Analysers, Continuous

Trimborn et al. (2000) describe a four-week field campaign to characterise an aerosol in a particular area, where the LAMPAS 2 instrument was continuously analysing the size and composition of individual particles in five size ranges between 0.2 pm and 1.5 pm. Some 10,000 single particle spectra were recorded during the measuring period, and one example of these is shown in Figure 3.13. 

A number of the previously cited investigators3->2>5 9 have employed UV spectroscopy as an analytical tool for following PC degradation. We have found the measurement of UV spectra of weathered PC films by difference from an unexposed reference sample to be an extremely simple and useful analytical method. This nondestructive analysis allows the repetitive return of a sample to the exposure conditions and thus enables one to essentially perform continuous analyses on the same sample. This technique, of course, will not detect the formation of non-chromophoric products such as aliphatic oxidation products which may form during the degradation. 

Regardless of the analytical property measured and the measuring technique used, a continuous analyser or a detector should satisfy certain general requirements ... 

The discrete method has the advantage that samples can he processed at a high rate. For example, commercial colorimetric analysers are capable of yielding between 100 and 300 measurements per hour, whereas for continuous analysers a processing rate of 20— 80 samples per hour is normal. However, the high-throughput discrete analysers are appreciably more expensive than the continuous analysers. 

Two parameters have been demonstrated to be fundamental in calculating the performance characteristics of a continuous analyser, the lag phase and the half-wash time they afford a correlation between the approach to steady state, fraction of steady state reached in a given time and the interaction between samples. The half-wash time (Wy ) is the time for the detector response to change from any value to half that value, the lag phase L is defined in the ensuing discussion. 

Table 19 continued Analyses results Rudolf/Fresenius ... 

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). 

The CO-TPD measurements were performed in a flow reactor, described elsewhere [12]. A quadropole mass spectrometer continuously analysed the gas composition after the monolith... 

The apparatus for kinetic tests is shown in Figure 1. It comprises a quartz tubular flow reactor 300 mm height and 20 mm internal diameter, heated by an electrical furnace. The reactor temperature was controlled by a programmer-controller (Ascon). Cylinder air (99.999 % purity) was fed to the reactor and the flow rate was controlled by mass flow controllers (Hi-Tec). Exhaust gas concentrations were determined by Hartmann Braun continuous analysers Uras lOE (for carbon monoxide and carbon dioxide) and Magnos 6G (for oxygen). The signals from the analysers were acquired and processed by a personal computer which also performed the control of the experiment. 

Table X Continued).—Analyses of Irish Pot Still Whiskies (Schidrowitz)... 

Several categories of analytical instruments were modified for continuous analyses in a process area. This usually involved interfacing the primary analytical instrument with a computer system, which would control the instrument, obtain the raw data, apply any calibration factors necessary and finally provide the chemist or engineer with the pertinent data in graphical and tabular form. A few of these instruments, such as gas chromatography, were already well accepted in the process areas and required only minimal development to capture and present the data in the desired form. Other instruments had rarely, if ever, been introduced in the process area and required extensive development for appropriate packaging as well as computer control, data acquisition, and graphical presentation. Examples... 

Continuous analysers make continuous measurements directly in the flowing process stream, in a bypass test stream or inside a reactor vessel. This works best when no pretreatment of the sample is required although simple manipulations, such as reagent addition, sample filtration or dilution, can be carried out prior to the analyser by a sample preparation chamber. Actual determinations are carried out in a flow-through sample cell. 

Often, direct sensing instruments are used, e.g. probes or electrodes. If there are a number of streams with similar composition, they can all be directed into a common manifold that leads to the flow-through analyser (Figure 9.4). Each stream can be sampled in sequence. This provides rapid analyses for several streams. Continuous measurement means that the instrument constantly measures some physical or chemical property of the sample and yields dynamic information that is a continuous function of time. Continuous analysers are used when speed and sensitivity to changes are key to the process. 

Continuous analysers are automated and operate on-line or in-line. They are usually incorporated into a control loop which operates by means of a feedback mechanism... 

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. 

Figure 9.5 Schematic diagram of a continuous analyser connected to a control loop that...

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). 

Continuous analysers, typically represented by process analysers [20, 21] use the continuously generated results to adjust an industrial process in... 

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. 

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... 

The manner in which liquid samples are introduced into a continuous analyser depends on Its nature. Figure 3.2 illustrates some of the commoner situations in this respect, which are also commented on below. 

In completely continuous analysers, samples are taking from an evolving system such as a pipe pouring out waste water or an industrial effluent. A peristaltic pump continuously aspirates the sample, In which the evolution of one or several analytes is monitored as a function of time. This is the simplest possible alternative and Is characterized by the absence of discontinuities however, the system can be programmed to operate In a discrete fashion if required. Reverse FIA and completely continuous assemblies (Fig. 3.2a) are representative examples. 

Fig. 3.2 Different ways of introducing liquid samples into continuous analysers (a) continuously (b) by injection (c) by aspiration without air (d) by aspiration with air.

Fig. 3.7 Sample introduction systems for multi-analyte determinations based on the splitting of the samples into accurately measured volumes. (1) Batch analysers use special multi-injection valves. (2) Continuous analysers employ a manifold directing an accurately measured sample volume to each of the determinative units (Ai, Az, etc.).

In the sample collection system. The magnitude of the effect Is similar In batch and continuous analysers In this case. It arises from the use of the same probe and conduits to take samples and/or reagents successively and can be minimized In four ways ... 

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. 

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