Continuous-flow automatic analyser

Figure S.2 shows a schematic diagram of the automatic hydride/vapour-generator system designed by P.S. Analytical. This has been widely used to determine hydrideforming elements, notably arsenic, selenium, bismuth, tellurium and antimony, in a wide range of sample types. To provide a wide range of analyses on a number of matrices the chemistry must be very well defined and consistent. Goulden and Brooksbank s automated continuous-flow system for the determination of selenium in waste water was improved by Dennis and Porter to lower the detection levels and increase relative precision [10, 11]. The system described by Stockwell [9] has been specifically developed in a commercial environment using the experience outlined by Dennis and Porter.

Table 1.2 gives some of the reasons for the LGC setting up its automation team. The primary motivation was economic. LGC was often subject to constraints on staffing in parallel with large increases in analytical commitments. The introduction of cost-effective analyses, using mechanical or automatic instruments, reduces staff involvement and allows well qualified people to be released for the development of new analytical requirements. The analysis of beer samples by multi-channel continuous flow analyser [S, 6, 7] and the introduction of a mechanical solvent extraction and identification system to analyse and measure levels of quinizarin in gas oil, both for duty purposes, were prime examples [8], Both systems involved commercially available components and/or instruments integrated with modules designed and built in-house. 

Since ISEs can be used in continuous flow systems or in flow systems with sample injection (flow injection analysis, FIA)21 their application is wide, not limited to discrete samples. Analysis time becomes shorter, with faster recycling. Additionally, in flow systems the experimental assembly and data analysis can be controlled automatically by microcomputer, including periodic calibration. Another development is the use of sensors for the detection of eluents of chromatographic columns in high-pressure liquid chromatography (HPLC). Miniaturization has permitted an increase in the use of sensors in foods, biological tissues, and clinical analyses in general. 

A continuous analyser is attached to a sampling line and thereafter continuously and automatically obtains a signal proportional to the instantaneous concentration of a selected component in the flowing stream. The information acquired is automatically used to set the process environment controllers and to take any corrective action needed to control the process. These actions might be to close a valve, cool the stream, allow more diluent to be added, speed up mixing etc. Thus continuous analysers carry out the function of the control laboratory but in real-time and more efficiently. Continuous analysers are employed in many situations such as routine analysis, monitoring and on-line process control. 

Fig. 1.3 Scheme of the different types of automatic analysers, classified according to the way In which sample transport Is effected. The examples Illustrate the determination of a single analyte In a liquid sample requiring dilution (0) and sequential addition of two reagents (Ri, R2) for the analytical reaction to develop, (a) Batch analyser, (b) Continuous analysers (SFA, segmented-flow FIA, flow-injection CCFA, completely continuous flow), (c) Robot station. Note that agitation is carried out by independent units in (a), is not required in (b) and is effected by a single unit in (c). (Adapted from [17] with permission of Ellis Horwood). 

As far as separation techniques are concerned, they can be implemented on automatic continuous analysers or robot stations as ancillary modules (dia— lysers, ion exchangers, liquid-liquid extractors). As stated in Chapter 12, chromatographic processes —particularly column (HPLC and GC), but occasionally also planar chromatographic purposes— are commonly the subject of automation. A conventional chromatograph furnished with a system for sequential introduction of samples —which can even be partially treated in a continuous fashion before of after column separation (derivatlration and post-column techniques)— markedly resembles continuous flow analysers. Gas and liquid chromatographs are often used as separative-determinative modules in robot-stalons. 

It is significant that Technicon, the pioneering and virtually exclusive developers of continuous automatic analysers, have recently devoted their production efforts to batch models. Probably, their RA-1000 model was created to compete with the centrifugal analysers introduced by other firms and which were gradually displacing continuous-flow AutoAnaiyzers, widely used until a decade or so. 

Figure 9.3. Working schemes of continuous automatic analysers of segmented flow analyser (SFA, top) and flow-injection analysis (FIA, bottom) types...

A type of automatic analysis in which samples are mixed with reagents by means of plastic tubes, the solutions then being continuously pumped through a number of modules in which reaction takes place, finally passing to a flow-through colorimeter where the intensity of the colour is measured. The Technicon company manufactured the first commercial continuous flow analyser and since then have produced more sophisticated machines. It is these that will be described ... 

The schemes used for reactor control depend on the process and the type of reactor. If a reliable on-line analyser is available, and the reactor dynamics are suitable, the product composition can be monitored continuously and the reactor conditions and feed flows controlled automatically to maintain the desired product composition and yield. More often, the operator is the final link in the control loop, adjusting the controller set points to maintain the product within specification, based on periodic laboratory analyses. 

Thermogravimetric analyses were carried out in 10°-30° temperature increments with 200-mg samples using a conventional (Mauer) TGA system. Automatic recording of weight change was used to follow reaction to equilibrium, but actual weighings were recorded only by manual operation. The sample was bathed continuously in air of controlled humidity (Pmo = 7.9 torr) flowing at 180 cc/min. Precautions were taken to minimize drafts and convective currents, and buoyancy correction curve was made to 950°C. Further details on experimental methods are available (12). 

Automatic continuous analysers. I. Ail—segmented flow anal ysers... 

Fig. 5.1 Scheme of an automatic continuous segnented flow analyser. 

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