Empirical adsorption kinetics: comparing linear and nonlinear regression analysis emphasizing the need for high throughput analysis.

This paper evaluates linear and nonlinear regression analysis to describe the empirical adsorption kinetics using pseudo-first-order (PFO) and pseudo-second-order (PSO) models. These models have been used to characterize the performance of adsorbents for environmental remediation and environmental modeling. Data were simulated using the PFO and PSO models with 1, 2, and 5% noise levels and fitted by nonlinear and linearized PFO and PSO equations. Nonlinear regression analysis provided rate constants and adsorption capacities with better accuracy than linearization. Besides the correlation coefficient, Chi-square and residual plot analysis helped choose the proper model to describe the adsorbent efficiency and validate the results. Both models and the NLR fitting were employed to reevaluate data obtained in our research group, including the adsorption of Hg(II) on thiol-modified vermiculite, glyphosate on soils rich in aluminum and iron oxides, phosphate on Fe(III) polyhydroxy cations modified montmorillonite, and paraquat on soil and vermiculite. While fitting the simulated data indicates an unequivocal and correct kinetic model, fitting the experimental data is not straightforward, suggesting mixed models rule the adsorption and that a large number of data points, especially at the initial steps of adsorption, provided by high throughput analysis, help to improve the kinetic modeling.

Simultaneous determination of benznidazole and itraconazole using spectrophotometry applied to the analysis of mixture: A tool for quality control in the development of formulations.

The aim of this work was the development of an analytical procedure using spectrophotometry for simultaneous determination of benznidazole (BNZ) and itraconazole (ITZ) in a medicine used for the treatment of Chagas disease. In order to achieve this goal, the analysis of mixtures was performed applying the Lambert-Beer law through the absorbances of BNZ and ITZ in the wavelengths 259 and 321 nm, respectively. Diverse tests were carried out for development and validation of the method, which proved to be selective, robust, linear, and precise. The lower limits of detection and quantification demonstrate its sensitivity to quantify small amounts of analytes, enabling its application for various analytical purposes, such as dissolution test and routine assays. In short, the quantification of BNZ and ITZ by analysis of mixtures had shown to be efficient and cost-effective alternative for determination of these drugs in a pharmaceutical dosage form.

Online sequential-injection chromatography with stepwise gradient elution: a tool for studying the simultaneous adsorption of herbicides on soil and soil components.

The adsorption of triazine herbicides simazine (SIM), atrazine (ATR), and propazine (PRO) as well as the metabolites deisopropylatrazine (DIA), deethylatrazine (DEA), and 2-hydroxyatrazine (HAT) on soil, humic acid, and soil modified with humic acidic was studied by sequential-injection chromatography with UV detection at 223 nm. An online monitoring system was assembled, which was composed of a tangential filter and a peristaltic pump for the circulation of the soil (25 g L(-1)) or humic acid (2.5 g L(-1)) suspensions. A stepwise gradient elution separated the compounds using three mobile phases whose compositions were 28, 40, and 50% (v v(-1)) methanol in 1.25 mmol L(-1) ammonium acetate buffer, pH 4.7. The sampling throughput was about six analyses per hour; the linear dynamic range was between 100 and 1000 µg L(-1) for all of the studied compounds. The detection limits varied from 9 µg L(-1) for ATR to 36 µg L(-1) for DEA. At contact times <2 h, humic acid was the material with a higher adsorptive capacity (from 1470 ± 43 µg g(-1) for DIA to 2380 ± 51 µg g(-1) for PRO). In soil, HAT exhibited the highest adsorption (23.8 ± 0.2 µg g(-1)). The presence of humic acid in the soil increased the adsorption of ATR (14 ± 1 to 23 ± 2 µg g(-1)) and PRO (21.5 ± 0.5 to 24.0 ± 0.2 µg g(-1)), decreased the adsorption of HAT (23.8 ± 0.2 to 18 ± 2 µg g(-1)), and did not affect DIA and DEA. The adsorption of SIM was negligible in all of the sorbents studied. Simazine is the herbicide with the greatest potential for leaching to water bodies followed by DEA and DIA.

Development of a sequential injection-square wave voltammetry method for determination of paraquat in water samples employing the hanging mercury drop electrode.

