Stephen R. Bysouth

Flow injection techniques of method development for flame atomic-absorption spectrometry

The flow injection introduction of samples that would be difficult by conventional nebulisation owing to dissolved solid content, viscosity or quenching effects on flame reactions is demonstrated. The various ways of achieving sample dilution by control of volume injected, tube dimensions and flow-rate are discussed and a manifold described for producing solutions for calibration purposes and diluting off-range samples. The important characteristics of the nebuliser and instrument for flow injection work are evaluated. A simple manifold for investigating interference effects is described and its use illustrated for some well known effects. The possibilities of achieving pre-treatment by solvent extraction and extension to include indirect methods are discussed and the role of flow injection procedures in the method development for, and the extension of the range of application of, flame atomic-absorption spectrometry is critically evaluated.

Lead preconcentration with flow injection for flame atomic absorption spectrometry

The flow-injecton preconcentration of lead with immobilised reagents under a variety of conditions is discussed. Timed sample loading and matrix removal without passing the matrix to the nebuliser were achieved simply with one valve. Reagent consumption and calibration time were reduced by the addition of further valves. A system design incorporating control of the timing of operations by a commercial autosample is described. The effects of pH and interferent ions were examined. Water samples were analysed against aqueous standards and as standard additions solutions. For an analysis time of about 3 min a preconcentration factor of about 40 was obtained for both peak height and area measurements. Detection limits of down to 1.4 ml−1 were obtained.

Network flow injection manifolds for sample dilution and calibration in flame atomic absorption spectrometry

Two flow injection manifolds for on-line dilution are described and evaluated. One consists of an asymmetrical two-branch network giving rise to three measurement points, namely the two maxima and the minimum between the partially overlapping peaks. With this manifold a concentration range for magnesium of 0.2–100 mg l–1 was covered with a relative standard deviation of about 2%. The relative dilutions at the measurement points were 1.0, 5.9 and 23, corresponding to absolute values of 3.4, 20 and 77, respectively. The other manifold consisted of a three-branch network giving three partially overlapping peaks and five measurement points, three maxima and two minima; dilution factors of 6.8, 10, 14, 22 and 28 were produced. A calibration generated by the injection of a 6 mg l–1 magnesium solution was used to analyse solutions in the range 0.2–10 mg l–1. The corresponding range of standard deviations was 0.017–0.30 mg l–1. This method of calibration is only possible if straight line calibration graphs are obtained. The method generates a family of calibration graphs of varying sensitivity covering the extended concentration range that are more readily visualised on log-log plots. For dilution of off-range samples and extending the conventional calibration range, the two-branch network is preferred as it is less susceptible to minor fluctuations in flow-rate in those branches of the manifold which gave rise to poor day to day reproductibility of the three-branch network.

A comparison of curve fitting algorithms for flame atomic absorption spectrometry

A comparison of six commercially available curve fitting algorithms for calibration in flame atomic absorption spectrometry (FAAS) has been made. Programs using the algorithms were written in BASIC for a microcomputer and, where possible, tested against the commercial version. Calibration curves for magnesium, chromium and nickel produced by the algorithms were compared, on the basis of the sum of squares of the percentage deviations and its root mean square, with each other and with linear interpolation and manual fitting. The standard deviations of the goodness of fit parameters were calculated to indicate significant differences between the fits obtained. With the exception of the simple parabola, linear interpolation and manual methods, which were significantly poorer, the algorithms performed similarly. In general the errors in curve fitting were well below 5% for the commercial algorithms.

Flow injection manifolds with membrane filters for pre-concentration and interference removal by precipitation flow injection flame atomic absorption spectrometry

The use of inexpensive, disposable, non-metallic filters in two manifolds for continuous precipitation, as a means of pre-concentration and matrix separation, is described. In one manifold, novel valve configuration and a simple timing device allowed the merging of sample and reagent streams, precipitation, and collection during the “load” part of the cycle. In the “inject” position, the stream switching flushed the dissolved precipitate through the filter. Copper was precipitated as the hydroxide and collected on a disposable cellulose acetate membrane filter. The degree of pre-concentration was shown to be proportional to the volume of sample passed. After sufficient precipitate had been collected, it was dissolved in a stream of hydrochloric acid. Each pre-concentration step was followed by a timed wash step before injection/dissolution of the precipitate. For a sample volume of 11 ml, a pre-concentration factor of 12 was achieved. Calcium (10 mg l–1) was shown to be selectively precipitated in the presence of an aluminium concentration of up to 1000 mg l–1, allowing the successful determination of calcium in the presence of aluminium, not normally achievable by atomic absorption spectrometry with an air-acetylene flame.

