Carlos M. C. Infante

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 Multi‐purpose Flow System Based on Multi‐commutation

Abstract A multi‐purpose flow system designed with solenoid valves and micropumps is proposed. The independent control of each active device was exploited to implement dilutions, calibration with a single solution, standard additions, titrations and strategies to increase sample residence time without changing the manifold hardware. Sample dispersion coefficients between 1 and 7800 were achieved by changing the sample volume and exploiting zone sampling, with coefficient of variation estimated as 6.5% for the highest dilution. Calibration curves obtained from a single standard showed slopes in agreement with those obtained by conventional batch dilutions with variations of lower than 2% between days. The possibility of implementing the standard addition method or the stopped‐flow approach at different points of the manifold was demonstrated for creatinine determination in urine by the Jaffe reaction. Analytical curves for different sample residence times can be obtained for detecting matrix effects or for the analysis of colored samples. Titrations by the continuous variation method or binary search, as well as other possible applications, are discussed. The authors were invited to contribute this paper to a special issue of the journal entitled “Spectroscopy and Automation”. This special issue was organized by Miguel de la Guardia, Professor of Analytical Chemistry at Valencia University, Spain.

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.

Determination of picloram in waters by sequential injection chromatography with UV detection

This paper describes a sequential injection chromatography procedure for determination of picloram in waters exploring the low backpressure of a 2.5 cm long monolithic C18 column. Separation of the analyte from the matrix was achieved in less than 60 s using a mobile phase composed by 20:80 (v v-1) acetonitrile:5.0 mmol L-1 H3PO4 and flow rate of 30 μL s-1. Detection was made at 223 nm with a 40 mm optical path length cell. The limits of detection and quantification were 33 and 137 μg L-1, respectively. The proposed method is sensitive enough to monitor the maximum concentration level for picloram in drinking water (500 μg L-1). The sampling frequency is 60 analyses per hour, consuming only 300 μL of acetonitrile per analysis. The proposed methodology was applied to spiked river water samples and no statistically significant differences were observed in comparison to a conventional HPLC-UV method.

Development of a Spectrophotometric Sequential Injection Methodology for Online Monitoring of the Adsorption of Paraquat on Clay Mineral and Soil

Abstract The reduction of paraquat by sodium dithionite in alkaline solution was explored to develop an automated, spectrophotometric, sequential injection method for online determination of the herbicide in suspensions of solid adsorbents. A tangential filter was used for interfacing the suspension and the analyzer. The linear dynamic range was between 0.1 and 20 mg L−1, with detection and quantification limits of 0.039 and 0.10 mg L−1, respectively. Sampling throughput was 102 hr−1. Precision at the 5 mg L−1 paraquat concentration was 2.9%, with a consumption of dithionite of 1 mg per analysis. The method revealed that adsorption of paraquat onto vermiculite is faster and quantitative in comparison with a tropical soil. The authors were invited to contribute this paper to a special issue of the journal entitled “Spectroscopy and Automation”. This special issue was organized by Miguel de la Guardia, Professor of Analytical Chemistry at Valencia University, Spain. The special issue was published in volume 39, issue 6.

Improving the Detectability of Sequential Injection Chromatography (SIC): Determination of Triazines by Exploiting Liquid Core Waveguide (LCW) Detection

Coupling a liquid core waveguide cell to a sequential injection chromatograph improved the detection limits for determination of triazine herbicides without compromising peak resolution. Separation of simazine, atrazine, and propazine was achieved in water samples by a 25 mm long C18 monolithic column. Detection was made at 238 nm using a type II LCW (silica capillary coated with Teflon® AF2400) cell with 100 cm of optical path length. Detection limits for simazine, atrazine, and propazine were 2.3, 1.9, and 4.5 µg L−1, respectively. Reduced analysis time and low solvent consumption are other remarkable features of the proposed method.

Development of a Fluorimetric Sequential Injection Analysis (SIA) Methodology for Determination of Quinine

A sequential injection analysis (SIA) methodology was developed for fluorimetric determination of quinine in soft drinks. Minimum dispersion and efficient mixing were achieved by aspirating 200 mL of sample between two 100 mL reagent zones (0.10 mol L-1 H2SO4) at flow rate of 250 mL s-1 and using a reaction coil length of 50 cm (0.8 mm internal diameter). Linear response for quinine concentrations (C) between 0.050 and 100.0 mg L-1 was described by: I = (532 ± 40) + (2.36 ± 0.04) C, with r = 0.999, where I is the relative fluorescence intensity. The coefficient of variation and limits of detection and quantification were 1.9% (0.50 mg L-1 n = 10), 2.3 and 4.5 mg L-1, respectively. The sampling throughput was 60 analyses per hour, consuming 2.2 mL of concentrated H2SO4 and producing 4 mL of wastes per analysis. No statistically significant differences were observed between the results obtained by the proposed SIA methodology and the ones obtained by the batch procedure.