Merging zones

Flow injection analysis (FIA) (Ruzicka and Hansen), since 1975 In continuous flow, stopped flow or with merging zones (FIA scanning or intermittent pumping) Adapted voltammetric electrodes Membranes for Partial dialysis Membrane amperometry (Clark) Differential techniques (Donnan) Computerization, including microprocessors Special measuring requirements in plant control (to avoid voltage leakage, etc., Section 5.5)... 

Flow injection analysis (F1A). In this technique, introduced by Ruzicka and Hansen, a small amount of sample is injected into a liquid flow (see Fig. 5.16), which apart from being automated is normally continuous, but can include the use of stopped-flow, merging zones extraction techniques in addition to FIA scanning and methods based on intermittent pumping89. The principles of FIA and the versions just mentioned will now be briefly discussed on the basis of the excellent review of Ruzicka and Hansen89 in order to understand the appli-cational possibilities of electrochemical detection in this technique. 

Figure 3.10 — Flow manifolds for implementation of flow-through biosensors. (A) Flow injection merging-zones manifold for the bioluminescence detennination of ATP. ATP standards (30 fiL) and luciferin (30 fiL) are injected into the buffered carrier streams, each pumped at 0.7 mL/min and synchronously merged 12.5 cm downstream. Distance from merging point to immobilized enzyme coil, 2.2 cm. (Reproduced from [59] with permission of Elsevier Science Publishers). (B) Completely continuous flow manifold for the determination of NADH. (Reproduced from [71] with permission of the Royal Society of Chemistry). (C) Segmented-flow manifold for the determination of L-(+)-lactate. (Reproduced from [65] with permission of Marcel Dekker, Inc.). (D) Single-channel flow injection manifold with immobilized reagent for the detennination of glucose. (Reproduced from [77] with permission of Elsevier Science Publishers).

Figure 4.9 — Merging-zones manifold coupled to a flow-through sensor for the determination of Bg vitamers. P peristaltic pump SV switching valve V Vj injection valves S sample R reagent C microcomputer W waste. (Reproduced from n031 with oermission of Elsevier Science Publishers ).

Flow injection analysis has been used to determine aluminium in soil. Reis et al. [2] studied the spectrophotometric determination of aluminium in soil using merging zones and sequential addition of pulsed reagents. 

E. C. Silva, M. C. U. Araujo, R. S. Honorato, J. L. F. Costa Lima, E. A. Guidetti Zagatto, S. M. Boscolo Brienza, Standard additions in flow injection analysis based on merging zones and gradient exploitation application to copper determination in spirits, Anal. Chim. Acta, 319 (1996), 153-158. 

H. Bergamin-Filho, E.A.G. Zagatto, F.J. Krug, B.F. Reis, Merging zones in flow injection analysis. Part 1. Double proportional injector and reagent consumption, Anal. Chim. Acta 101 (1978) 17. 

J. Ruzicka, E.H. Hansen, Stopped-flow and merging zones. A new approach to enzymatic assay by flow injection analysis, Anal. Chim. Acta 106 (1979) 207. 

C.S. Lim, J.N. Miller, J.W. Bridges, Automation of an energy-transfer immunoassay by using stopped-flow injection analysis with merging zones, Anal. Chim. Acta 114 (1980) 183. 

C.D. Crowe, H.W. Levin, D. Betteridge, A.P. Wade, A random-walk simulation of flow injection systems with merging zones, Anal. Chim. Acta 194 (1987) 49. 

Intermittent stream addition is often exploited to achieve merging zones [106], to speed up the flushing time in a flow analyser [107] or to implement multi-analyte determinations [108]. At the confluence point, the sample/carrier stream concentrations undergo sudden variations caused by the addition of the merging stream (Eq. 3.10). Consequently, the Schlieren effect can manifest itself as a spurious transient signal, usually observed before sample arrival in the detector flow cell. This effect is more likely to occur in highly versatile flow systems such as multi-commuted flow systems [109]. 

Finally, it should be stressed that there are some specific strategies that can strongly influence sample dispersion in flow injection analysis, e.g., merging zones, zone sampling, stream splitting and closed-loop arrangements. These strategies are discussed in Chapter 7. 

With two rotary valves or a sliding bar valve with two commutation sections [110], two solutions can be sampled and inserted, and this is the essence of flow systems exploiting, e.g., merging zones or zone sampling (see 7.1.1 and 7.2.1). 

Merging Zones Relying on Convergent Carrier Streams 245... 

Merging Zones Relying on a Single Carrier Stream 246... 

The merging zones approach was conceived independently by Bergamin-Filho et al. [1] and by Mindegaard [2] as the first strategy exploiting multiple injections in the context of flow injection analysis. As... 

The merging zones strategy has been implemented by exploiting two convergent carrier streams, a single carrier stream or an intermittent stream, and the main characteristics of the resulting systems are discussed below. 

---