Automatic calibration in continuous flow analysis

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A manifold which permits calibration graphs to be obtained automatically during continuous flow analysis is discussed. The entirely computer-controlled system is based on the use of two variable-speed peristaltic pumps. One of the pumps moves a standard solution of the analyte and the other displaces a diluent solution, both solutions being merged before reaching the detector. The flow rate delivered by the first pump is increased linearly with time, while that of the second pump is decreased linearly at the same rate. In this way, an on-line concentration gradient is obtained, the flow rate through the detector remaining constant. The signal-time profiles thus obtained can be used for calibration graphs after suitable treatment of the data. A sample solution can then be quantified by pumping it continuously through the manifold. The system is versatile and easy to handle since it is easily adapted to determinations which need a previous chemical reaction, and the same computer that controls the pumps also acquires and treats the signal-time data. The performance of the system is first discussed and then experimentally checked by using two common spectrophotometric determinations. The experiments prove that the approach, which can be extended to other detection systems, is highly reproducible and accurate. In addition, the manifold permits the on-line dilution (up to ten times) of samples which are too concentrated to be measured directly. A saving of time and reagents can be achieved since, if needed, reliable calibration can be obtained in 50 s while only 145 μl of a single standard solution is consumed.