Jacobus Frederick van Staden

Automated simultaneous determination of nitrate and nitrite by pre-valve reduction of nitrate in a flow-injection system

Abstract Nitrate is reduced to nitrite by using the pre-valve in-valve reduction technique prior to the sampling system. One loop of a two-position sampling valve is replaced by a copperised cadmium column. Nitrite from the samples as well as nitrite formed in the reduction procedure is sampled by a second valve and introduced into the flow system. The two sampling valves are synchronised in such a way that two peaks are obtained, one corresponding to the nitrate plus nitrite and the other to the nitrite only. The method is suitable for the simultaneous determination of nitrate and nitrite at a sampling rate of up to 72 determinations per hour with coefficients of variation better than 1.96% for nitrate and 0.83% for nitrite.

Sequential injection spectrophotometric determination of iron as Fe(II) in multi-vitamin preparations using 1,10-phenanthroline as complexing agent.

A sequential injection analysis (SIA) system is proposed for the determination of iron (II). Fe(II) was determined by SIA based on the reaction between 1,10-phenanthroline and iron (II), yielding an orange-red colour complex with absorption maximum at 512nm. The method involved aspiration of 187mul sample/standard zone followed by a zone of a reagent solution containing 140mul of 7.8 x 10(-4)moll(-1) 1,10-phenanthroline into a carrier stream to be stacked inside a holding coil and flow reversed through a reaction coil to a detector. The optimum condition was evaluated and the calibration curve is linear over a range of 0.25 to 5.0mgl(-1) of Fe(II) with detection limit of 18mugl(-1). A sample throughput of 40h(-1) was established. This technique is found to be simple, accurate, reproducible and sensitive. The proposed method was successfully applied for the determination of total iron as Fe(II) in pharmaceutical products (multi-vitamin tablets) and is especially useful for the determination of iron (II) in tablets with lower iron (II) contents. The results were found to be in good agreement with the results obtained by manual UV/Vis spectrophotometry and flame atomic absorption spectrometry (FAAS) and with claimed values by the manufacturers.

Recent Developments and Applications of Chemiluminescence Sensors

ABSTRACT Chemiluminescence sensors are important tools in analytical chemistry due to their high sensitivity selectivity. This review presents the instrumentation involved in their design, including light detection and flow injection analysis system used. Various applications for the analysis of inorganic and organic compounds from gaseous samples and solutions indicate that these sensors are used with good reproducibility and selectivity of the analytes at low concentration level.

Simultaneous detection of S and R captopril using sequential injection analysis

A simultaneous detection sequential injection analysis (SIA) system is proposed for the determination of S and R captopril using a potentiometric, enantioselective membrane electrode based on maltodextrin (DE=14-17) for the assay of S-captopril and an amperometric biosensor for the assay of R-captopril. The proposed SIA system can be utilized reliably for the on-line simultaneous detection of the enantiomers in the synthesis process at a rate of 38 samples per hour in the following linear concentration ranges: 100-1000 nmol/l (R-captopril) and 1-1000 mumol/l (S-captopril) with a RSD better than 0.009% (n=10).

Amperometric biosensors/sequential injection analysis system for simultaneous determination of S- and R-captopril

Two amperometric biosensors based on L- and D-amino acid oxidase, respectively, are proposed for the simultaneous detection of S- and R-captopril in a sequential injection analysis system (SIA). The linear concentration ranges are: 0.4-1.6 micromol/l (S-captopril) and 120-950 nmol/l (R-captopril) with detection limits of 0.2 and 15 nmol/l, respectively. The biosensors/SIA system can be used reliably on-line in synthesis process control, for the simultaneous assay of S- and R-captopril with a frequency of 34 samples/h.

Analysis of Chiral Drugs with Enantioselective Biosensors. An Overview

The opportunity to use amperometric biosensors based on L-aminoacid oxidase in enantioselective analysis of chiral drugs is discussed. The designed biosensors combine the selectivity of the enzyme with the sensitivity of the amperometric device. To obtain a reliable amperometric biosensor, graphite paste is used as support for the enzyme. The parameters obtained for amperometric biosensors are compared with those obtained for the potentiometric enantioselective membrane electrodes based on β-cyclodextrin derivates impregnated in graphite paste for several angiotensin converting enzyme (ACE) inhibitors–S-captopril, S-enalapril, S-ramipril, S-cilazapril, S-trandolapril, S-pentopril, and S-perindopril–in terms of sensitivity, selectivity, limit of detection, working concentration range, lifetime, response time, accuracy and precision.

Electrochemical Sensor Arrays

The importance of sensor arrays in environmental, food and clinical analysis is discussed. The possible designs of sensor arrays is shown. The most reliable mathematical models for data processing are presented. The importance of different types of electrochemical sensor arrays in analytical chemistry as well as their performances are shown.

Computer-Aided Flow-Analysis for Laboratory Use and Process Analysis

ABSTRACT Flow-based methods of analysis such as flow-injection analysis and sequential-injection analysis are being used increasingly in the automation of laboratory methods and for process analysis. Full exploitation of these techniques in automated modes of operation necessitates computer control and data acquisition. Although most commercial systems are microprocessor driven, flexibility is seldom built into the software, thereby severely limiting the ingenuity of the analysts in the use of these instruments in novel and imaginative applications. A software package is described which allows for easy automation of flow-based analysis systems. The emphasis in the design of this package has been on flexibility, with numerous user-selected options and a wide variety of features. Applications to laboratory and plant analysers are described.

Flow-Injection Analysis of Chloride in Milk with a Dialyzer and a Coated Tubular Inorganic Chloride-Selective Electrode

Abstract A rapid and reliable automated procedure for the direct measurement of the chloride content in milk is described. The method is based on the principles of the flow-injection technique; a dialyzer is used to eliminate interferences and the dialyzed chloride is measured with a coated tubular chloride-selective electrode. With this system potential change caused by the interference of casein is obviated, giving a stable baseline. Results obtained for chloride in milk at a sampling rate of 120 samples per hour compared well with data obtained by the standard Volhard titration method. With 30-μl samples a working range of 250–5000 mg/1 chloride is covered with a coefficient of variation of better than 0.50%.

Analysis of Several Angiotensin-Converting Enzyme Inhibitors using Potentiometric, Enantioselective Membrane Electrodes

The construction of potentiometric, enantioselective membrane electrodes for several angiotensin-converting enzyme inhibitors as well as the mechanism of potential development and enantioselectivity are described. The response characteristics for angiotensin-converting enzyme inhibitors and the reliability of their enantiopurity tests are highlighted.