Automatic analytical methods for environmental monitoring and control
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- Figure III.2.5.
- Figure III.2.6.
Figure III.2.4. Scheme of a spectrophotometric flow-cell with debubbler. Principles and kinetic aspects of SFA An operational feature of the SFA system is that, in addition to sample and reagents, air is drawn into the system through one of the pump tubes and it produces segmentation of the liquid stream once it has been merged with it. This segmentation is maintained through the succeeding stages of the analysis up to the detection unit where the air is removed and a continuous solution phase is reformed. The air introduction causes each individual sample to be divided into a number of small discrete liquid slugs and this presents several advantages. First, the air segments are responsible for maintaining a sharp concentration profile at the leading and following edges of each individual sample. Second, the presence of air bubbles promotes the mixing of the phases. Each stream slug can invert efficiently as it rises and falls through each turn of the mixing coil. For maximum mixing efficiency the length of each liquid slug must be less than half the diameter of the coil. In addition, the wiping action of the air along the tube wall prevents the build-up at the surface of residues from the preceding liquid slug. The measurements carried out in SFA systems are made under physical (homogenization of the sample-reagent slug between two consecutive bubbles) and chemical (analytical reaction reaches the equilibrium state before the reacting slug reaches the detector) equilibrium. Therefore, in these systems the steady-state signals are recorded and hence their design and the operation should achieve these equilibria. The main advantage of a determination realized in a SFA analyzer, namely precision and rapidity are drastically influenced by operational parameters such as the extent of carry-over and mixing reactant, the time spent by reacting mixture in the system, etc. Analytical signal T  he performance of a system that processes discrete samples at intervals is related to the dynamics of the flowing stream. A continuous stream of liquid flowing through tubing exhibits a velocity profile, the flow being faster at the center and slower at the tubing surface where frictional retardations occur (Figure III.2.5.a). If part of the fluid races ahead and part lags behind, this can lead to contamination from one sample to the next. Segmentation of the liquid stream by air-bubbles reduces contamination by providing a barrier to mixing. As shown in Figure III.2.5.b, each liquid slug between two bubbles is well mixed by turbulence due to wall friction, and laminar flow and contamination between samples is prevented by complete separation between each pair of liquid slugs. The bubbles continually clean the system by wiping the walls of the tubing and driving forward any stationary liquid film that might contaminate following samples. However, the air-bubbles do not entirely prevent the sample carry-over, because mixing in the surface layer can still occur. (a) (b)
The standard response of the detector for a SFA system and its characteristic parameters are shown in Figure III.2.6. It is obtained upon passage of the reacting mixture zone, placed between two reagent or washing solution zones, the air bubbles having been previously removed, through the detector flow-cell. It is composed of three parts: a rising portion, a plateau (steady-state signal) and a falling portion, the inverse of the rising portion, merging again with the baseline. Detailed studies reveal that the rising portion of the SFA signal is exponential, thus the measured concentration C as a function of time, t, is given by the equation: 
where C is the equilibrium concentration ant Ct that corresponding to a given time, t.  Figure III.2.6. Standard signal obtained in SFA systems. tr – residence time, delay time between sample start and detection unit; tin – time for which probe is in the sample vial; tout – time for which probe is out of sample vial. The significance of the signal parameters presented in Figure III.2.6 are: tr – time elapsed between the start of the sample aspiration and its arrival at the flow-cell, also known as residence time; tin – aspiration time over which the probe is submerged in the sample vial, and tout – interval during which the aspirating probe remains outside the sample vial withdrawing air and washing solution. Additional to these parameters, there are another two of great significance, namely the lag phase (tL) and the half-washing time (tw1/2), which have been demonstrated to be fundamental in calculating the performance characteristics of a SFA system. They afford a correlation between the approach to steady-state, fraction of steady-state reached in a given time and contamination between samples. A plot of log Ct against time is presented in Figure III.2.7. The lag – phase is related to the first portion of the signal and it is defined as the interval elapsed between the signal start and the obtainment of the signal plateau and is expressed numerically as the value of the intercept of the linear portion on the time axis. The half-washing time is defined as the time required for the signal at a given point to change from its value to half of its value and is calculated directly from the slope of the linear portion of the plot. 
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