Reaction detectors mixing devices

Automatic systems based on FIA were also implemented for wine analysis. In a comprehensive review, Ferreira and coworkers [97] remarked that some systems had limitations as many of them were only tested with a particular wine type or demanded a previous treatment of the sample before injection. The lack of robustness of some manifold components (tubing of peristaltic pumps, some types of injection devices) was also thought to prevent the extensive use of FIA in industrial laboratories [98]. SIA has been proposed as a mechanically simple alternative to FIA [99]. As previously stated, SIA is based on the sequential aspiration of well-defined sample and reagent zones into a holding coil by means of a multiposition valve. The flow is then reversed and the stacked zones are mixed and propelled to the detector, where the reaction product is monitored. As already described for other beverage matrices (water, juices), the SIA of wine has been developed in recent years for the determination of more than 20 species and several aspects of these systems were reviewed by Segundo et al. [100] in a recent paper. The authors focused on the implementation of in-line treatment and the adaptation of system operation through software control to enable determination in different kind of wines. 

To test the response time of any detector, one normally challenges the detector with a step function. For example, consider a flow system (Balko et al., 1981) with a mixer and two driving syringes, an observation tube, and a thermistor as the detector as shown in Figure 9. If acid and base are mixed, a step increase in heat occurs because the reaction is so fast. Thus, 0.02M NaOH and 0.0 M HC1 mixed in the ratio of one to one will give a temperature rise of about 69 mK since Mi for the heat of formation of water at 25°C and in 0.1M KC1 is about 13.75 x 103 cal/mol (Hale et al., 1963). The time it takes the temperature of the sensor to rise to Me of its total change is called the sensor s time constant, t. The time to rise half way is ti/2. Since most physical devices respond exponentially, at least to a first approximation, the output E may be expressed as... 

Analogously to process engineering, the reactor is one of the most important components of the flow system, inside which almost all the sample processing steps take place. When this component is a simple conduit to promote the transport of a sample to the detector without any chemical reaction, the device is also called a transmission line or sample delivery line. If only physical mixing is performed inside a coiled reactor,... 

Some of the interesting applications of LOV inclnde the manipnlation of beads and microcolnmns for sample clean-up or separation. In particnlar, one version of the system has received considerable attention and is called microseqnential injection-lab-on-valve (pSl-LOV). The J,SI-LOV device can be used for sample pretreatment alone or in seqnence with analysis and detection (usually absorbance or flnorescence). It has also been employed as the front end of a miniaturised CE system. An interesting nse of J,SI-LOV has been in conjunction with suspensions of specially coated beads. These beads can be manipulated by the multiport valve to flow into the holding coil and from there into the detector flow cell to form a temporary microcolnmn. The sample is then mixed with the bead column and any reactions are detected, e.g. adsorption over time. After measurements have been made, the beads are sent to waste. The formation and removal of these temporary microcolumns has advantages over the nse of fixed colnmns snch as ... 

Lab-on-valve is similar in concept to sequential injection, but involves integration of all flow manifold elements and the detector flow cell into a microconduit attached to the back of the selection valve [151]. Only one method based on this methodology for AA determination has been published. Sorouraddin et al. [152] applied an on-chip microdetermination device, integrating the FIA system, to l-AA and DHAA determination in urine and pharmaceutical samples. The manifold components are used for flow, mixing, reaction, and detection. The reaction system is a coupled redox-complexation reaction between AA and a l,10-phenanthroline-Fe(III) and a photothermal microscope is used for the ultrasensitive detection of the nonfluorescent reaction product (ferroin). For DHAA determination, dithiothreithol is used as reducing agent and the total AA is determined. 

---