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.