Flow system based on a binary sampling process for automatic dilutions prior to flame atomic spectrometry

A microcomputer-controlled flow system to perform automatic dilutions exploiting a binary sampling process is proposed. Introduction of precise volumes of sample and solvent was achieved by solenoid valves. The possibility to get different volumetric fractions was realized by software, which defined the time delays of the valves and could be used, e.g., for samples with analyte concentrations exceeding the analytical range. A noteworthy feature is that sample and standards processing is not necessarily the same. The analytical performance was exhaustively investigated, in particular the precision of standard measurements generated after application of different dilutions, or repeatability of results of 20 and 40 times diluted real sample solutions. The feasibility of the approach was demonstrated by the on-line automatic dilution of plant digests for determining Ca and K by flame atomic absorption and emission spectrometry. Precision of measurements performed on a solution produced by the automatic 10-fold dilution of a 100 mgl solution was characterized by a R.S.D. < 1%. Accuracy was assessed by analysing vegetal reference materials after diluting 10 or 20 times. Results were in agreement at 95% significance level with the certified values. Comparison of automatic and manual dilutions by using a paired t-test on a set of samples with varying composition presented no statistical difference at the 95% significance level.