Flow cell analyte transport

FTA [5-7] is a version of continuous-flow analysis based on a nonsegmented flowing stream into which highly reproducible volumes of sample are injected, carried through the manifold, and subjected to one or more chemical or biochemical reactions and/or separation processes. Finally, as the stream transports the Anal solution, it passes through a flow cell where a detector is used to monitor a property of the solution that is related to the concentration of the analyte as a... 

A typical extraction manifold is shown in Figure 13.2. The sample is introduced by aspiration or injection into an aqueous carrier that is segmented with an organic solvent and is then transported into a mixing coil where extraction takes place. Phase separation occurs in a membrane phase separator where the organic phase permeates through the Teflon membrane. A portion of one of the phases is led through a flow cell and an on-line detector is used to monitor the analyte content. The back-extraction mode in which the analyte is returned to a suitable aqueous phase is also sometimes used. The fundamentals of liquid liquid extraction for FIA [169,172] and applications of the technique [174 179] have been discussed. Preconcentration factors achieved in FIA (usually 2-5) are considerably smaller than in batch extraction, so FI extraction is used more commonly for the removal of matrix interferences. 

Dynamic factors are among the key variables to be optimized in an SFE process. In addition to extracting the analytes, the primary function of the supercritical fluid is to transport the solutes to the collecting vessel or to an on-line coupled chromatograph or detector. Ensuring efficient transportation of the analytes following separation from the matrix entails optimizing three mutually related variables, namely the flow-rate of the supercritical fluid, the characteristics of the extraction cell and the extraction time. These factors must be carefully combined in order to allow the flow-cell to be vented as many times as required. 

Another option is to place the filtration unit inside the flow cell, as demonstrated in the spectrophotometric flow injection determination of hydrogen peroxide [297]. The analyte interacted with titanium(IV) and 2-((5-bromopyridyl)azo)-5-(N-propyl-N-sulfopropylamino) phenol (PAPS) yielding a red-purple complex. After ion pairing with CTAB, the complex was adsorbed and concentrated on a very small area of the membrane filter positioned inside the flow cell. The analyte was quantified directly in the membrane by solid-phase spectrophotometry (see 4.1.1.4). Thereafter, ethanol was injected in order to solubilise the complex and transport it to waste. 

PDE Fundamental equation of analyte transport inside flow cell (partial differential equation)... 

To model the effect of an analyte injection, the equation has to be solved for an initial condition of zero analyte concentration inside the flow cell at f = 0 and a boundary condition of analyte concentration o at the entrance of the flow cell a (0,y, t) = ao. Results are shown in Fig. 7. It can be seen that for the central part of the vertical profile the injected analyte concentration o is achieved relatively rapidly (depending on the flow rate), but the analyte concentrations remains zero in close proximity of the cell walls. This is a direct consequence of laminar flow - analyte transport to the active surface layer by laminar flow alone is very ineffective. 

The first simpUfication of Eq. 37 is based on the assiunption that the analyte transport in the x direction is mainly conductive, i.e., it is controlled by the flow in the cell. The relation between conductive transport and diffusion in y direction is often characterized by the Peclet number, which reflects the ratio of the ideal time required for an analyte molecule to diffuse from the cell... 

Equations 46 have been directly derived from the full model in [19]. On the other hand, they are almost identical with the relations obtained from the so-called two-compartment model (the only difference is that the numerical coefficient is a little bit lower). The two-compartment model was first developed for sensors with receptors placed on small spheres [23]. In [24-26] it was adapted for the SPR flow cell and in [ 18] it was approved and verified by comparison of munerical results with those obtained from the full model. The two-compartment model approximates the analyte distribution in the vicinity of the receptors by considering two distinct regions. The first is a thin layer around the active receptor zone of effective thickness fiiayer> and the second is the remaining volume with the analyte concentration equal to the injected one, i.e., a. While the analyte concentration in the bulk is constant (within a given compartment), analyte transport to the inner compartment is controlled by diffusion. The actual analyte concentration at the sensor surface is then given by the difference between the diffusion flow and the consump-tion/production of the analyte via interaction with receptors. For the simple pseudo first-order interaction model we obtain ... 

Schulz H.D. and Reardon E.J. (1983) A combined mixing cell/analytical model to describe two-dimensional reactive solute transport for unidirectional ground-water flow. Water Resour. Res, 19, pp. 493-502... 

Integrated sorption-detection units are based on the placement of an inert or active support in the flow cell of a nondestructive spectroscopic detector where the analytes or their reaction products are retained temporarily for sensing immediately after their elution. The equipment required to develop this type of sorption methodology is very simple and closely resembles that used in ordinary flow injection analysis (FIA) manifolds. The only difference lies in the replacement of the packed reactor located in the transport-reaction zone with a packed flow cell (usually photometric or fluorimetric) situated in the detector. 

Two methods have been described in the literature for FTIR detection after SFC a flow-cell approach, in which the column effluent is monitored by the FTIR beam as it flows through the cell [21,22,23] and a solvent-elimination approach. With this interface the column effluent is sprayed on to an infrared transport support from a restrictor. The mobile phase evaporates and leaves the analyte as a concentrated spot on the surface, which is later analysed using an FTIR microscope [23]. 

The analyte transport through such membranes is based on a solvation/diffusion mechanism in a lipophilic phase. The liquid membranes have to be supported by a microporous and hydrophobic layer. The flow separation cells shown in Figure 5 can be used to apply these membranes for separation procedures that can be coupled online to flow analytical, liquid, and gas chromatographic setups. 

The FIA system is defined as an automated method of an analytical procedure in which a liquid sample is injected into a continuous carrier flow, and this mixture is driven to the detection cell. During transport, the sample is subjected to several operations such as dilution, incubation, mixing, and dialysis (Ruzicka and Hansen, 1988). The monose-gmented FIA is a kind of flow system in which the sample is transported between two air bubbles, but the flow is not continuous and the bubbles need to be purged before they enter the detection cell (Calatayud, 1996). The most sophisticated FIA manifolds are those based upon computer-controlled automated systems, such as SIA and MSFIA (Magalhaes et ah, 2009). 

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