This work describes the development and optimization of a sequential injection method to automate the determination of paraquat by square-wave voltammetry employing a hanging mercury drop electrode. Automation by sequential injection enhanced the sampling throughput, improving the sensitivity and precision of the measurements as a consequence of the highly reproducible and efficient conditions of mass transport of the analyte toward the electrode surface. For instance, 212 analyses can be made per hour if the sample/standard solution is prepared off-line and the sequential injection system is used just to inject the solution towards the flow cell. In-line sample conditioning reduces the sampling frequency to 44 h(-1). Experiments were performed in 0.10 M NaCl, which was the carrier solution, using a frequency of 200 Hz, a pulse height of 25 mV, a potential step of 2 mV, and a flow rate of 100 µL s(-1). For a concentration range between 0.010 and 0.25 mg L(-1), the current (i(p), µA) read at the potential corresponding to the peak maximum fitted the following linear equation with the paraquat concentration (mg L(-1)): i(p) = (-20.5 ± 0.3)C (paraquat) - (0.02 ± 0.03). The limits of detection and quantification were 2.0 and 7.0 µg L(-1), respectively. The accuracy of the method was evaluated by recovery studies using spiked water samples that were also analyzed by molecular absorption spectrophotometry after reduction of paraquat with sodium dithionite in an alkaline medium. No evidence of statistically significant differences between the two methods was observed at the 95% confidence level.

Development of a sequential injection chromatography (SIC) method for determination of simazine, atrazine, and propazine.

This paper describes the development of a sequential injection chromatography (SIC) procedure for separation and quantification of the herbicides simazine, atrazine, and propazine exploring the low backpressure of a 2.5 cm long monolithic C(18) column. The separation of the three compounds was achieved in less than 90 s with resolution >1.5 using a mobile phase composed by ACN/1.25 mmol/L acetate buffer (pH 4.5) at the volumetric ratio of 35:65 and flow rate of 40 microL/s. Detection was made at 223 nm using a flow cell with 40 mm of optical path length. The LOD was 10 microg/L for the three triazines and the quantification limits were of 30 microg/L for simazine and propazine and 40 microg/L for atrazine. The sampling frequency is 27 samples per hour, consuming 1.1 mL of ACN per analysis. The proposed methodology was applied to spiked water samples and no statistically significant differences were observed in comparison to a conventional HPLC-UV method. The major metabolites of atrazine and other herbicides did not interfere in the analysis, being eluted from the column either together with the unretained peak, or at retention times well-resolved from the studied compounds.

A multicommuted flow system with solenoid micro-pumps for paraquat determination in natural waters.

A flow system designed with solenoid micro-pumps is proposed for the determination of paraquat in natural waters. The procedure involves the reaction of paraquat with dehydroascorbic acid followed by spectrophotometric measurements. The proposed procedure minimizes the main drawbacks related to the standard chromatographic procedure and to flow analysis and manual methods with spectrophotometric detection based on the reaction with sodium dithionite, i.e. high solvent consumption and waste generation and low sampling rate for chromatography and high instability of the reagent in the spectrophotometric procedures. A home-made 10-cm optical-path flow cell was employed for improving sensitivity and detection limit. Linear response was observed for paraquat concentrations in the range 0.10-5.0 mg L(-1). The detection limit (99.7% confidence level), sampling rate and coefficient of variation (n=10) were estimated as 22 microg L(-1), 63 measurements per hour and 1.0%, respectively. Results of determination of paraquat in natural water samples were in agreement with those achieved by the chromatographic reference procedure at the 95% confidence level.

A green flow-based procedure for fluorimetric determination of acid-dissociable cyanide in natural waters exploiting multicommutation.

A flow system designed with solenoid valves is proposed for determination of weak acid dissociable cyanide, based on the reaction with o-phthalaldehyde (OPA) and glycine yielding a highly fluorescent isoindole derivative. The proposed procedure minimizes the main drawbacks related to the reference batch procedure, based on reaction with barbituric acid and pyridine followed by spectrophotometric detection, i.e., use of toxic reagents, high reagent consumption and waste generation, low sampling rate, and poor sensitivity. Retention of the sample zone was exploited to increase the conversion rate of the analyte with minimized sample dispersion. Linear response (r = 0.999) was observed for cyanide concentrations in the range 1-200 microg L(-1), with a detection limit (99.7% confidence level) of 0.5 microg L(-1) (19 nmol L(-1)). The sampling rate and coefficient of variation (n = 10) were estimated as 22 measurements per hour and 1.4%, respectively. The results of determination of weak acid dissociable cyanide in natural water samples were in agreement with those achieved by the batch reference procedure at the 95% confidence level. Additionally to the improvement in the analytical features in comparison with those of the flow system with continuous reagent addition (sensitivity and sampling rate 90 and 83% higher, respectively), the consumption of OPA was 230-fold lower.