Nowadays, a great deal of attention is given to the use of automatic systems coupled with various detection schemes. The most promising one is a flow-based system that increases the sample throughput while maintaining the reproducibility and repeatability of the method. This review focuses on electrochemical detection methods including amperometry, pulse amperometry, voltammetry, potentiometry, and miscellaneous detections for flow-based systems. Fundamental concepts and progress in the field of electrochemical detection with flow-based systems that have occurred within the past six years, including new methodologies and unique applications, are highlighted. The references cited herein were selected from the period between 2003 and 2009.
A sequential injection analysis (SIA) using a specific condensation reaction between malonic acid and ethylenediamine with 1–(3–dimethylaminopropyl)–3–ethylcarbodiimide hydrochloride (EDC･HCl) in aqueous media was developed for EDC･HCl determination. EDC･HCl can act as a dehydration or condensation reagent for the formation of amide (peptide). The proposed system, whose operation was fully automated by a lab–made computer–controlled program, consisted of a syringe pump, a selection valve, a spectrophotometer and a computer. The product in the condensation reaction accelerated at 60oC could be detected at 262 nm. The calibration graph of EDC･HCl showed a good linearity: the regression equation and correlation coefficient (r) were y = 1.5 × 10-3 x – 3 × 10-4 (y, absorbance; x, M concentration of EDC･HCl) and r = 0.998, respectively. The limit of detection (LOD) was 2.0 × 10-5 M and the sample throughput was 17 samples per hour. The automated SIA for EDC･HCl determination has improve the reaction efficiency and the sensitivity by coupling with forward and backward mixing in the reaction coil.
A continuous flow system for mutual separation of aluminum(III), gallium(III) and indium(III) was developed based on 4,4,4-trifluoro-1-(2-thientl)-1,3-butanedione (TTA) extraction. The proposed flow system has two extraction coils, a forward-extraction coil and a back-extraction coil. In the forward-extraction coil, gallium(III) and indium(III) are extracted into the organic phase while aluminum(III) is hardly extracted. In the back-extraction coil, indium(III) is back-extracted into the aqueous phase while gallium(III) remains in the organic phase. Consequently, when a sample solution is introduced into the system, aluminum(III), gallium(III) and indium(III) are separated each other. The effects of the length of the extraction coils and the flow rate of the pump on the separation efficiency were examined. The proposed flow system with extraction coils, 10 m in length for forward-extraction and 1 m in length for back-extraction, enabled the quantitative separation of aluminum(III), gallium(III) and indium(III).
The proposed FIA is based on a modified Berthelot reaction of ammonium ion with 1-naphthol and dichloroisocyanurate to form an indophenol blue derivative. We study to improve the sensitivity of the modified Berthelot reaction, and propose the addition of acetone. The FIA consists of two line flow system. Reagent solution 1 is an alkaline (14.5 g L-1 of NaOH) – sodium dichloroisocyanurate (1.2 g L-1) solution containing of trisodium citrate dehydrate (4 g L-1). Reagent solution 1 plays a sample carrier role in the FIA. Reagent solution 2 is 1-napthol (24 g L-1) solution (water : acetone : ethanol = 4 : 1 : 5). The interference of foreign ions in the sample is studied and removed by the addition of the complexing reagent, such as citrate. A linear range of the FIA is 0 to 4.0 μg mL-1 of ammonium ion. The relative standard deviation is 0.6% for 2.0 μg mL-1 of ammonium ion (n = 10) and the detection limit is 0.013 μg mL-1 of ammonium ion. The FIA is applied to environmental water samples, such as river, lake, and sea water.
An electrochemical anodic stripping voltammetry (ASV) coupled with auto-pretreatment system (Auto-Pret system) as an automated flow system was applied for trace metal analysis. A micro-flow sensor consisted of preconcentration membrane and three-eletrode electrochemical part was used as a detector. The characterization of the carbon electrode in micro-flow sensor such as life time, reproducibility and sensitivity were performed. The micro-flow sensor was tested with 50 ppb of Cd(II) and Pb(II) in 1.0 M HCl, as a supporting electrolyte, by on-line differential pulse anodic stripping voltammetry. It was used for 160 samples with the relative standard deviation less than 10 %. Analytical parameters for quantitative determination of Cd(II) and Pb(II) were investigated by differential pulse anodic stripping voltammetry (DPASV) and square-wave anodic stripping voltammetry (SWASV). Detection limits were 2.37 ppb and 0.15 ppb for the determination of Cd(II) and Pb(II) by DPASV and 0.02 ppb and 0.01 ppb for the determination of Cd(II) and Pb(II) by SWASV. The relative standard deviation less than 8 % for Cd(II) and less than 5 % for Pb(II) were achieved with each 50 ppb of metal ion solution. The results indicated that the carbon electrode in micro-flow sensor coupled with on-line Auto-Pret system have a good efficiency for the metal ion determination.
An automatized technique of stepwise injection potentiometric determination of ammonium-ions in water is developed. The technique includes consecutive stages of evolution of ammonium-ions in the form of ammonia and its liquid absorption into water phase. The determination range is of 5 to 2000 μg/l and efficiency of 7 determinations per hour.
Sequential injection analysis technique was utilized for automatic titration of vitamin C in drug formulation. The method is based on the oxidation reaction of vitamin C with potassium permanganate in sulfuric acid media. The titration reaction was monitored by the decrease of absorbance of the permanganate at its maximum wavelength at 525 nm. The super modified simplex computer program was successfully utilized for optimizing the reaction parameters such as sulfuric acid and potassium permanganate concentration as well as determining the stoichiometry of the reaction. The optimized operating conditions were 0.418 μmol/L potassium permanganate and 7.49 μmol/L sulfuric acid with the injection of 0.25 μmol/L vitamin C thus verifying the stoichiometry of 2:5:16 for permanganate, vitamin C and sulfuric acid respectively. In addition a (3f) factorial design was employed for studying interaction effects. The method was found to be applicable for the detection of vitamin C in the range between 100 to 350 ppm and it was found to be accurate when statistically compared with the BP standard method. The method exhibited no interferences from excepients added in drug formulation. The method was also found to be faster and more economical compared to the previous reported methods.