Continuous-flow techniques

Jas G, Kirschning A (2003) Continuous flow techniques in organic synthesis. Chem Eur J 9 5708-5723... 

Coakley, W. A. Handbook of Automatic Analysis. Continuous Flow Techniques Dekker New York, 1982. 

Results presented by Stockwell [9] for some of the hydride-forming elements and for mercury illustrate the enormous increase in sensitivity achieved with automated analytical chemistry methods (Table 5.2). Earher developments centred on the batch approach. These methods have recently been dropped (in favour of continuous-flow techniques) because they were not easy to use, were very dependent on operator abihty, and were difficult to automate. 

Table 5.3 sets out the advantages and disadvantages of the batch and continuous flow techniques. The introduction of continuous-flow hydride/vapour-generation has substantially advanced the value and acceptance of the technique for trace elemental analysis. Appfied Research Laboratories (now part of Fisons Elemental), P.S. Analytical and Varian have all introduced continuous-flow hydride/vapour-generation systems, whilst Perkin Ehner has used the flow injection modification to automate the techniques with their instrumentation. 

Wiles C. and Watts, P. and Haswell, S. J. (2007). An efficient, continuous flow technique for the chemoselective synthesis of thioacetals. Tetrahedron Letters, 48, 7362-7365. 

Burgoyne and Hayes (1998) and Sessions et al. (1999) introduced the continuously flow technique for the D/H measurement of individnal organic compounds. The precise measurement of D/H ratios in a He carrier poses a number of analytical problems, related to the tailing from the abundant " He+ onto the minor HD+ peak as well as on reactions occurring in the ion source that produce H3+. However, these problems have been overcome and precise hydrogen isotope measurements of individnal organic compounds are possible. 

Peracetic Acm-AcErALDEHYDE Reaction. The cobalt- and manganese-catalyzed reactions of peracetic acid with acetaldehyde were studied by a continuous flow technique (9). Peracetic acid (0.15M in acetic acid) and acetaldehyde-catalyst solutions were metered through rotameters to a mixing T (standard 0.25-inch stainless steel Swagelok T) and... 

All these results were obtained using screen-printed electrodes with batch amperometric or continuous flow techniques (for more details see Procedure 17 in CD accompanying this book). 

There are, however, a number of disadvantages to using continuous flow techniques to study the kinetics of reactions on soil constituents. Often the colloidal particles are not dispersed—for example, the time required for an adsorptive to travel through a thin layer of collodial particles is not equivalent at all locations of the layer. Consequently, mass transfer can be significant if the sample is not dispersed. Skopp and McAllister (1986) note that even if the sample is dispersed, different pore and particle sizes of the adsorbent may result in nonuniform tracer transit times. The thickness of the disc of colloidal particles should be thin and measured to establish that perfect mixing is operational. 

Figure 3.4. Apparent adsorption and desorption of boron (B), determined using the continuous flow technique (Sparks et at., 1980b) with no adsorbent. [From Carski and Sparks (1985), with permission.]...

The methods described here are illustrated in Figs 6.11 and 6.12 for the continuous flow technique, and in Fig. 6.13 for leakage compensation. 

If one of the species is predominantly hydrated (e.g. the quinazoline cation ), then, by using the rapid-reaction apparatus and allowing an acid solution of the substance and a near-neutral buffer to run through the cell continuously [continuous-flow technique ), the spectrum of the predominantly hydrated (unstable) neutral species can be obtained. This is possible if the newly formed species is not too unstable. 

Has lower sample consumption compared with continuous-flow techniques... 

Wang (4) et al studied regeneration kinetics on a zeolite catalyst using pulse and continuous flow techniques Theyproposed a parallel-consecutive reaction mechanism... 

Two other filtration modules, which both formed part of automated radioimmunoassay systems, have been developed. - One of these, was based on conventional continuous flow techniques. At the filtration stage, a continuously moving strip of glass fiber filter paper was strengthened and wetted, the reaction mixture was filtered, and the precipitate was washed by two streams of buffer. The strip was dried and overlaid with cellulose adhesive tape before being counted as it passed between two end-window radioactivity detectors and wound onto a take-up spool. In the second system, glass fiber filter disks are mounted at intervals over perforated segments of a flexible plastic carrier tape. The contents of five... 

W. Coakley, Handbook of Automated Analysis. Continuous Flow Techniques, Marcel Dekker, New York, 1981. 

One of the most interesting and green continuous flow techniques to be developed emerged from the ability to perform triphasic reactions, whereby a gas-liquid phase is pumped through a packed ed containing a heterogeneous catalyst. Using this... 

The continuous flow technique was used to measure the catalytic activity of zeolites in the reaction between methanol or ethanol and hydrogen sulphide (see ref.2). The catalysts were activated in helium at 674K for 4 hours. The reaction was carried out at 523K by using a reagent mixture containing... 

Adsorption isotherms of cyclopentane vapours were measured at 20°c. Temperature programmed reduction profiles were measured by the continuous flow technique with a thermal conductivity detector. Simultaneous TG, DTG and DTA were carried out in an argon atmosphere at the heating rate of 10 K.min Catalytic measurements were performed in a continuous flow integral fixed bed reactor under 0.1 MPa at 200°C. The inlet C0/H ratios ranged from 0.5 to 2.0, the space velocity was... 

Continuous-flow techniques offer different advantages on application to clinical chemistry. The differences between one another are most evident when they are applied to the determination of the same analyte. Thus, let us consider the determination of albumin by all three techniques. The Bromocresol Green (BCG) method proposed by Rod key [29] is specific for this substance. At pH 4.5, BCG binds to albumin to form a stable complex which is monitored at 630 nm. The corresponding SFA method is fairly fast (150 samples/h) and precise, and has few Interferences —chiefly from ferrlln and some proteins at high concentrations. The problem of the sample turbidity or the presence of lipaemic substances is minimized by Increasing the sample dilution to 236 1. The manifold used for this determination is shown in Fig. 14.9a. 

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