Automatic continuous analysers

These analysers resemble hquid and gas chromatographs, although their background is markedly different. They differ from discrete automatic analysers (DAA) in various respects, namely the fashion in which samples are transported and mixed with diluents and reagents, the manner in which carry-over between samples and reagents is avoided and the type of detection used. 

The classification of automatic continuous methods is based on the way in which carryover between samples successively introduced into the analyser is avoided. Two general groups have been described by Valcarcel and Luque de Castro [20]. These are illustrated in Table 2.4. 

Continuous segmented methods avoid carry-over by use of air bubbles establishing physical separations (segments) along the continuous flowing stream. These methods were invented by Skeggs [1] and formed the basis of the Technicon AutoAnalyzer. They are now also implemented on Skalar assembhes. Samples are introduced sequentially by aspiration with a moving articulated pipette. 

Segmented By aspiration Sequential Continuous Segmented flow analysis (SFA) 

Unsegmented By aspiration Continuous Continuous Completely continuous-flow analysis (CCFA) 

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Fig. 1.9 Automation of the first and third stages of the analytical process (Type 5 analyser). Scheme of automatic continuous analyser for determination of pollutants in waste water, based on a reversed FIA configuration. (Reproduced with permission of the copyright holders).

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. 

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

Most samplers are electronically or computer-controlled, so that they allow programming of the aspiration probe and the turntable. Thus, the Techni-con Sampler II permits the selection of the sample-to-washing solution volume ratio, which can be varied between 1 6 and 6 1, and the sampling rate (20, 40 or 60 samples/h). Later models such as the Sampler IV are even more flexible and work over wider ranges of the above-mentioned parameters. The SOLIPpepII module (also manufactured by Technicon), described in Chapter 3, allows direct sampling of solid samples in automatic continuous analysers. 

The amount of some gases, such as SO2 [73], CO [74], can be automatically continuously measured by polarographic analyzers [75]. Measuring electrodes with a constant surface, e.g. carbon, platinum, gold disks or rods are most often used. Present research is concentrated on the so-called coulometric analyzers [73]. The analysed gas is brought by the shortest route directly to the measuring electrode. The substance to be determined is completely reduced or oxidized electrochemically at a large area electrode. 

Catalytic activity data were obtained by using a conventional fixed-bed reactor at atmospheric pressure. A stainless steel tube with an inner diameter of 12 mm was chosen as the reactor tube. Catalyst (3.5 cm, ca. 1.8 g) was placed on ceramic wall at the lower part of the reactor. The upper part of the catalyst bed was packed with 10 cm of inactive ceramic spheres (2 mm O.D.) to preheat the gas feed. The furnace temperature was controlled with a maximum variation of 2°C by an automatic temperature controller. The gas exiting the reactor was led to a condenser to remove water vapour. The remaining components were continuously analysed by non dispersive infrared (CO and CO2), flame ionisation (HC), magnetic susceptibility (O2), and chemiluminiscence (NOx). 

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). 

In contrast to the two operations described above, the incorporation of the solid sample Into the analyser or Instrument Is comparatively easy to automate. Samplers with cups or vials holding each sample separately are relatively Inexpensive. In batch analysers, samples are treated and transferred separately continuous analysers, which are much commoner, involve Intermediate operations (dissolution, extraction, etc.) and do not have many automatic systems available for incorporation of solid samples. One such system Is the... 

Analytical separation techniques play a major role in the above-mentioned preliminary operations. Their implementation on automatic systems can be achieved in a variety of ways, although most often it is done in one of two ways, namely discontinuously or off-line and continuously or on-line. The former is better suited to continuous analysers (SFA, FIA), and the latter is equally suited to continuous and batch analysers. 

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

Fig. 5.2 Characteristic profile of transient signal provided by an automatic continuous segmented analyser and parameters defining it. The dotted line represents the theoretical response.

The applications of automatic continuous segmented analysers can aiso be classified according to the type of detection system involved. Thus, 70-75 of all the methodologies described on this topic used molecular UV absorption spectroscopy (spectrophotometry, photometry), followed by ISE potentiometry (10-15 ) and, much less often, nephelometry, fluorimetry, etc. The applications described below were mostly developed with the aid of Technicon technology and are classified according to this criterion —other applications to specific problems related to laboratory processes are described in the corresponding chapters. 

Analytical processes performed by DAAs are similar to those carried out manually. Hence, they allow readier adaptation of manual methods than do continuous analysers insofar as the latter require stricter optimization of the different chemical and physico-chemical variables involved. As can be seen from Fig 8.2, of the three types of automatic analysers, robotic types bear the strongest resemblance to manual configurations. 

In a test chamber with a capacity of approximately 155 litres, air is circulated in a closed circuit with an adjustable flow so that there are 0.5 to 3 air changes per minute. The air temperature can be set from 20 to 7.5 °C and is controlled with a precision of 1 "C. The ozone concentration can be set from 0.1 to 5.5 ppm and an electrical signal is emitted, proportional to the concentration being used, by means of a continuous analyser, based on the bubbling of the air-ozone mixture in a potassium iodide buffer solution. This electrical signal acts automatically on the regulator of an UV lamp that generates the ozone. 

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. 

On-line sensor/analysing equipment is an automatic measuring device giving an output signal linked to the value of one determinant (or more) from a medium, continuously or at regular time intervals. The choice and implementation of an on-line sensor must take into account several items or properties related to use constraints, which are presented in Table 7 (derived from [11]). 

Discontinuous methods are performed in conventional separating funnels in one or more steps. In ultratrace analyses, tapered or specially profiled quartz tubes are recommended because of their easier cleaning (more compact size), the introduction of less contaminating material, and easier centrifugation in the case of difficulties with phase separation. Shaking must be continued until equilibrium is reached, which may last seconds, minutes, or (rarely) hours, depending on the physicochemical properties of the system more than 2-5 min requires a mechanical shaker. Microscale extraction carried out in autosampler tubes, followed by direct automatic introduction of the organic phase into the atomizer, is recommended. 

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