Integrated microconduits

When reviewing the development of FIA over the past 10 years, several patterns emerge, of which the trends toward miniaturization, exploitation of the possibilities inherent in the concept of controlled dispersion, and application and introduction of new flowthrough detectors are the most striking. While the first FIA system [1] used more than 10 mL of reagent and 0.5 mL of sample for a single measurement, contemporary designs 

In Fig. 4.68 is shown a microconduit incorporating two potentiometric pH electrodes and a common reference electrode, the introduction of sample solution being executed by means of an exteriorly placed injection port. The measurement of pH requires a system with limited dispersion coefficient, and no chemical reaction is needed in the flow channel. Consequently, a short residence time was chosen, and the two pH-sensitive PVC-based membrane electrodes, containing as electroactive material tri- -dodecylamine [778], were placed in a single-line system and very close to the injection position (cf. Fig. 4.3), the Ag/AgCl wire reference electrode being situated in a side channel and connected to the main channel downstream from the indicator electrodes. The manifold construction is such that the reference solution and thus the liquid junction are renewed 

In all the previous examples, the sample zone was injected into the microconduit channel from an external sample valve, yet ultimately this function, of course, ought to be integrated into the microconduit. Miniaturization of a rotary valve is one possibility (see below), while another is the use of the hydrodynamic injection principle ([338 cf. also Section 5.1.3], which involves a combination of hydrodynamic and hydrostatic forces to aspirate, meter, and inject the sample solution in the form of a well defined plug into the carrier stream. 

Recently, a novel type of integrated optical detectors based on the interaction of radiation with a surface situated in a flowing stream was introduced [848]. Exploiting absorbance, reflectance, and fluorescence of visible and/or UV light as it changes due to chemical reactions taking place at or in close proximity of a surface surrounded by a flowing stream, 

Guldager Petersen, FIA Gradient Techniques. Development of a Procedure for Standard Addition [in Danish]. Chem. Depart. A, Tech. U. Denm., (1983). (M.Sc. Thesis, Part II). 

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Ruzicka, J. Plow Injection Analysis Prom Test Tube to Integrated Microconduits, Anal Chem. 1983, 55, 1040A-1053A. 

Figure 3.37 — (A) Optosensor with fibrous flow-through structure on the surface of which a pH indicator is covalentiy bound C Carrier stream d thickness of indicator-containing celiuiose pad r reflector o opticai fibre. (B) Integrated microconduit for measurement of pH comprising injection valve and optosensor S sample solution, PI and P2 tubes leading to the peristaltic pumps W waste tube. (Reproduced from [262] with permission of Elsevier Science Publishers).

Figure 4.17 — (A) Exploded view of a tubular flow-cell integrated microconduit system. I Ag/AgCl inner reference electrode M sensitive membrane S internal reference solution. (B) Detail of the integrated microconduit shown within the dotted lines in C. (C) Integrated-microconduit FI manifold for potentiometric measurements C carrier stream R reference electrode solution P pump V injection valve I indicator electrode R reference electrode I pulse inhibitor G ground W waste. (Reproduced from [140] with permission of Pergamon Press).

J. Ruzicka, Flow injection analysis. From test tube to integrated microconduits, Anal. Chem. 55 (1983) 1040A. 

One of the first prototypes involved the use of integrated microconduits [104]. The flow channels, the injection device and the outlet to the detector were mechanically engraved into a flat PVC block using channels with a cross sectional area of 0.8 mm2 and covered with a flat plate glued on top of the machined block. The system was operated with a conventional peristaltic pump. A similar system, but using a piston pump, was proposed in the 1990 s [105]. 

J. Ruzicka, E.H. Flansen, Integrated microconduits for flow injection analysis, Anal. Chim. Acta 161 (1984) 1. 

As with other analytical techniques, there is a trend in FIA towards significant reductions In size, which ultimately result in considerable advantages. In this way miniaturized FIA In its two versions was conceived capillary FIA and integrated microconduits. 

To summarize, the three scaling factors are the dispersion coefficient D, the residence time t, and the dispersion factor Pi/2 = SmlVr- These factors were used for optimization of the design of integrated microconduits [608] (Section 4.12). 

The present trend toward miniaturization in modem analytical chemistry has affected FI in two main ways (1) size reduction of the different units, especially tubing diameters and (2) the design and commercialization of the so-called integrated microconduits , which include in a single plastic block smaller than a cigarette box almost the whole FI manifold (injection, reaction/transport, and detection units). 

The most recent variation of FIA is the so-called lab-on-a-valve (LOV) technology introduced by Ruzicka. The LOV idea incorporates an integrated microconduit on top of the selection valve u.sed in SIA. The nii-croconduit is designed to handle all the unit operations needed for a given analytical procedure. Mixing points for analyte and reagents, column reactors, bead reactors, separation columns, and membranes can all be accommodated within the LOV system. In some cases, microfluidic flow systems are used in ways similar to those described in the next section. 

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