Automatic analytical methods for environmental monitoring and control
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Figure III.2.7. Graphical response of lag phase tL and half-washing time tw1/2. Sample carry-over The sample carry-over occurs mainly in unsegmented streams and in terms of the standard SFA system design this implies: the aspiration device, the probe is contaminated internally and externally by the previous sample unless a washing mechanism is employed; flow system, a static, thin liquid film prevents direct contact of air with the tubing walls, thereby generating a heading sample residue. Upon arrival at the same point, part of the first sample is mixed with and incorporated into the segment of the second (Figure III.2.8.a). connection between the debubbler and the detector input. It favors mixing through the absence of any physical separation between successive samples. The connection must be as narrow and short as possible in order to avoid axial diffusion The effect of carry-over on the SFA signals compared with the theoretical situation is presented in Figure III.2.8.b. Substantial carry-over results in strongly overlapped, analytically unusable signals. Sample contamination (Figure III.2.8) can be determined by sequentially introducing three samples (S1, S2, S3) the first and the last of the same concentration and S2 of a much higher concentration. If contamination does not occur, the steady-state signals of S1 and S3 should be identical; if the third is higher then the first, the SFA system is subject to carry-over. The fact that the signal between samples may not reach the baseline is normal in routine determination as long as the signal attains equilibrium, i.e. provides a plateau. A measure of sample carry-over can be quantitatively expressed by using tw1/2 and tL. If the time elapsed between the aspiration of two successive samples is measured, the value of the equation  gives the so-called degree of interaction,  . Clearly, the smaller the value of tL and tw1/2 the higher is the performance of the analyzer.
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