Gas diffusion, FIA

Gas diffusion FIA has also been used for the online determination of chlorine dioxide in potable water. In this example, chlorophenol red was used as the chromogenic reagent and a detection limit of 0.02mgl was achieved. 

Gas-diffusion FiA-fiuorescence manifoid for the determination of totai cyanide. Detector fluorimeter IV ... 

Cream of asparagus soup, broccoli sauce GIAD pH adjustment to 4.0, ultrasonic extraction, filtered Gas diffusion FIA Spectrophotometric 30 2.0-20 N/A 2.8 27... 

The GAD reactor system consists of a gas-diffusion FIA system in which GAD is immobilized in a reactor and placed in a pyridoxal 5 -phosphate/phosphate buffer pH 4.0 carrier stream. The sample is injected into the carrier stream and passed through the GAD reactor, where MSG is decarboxylated and CO2 is released. This stream merges with a phosphoric acid solution and is diffused through the gas-permeable Teflon membrane. The CO2 reacts with an acid-base indicator solution that acts as acceptor and is detected spectrophotometrically at 430 nm. The optimization of the sample flow rate is crucial, since it affects the contact time with the GAD enzyme and the diffusion time of the CO2 through the membrane. Sample throughput is 30 h. ... 

In one example the determination of ammonia and urea is performed by combining a gas diffusion unit and a fiber-optic-based spectrophotometer (figure 21.7). This is an excellent example of how FIA improves the selectivity of the sensor system. The ammonia and the ammonia generated by the... 

The combination of chemical and biological sensors with flow injection has been demonstrated. Both more-traditional-type sensors such as pH electrodes and newer sensors such as fiber optics and surface acoustic wave detectors have been incorporated into FIA systems with success. An advantage that FIA brings to the sensor field is the possibility of turning a moderately selective sensor into a selective sensor by incorporating into the FIA system some type of selectivity enhancement technique such as gas diffusion, dialysis, and reactors. Finally the FIA systems permit renewable systems since sensor surfaces and reaction cells can be washed, surface regenerated, and reagents replenished on demand. 

Until recently, samples for FIA were already extracted. Altered, centrifuged or pretreated in some way prior to assay. However, some sample preparation and preconcentration steps can now be accommodated in FIA. Some examples are on-line liquid-liquid extraction, solid phase extraction and ion-exchange procedures. In this way, FIA is managing to convert some traditionally labour-intensive steps into automated operations that have higher precision and faster throughput. FIA can also tolerate other sample types, such as fermentation broth samples and even gases through the use of silicon membrane separators and gas diffusion systems, respectively. 

Two-line and multiline manifolds are, of course, now commonplace for FIA methods. In fact, most of the procedures described in the FIA literature (Chapter 7) utilize this approach. Thus, in Ref. 52 is described a turbidimetric procedure for the determination of ammonia in low concentrations with the use of Nessler s reagent, while Ref. 253 recounts the spectrophotometric determination of chromium(VI). Besides being based on one-phase equilibria, multiline manifolds may also involve gas diffusion, solvent extraction, and liquid-liquid phase reactions in packed reactors (see the following sections). It should be emphasized, however, that a FIA system should always be kept as simple as possible, and that a well-designed chemical analysis will often require only the use of a two-line manifold. 

By incorporating gas diffusion into a FIA system, physically separating... 

The use of the dual-phase gas-diffusion cell allows the hydride to pass immediately through the membrane into a hydrogen acceptor stream. The effect of this process is a decrease of the contact time between the hydride and any transition metal precipitate. The end result is that the interferences observed for this system versus other flow methods generally are significantly diminished. Since the reduced contact time also favorably influences the interference due to hydride competition, the observed reductions are therefore really a combination of the separation techniques obtained in the FIA system and that of kinetic discrimination. 

FIA systems comprising dialysis (or microfiltration) employ modules similar to those used for gas diffusion, the hydrophobic microporous membrane being replaced by a hydrophilic dialysis membrane (such as cu-prophan, cellulose aceate, or cellulose nitrate). Basically, the incorporation of a dialysis unit into a FIA manifold is to serve one (or several) of the following objectives (1) separation of an analyte species from unwarranted matrix constituents, (2) as an exact and reproducible means of dilution, or (3) microfiltration performed continuously by transferring species from one stream (the donor) to another stream (the acceptor). 

The most recent experimental evidence obtained as a part of a graduate programme at the Chemistry Department A, in cooperation with Ib Andersen of Herlev County Hospital, confirms that determination of urea in full blood yields results which are in agreement with the more laborious urea assay performed on plasma samples [8.1]. The FIA manifold used incorporated a flow cell combining gas diffusion and optosensing (cf. Fig. 4.35), the separating barrier between the donor and accepting streams... 

Flow-injection methods Flow-injection techniques have been developed and studied for the determination of free chlorine in industrial formulations and water samples. Figure 5 shows a schematic diagram of a flow-injection analysis (FIA) assembly that employs a gas-permeable membrane (0.5 pm Fluoropore, Milli-pore) as a gas diffusion unit to separate dissolved chlorine from other potential matrix interferents. 

Gas-diffusion membranes Hydrophobic porous polymer membranes with air filling the membrane pores have been used successfully in the online separation of volatile and semivolatile analytes between two miscible liquid streams in flow injection analysis (FIA) systems. The corresponding technique is frequently referred to as gas-diffusion EIA. The mass transfer of an analyte across a gas-diffusion membrane is controlled by the membrane pore size and the solubility of the analyte in the feed and receiver solutions. The latter can be manipulated by appropriately modifying the chemical composition of the two solutions. In this way it is possible to enhance both the evaporation of the analyte from the feed solution into the membrane pores and its subsequent absorption into the receiver solution. 

The current definition of FIA expands its scope because, besides automating chemical reactions, flow injection systems foster online sample treatment, namely solvent extraction, gas diffusion, dialysis and solid-phase extraction, as found in many applications. 

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