosate in water samples using a multi-pumping flow system. The proposed method is based on the reaction between glyphosate and p-dimethylaminocinnamaldehyde (p-DAC), in an acid medium where the reaction product can be measured spectrophotometrically at λ(max) = 495 nm. An experimental design methodology was used to optimize the measurement conditions. The proposed method was applied to the determination of glyphosate in water samples in a concentration range from 0.5 to 10 µg mL(-1). The limit of detection and quantification were 0.17 and 0.53 µg mL(-1), respectively. The results obtained (88.5 to 104.5%) in recovery studies for the determination of glyphosate in different water samples indicated good accuracy and no matrix effect for the developed method. Samples were also analyzed by a confirmatory HPLC method, and agreement within the two set of results was found.

Sequential spectrofluorimetric determination of free and total glycerol in biodiesel in a multicommuted flow system.

A new procedure for spectrofluorimetric determination of free and total glycerol in biodiesel samples is presented. It is based on the oxidation of glycerol by periodate, forming formaldehyde, which reacts with acetylacetone, producing the luminescent 3,5-diacetyl-1,4-dihydrolutidine. A flow system with solenoid micro-pumps is proposed for solution handling. Free glycerol was extracted off-line from biodiesel samples with water, and total glycerol was converted to free glycerol by saponification with sodium ethylate under sonication. For free glycerol, a linear response was observed from 5 to 70 mg L(-1) with a detection limit of 0.5 mg L(-1), which corresponds to 2 mg kg(-1) in biodiesel. The coefficient of variation was 0.9% (20 mg L(-1), n = 10). For total glycerol, samples were diluted on-line, and the linear response range was 25 to 300 mg L(-1). The detection limit was 1.4 mg L(-1) (2.8 mg kg(-1) in biodiesel) with a coefficient of variation of 1.4% (200 mg L(-1), n = 10). The sampling rate was ca. 35 samples h(-1) and the procedure was applied to determination of free and total glycerol in biodiesel samples from soybean, cottonseed, and castor beans.

Fast batch injection analysis of H(2)O(2) using an array of Pt-modified gold microelectrodes obtained from split electronic chips.

A fast and robust analytical method for amperometric determination of hydrogen peroxide (H(2)O(2)) based on batch injection analysis (BIA) on an array of gold microelectrodes modified with platinum is proposed. The gold microelectrode array (n=14) was obtained from electronic chips developed for surface mounted device technology (SMD), whose size offers advantages to adapt them in batch cells. The effect of the dispensing rate, volume injected, distance between the platinum microelectrodes and the pipette tip, as well as the volume of solution in the cell on the analytical response were evaluated. The method allows the H(2)O(2) amperometric determination in the concentration range from 0.8 µmolL(-1) to 100 µmolL(-1). The analytical frequency can attain 300 determinations per hour and the detection limit was estimated in 0.34 µmolL(-1) (3σ). The anodic current peaks obtained after a series of 23 successive injections of 50 µL of 25 µmolL(-1) H(2)O(2) showed an RSD<0.9%. To ensure the good selectivity to detect H(2)O(2), its determination was performed in a differential mode, with selective destruction of the H(2)O(2) with catalase in 10 mmolL(-1) phosphate buffer solution. Practical application of the analytical procedure involved H(2)O(2) determination in rainwater of São Paulo City. A comparison of the results obtained by the proposed amperometric method with another one which combines flow injection analysis (FIA) with spectrophotometric detection showed good agreement.

A flow injection procedure based on solenoid micro-pumps for spectrophotometric determination of free glycerol in biodiesel.

A flow system designed with solenoid micro-pumps is proposed for fast and greener spectrophotometric determination of free glycerol in biodiesel. Glycerol was extracted from samples without using organic solvents. The determination involves glycerol oxidation by periodate, yielding formaldehyde followed by formation of the colored (3,5-diacetil-1,4-dihidrolutidine) product upon reaction with acetylacetone. The coefficient of variation, sampling rate and detection limit were estimated as 1.5% (20.0 mg L(-1) glycerol, n=10), 34 h(-1), and 1.0 mg L(-1) (99.7% confidence level), respectively. A linear response was observed from 5 to 50 mg L(-1), with reagent consumption estimated as 345 µg of KIO(4) and 15 mg of acetylacetone per determination. The procedure was successfully applied to the analysis of biodiesel samples and the results agreed with the batch reference method at the 95% confidence level.

A computer-controlled apparatus for micrometric drop deposition at liquid surfaces.

A low-cost, automated apparatus has been used to perform micrometric deposition of small pendant drops onto a quiet liquid surface. The approach of the drop to the surface is obtained by means of discrete, micron-scale translations in order to achieve deposition at adiabatically zero velocity. This process is not only widely used in scientific investigations in fluid mechanics and thermal sciences but also in engineering and biomedical applications. The apparatus has been designed to produce accurate deposition onto the surface and minimize the vibrations induced in the drop by the movement of the capillary tip. Calibration tests of the apparatus have shown that a descent of the drop by discrete translational steps of approximately 5.6 microm and duration of 150-200 ms is sufficient to minimize its penetration depth into the liquid when it touches the surface layer and reduce to a level of noise the vibrations transmitted to it by the translation of the dispenser. Different settings of the experimental setup can be easily implemented for use in a variety of other applications, including deposition onto solid surfaces, surface tension measurements of pendant drops, and wire bonding in microelectronics.