Conventional flow injection (FI) has been utilised for the development of the sample-to-standard additions method as an alternative to the reverse FI standard additions method. An equation has been derived for the proposed method, allowing the calculation of sample concentration from two transient signals obtained from the injection of the sample solution and the blank, which must possess matched matrix composition. For these conditions the sample-to-standard additions method was as effective as the standard additions method. Two conventional FI methods for the determination of hydrochloric acid and orthophosphate in phosphate rock digests were examined by the sample-to-standard additions method. Satisfactorily accurate and precise results were demonstrated. Because the proposed method was operated in the conventional FI mode, an autosampler was incorporated to obtain a throughput of 80-100 samples h-1.
Steady-state concentrations have been achieved by on-line dilution, flow injection techniques with conventional instrumentation. An approach employing tandem injection for on-line dilution was applied to obtain a steady-state concentration. The deviation from signal stability resulting from concentration variations or ripples hampered exploitation of this approach for on-line dilution except over a limited range. However, when tandem injection was coupled with a single-or, especially, a double-confluence for the diluent, dilution was achieved over a wide range (up to 900-fold). Further, steady-state concentrations were obtained without concentration ripples. With the combined tandem injection and merging stream the emission signal variation with inductively coupled plasma (ICP) spectrometry compares well with that obtained for direct sample nebulisation. Simple relationships for calculation of the anticipated dilution have been derived and validated experimentally. Flow configurations were designed to implement on-line dilution approaches under optimum conditions. Although these developments for on-line dilution are initially intended for sequential multi-element ICP spectrometric analysis, they should prove to be of general use.
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