A microcomputer-based peak-width method of extended calibration for flow-injection atomic absorption spectrometry

Abstract In the proposed method of extended calibration based on peak widths, all data collection and reduction are done by a microcomputer interfaced to the spectrometer. The method produces an estimate of concentration without dilution of the off-range samples. Calibrations covering the ranges 40/2-1000 mg l−1, 1.0/2-50 mg l−1 and 20/2-1000 mg l−1 were obtained for chromium, magnesium and nickel, respectively.

Study of the possible benefits of flow injection sample introduction for flame atomic absorption spectrometry

Flow injection techniques for use with atomic spectrometry are briefly reviewed, from which it is apparent that there has been a considerable increase in interest recently in on-line pre-concentration and matrix isolation methods. In addition to the use of immobilised reagents and solvent extraction, methods based on precipitation and collection of the precipitate in the manifold have been reported. The possible benefits of running the nebuliser under starvation conditions both with and without air compensation are examined, together with the benefits of peak-area as opposed to peak-height measurements, claimed in some previous studies to lead to improved detection limits. It is shown that, with a Baird Atomic A3400 nebuliser, the depressive effect of phosphate on calcium may be removed at low flow-rates and that with a Philips SP9 nebuliser the best detection limits are obtained when strategies for maximising the peak height are adopted. Preliminary results for the generation of a volatile metal chelate (copper trifluoroacetylacetonate) as the basis for matrix isolation show a considerable improvement in peak-area sensitivity over conventional nebulisation, although the peak-height sensitivity is similar.

Supercritical carbon dioxide as a carrier for sample introduction in atomic absorption spectrometry

Abstract The use of a supercritical carbon dioxide carrier to transport organic solutions of metal complexes to a heated restrictor is demonstrated as a means of aerosol or vapour generation for sample introduction in flame atomic absorption spectrometry. Calibrations obtained using conventional pneumatic nebulisation of continuously aspirated solutions or using flow injection into a water carrier are compared with those obtained using the carbon dioxide carrier. Injection of solutions of copper pyrrolidinedithiocarbamate in 4-methyl pentan-2-one into a carbon dioxide carrier at 1000 p.s.i. was shown to 1.2 times more sensitive than injection of the same solutions into a water carrier with conventional nebulisation. The sensitivity of continuous nebulisation of the solutions, however, was not achieved. Dissolution of solid copper acetyl acetonate in supercritical carbon dioxide is demonstrated and subsequent vaporisation followed by atomisation in a heated quartz furnace is shown to occur.

Flow Manifold for Automated On-Line Dilution of Standards for Flame Atomic Absorption Spectrometry and Its Use in a Null Measurement Method

A flow manifold that automatically dilutes solutions presented to a flame atomic absorption spectrometer while maintaining a constant flow to the nebuliser is described. The manifold is based on a fixed-speed pump, together with a computer-controlled stream switching valve and pump. The computer is also used for data acquisition and handling. In addition to the production of solutions for a conventional calibration procedure, the system is also used for a novel calibration procedure whereby the stock standard solution is automatically diluted by a known factor until its absorbance matches that of the sample. This null measurement method avoids the need to fit a curve to a set of calibration points. The performance of both methods compares favourably with the conventional procedure with errors ranging from 0.0 to 4% and no significant bias at the 99% confidence level. Problems with pulsations in the flow caused by peristaltic pump rollers were encountered with the consequent difficulties of accurate flow-rate measurement. Attempts to improve the speed of the method by the use of flow injection introduction were not successful owing to these pulsations in the flow.

Design and Operation of an Autosampler Controlled Flow-Injection Preconcentration System for Lead Determination by Flame Atomic Absorption Spectrometry

Flow-injection manifolds are described which allow the preconcentration of lead for flame atomic absorption determinations, using columns contained within the sample loop of an injection valve. An interface was designed which allowed the valves and pump in the system to be controlled by an autosampler which enabled precise timing of preconcentration and elution steps. The effects of sample flow rate, buffer pH and buffer type for preconcentration and eluent concentration and flow rate were investigated in order to obtain optimum performance of the system. A 50-times improvement in detection limits over conventional sample introduction was obtained for a sample volume of approximately 12 ml, preconcentrated for 150 s. The injection of eluent, as opposed to the use of a continuously flowing eluent stream, enabled this reagent to be conserved.