EXTENDING ONLINE DILUTION STEADY-STATE CONCENTRATION RANGE BY MODIFICATION OF THE MERGING STREAM AND TANDEM INJECTION CONTINUOUS-FLOW METHODS

The range of steady-state concentrations, which had been achieved previously by on-line dilution constant-flow methods including flow injection, has been extended by the introduction of new flow configurations. The extent of dilution achieved depends on their combined arrangement. The replacement of constant-flow channels by either completely or partially variable flow channels has extended the dilution range. Moreover, a flexible flow approach for on-line dilution is also achieved. With a totally variable speed flow, an on-line dilution system can be operated at a constant over-all flow-rate over all of the achievable dilutions. Hence the precision of the steady-state concentration section is improved, as the choice of a single efficient coil or coil assembly becomes possible. The most effective means of extending the dilution range is stream flow splitting of the diluted sample produced by the merging stream or tandem injection approach followed by remerging the split stream with the diluent. This combination can in practice provide any desired dilution irrespective of whether constant or partially variable speed flow is adopted. On-line dilution of more than 300 000 is demonstrated. Hydrodynamic injection was as effective in performing tandem injection on-line dilution as conventional tandem injection and yielded identical precision of the steady-state concentration. Each of these dilution techniques can be treated as a module with specific dilution features which can be combined with each other to provide specific steady-state concentration on-line dilution.