On-line analysers

All of the laboratory instrumental methods have been practically adapted for on-site analytical procedures, from pH measurement to NMR analyses. The instruments are generally less versatile than their laboratory counterparts. Dedicated to particular measurements and suitable for harsh environments or hazardous areas, they must be robust, so their design is different. They are configured for industrial process control. Many of them are process analysers for concentration measurements. A continuous sample is introduced via a diaphragm pump into a small tank. At intervals the sample or calibration standards are pumped into the chemistry stream where they are treated depending of the method of analysis (UV, IR). 

The use of a selective sensor plunged into the centre of a product to be analysed is not always possible. The preparation of the analyte prior to measurement leads to the removal of a small fraction in order to ensure that the applied treatment is adequate. 

In chemical analysis, as in many other sciences, statistical methodologies are unavoidable. The calibration curve constitutes an everyday application, just as an analytical result can only be ascertained if an estimation of the possible error has been considered. Once a measurement has been repeated, a statistical exploitation becomes possible. However, the laws of sampling and tests based upon hypotheses must be understood to avoid non-value conclusions, or to ensure the meaningful quality tests. Systematic errors (user-based, instrumental) or gross errors which lead to results beyond reasonable limits do not enter into this chapter. For the tests most frequently met in chemistry only indeterminate errors are considered here. 

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Enzyme immunosensors are used in flow injection systems and Hquid chromatography to provide automated on-line analyses (71—73). These systems are capable of continuously executing the steps involved in the immunoassays, including the binding reactions, washing, and the enzyme reaction, in about 10 minutes. 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

Top temperatures are usually controlled by varying the reflux ratio, and bottom temperatures by varying the boil-up rate. If reliable on-line analysers are available they can be incorporated in the control loop, but more complex control equipment will be needed. 

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. 

Catalytic activity was determined with a fixed bed microreactor which consisted of two coassial quartz tubes (i.d. 35 and 16 mm) to allow feed gas preheating and heated in an electrical oven (Watlow) with a temperature controller. The bed temperature was monitored by A K-type thermocoupling. On-line analysers (ABB) for CO, C02, CH4, ... 

Instrnmentation for UV-vis process analysis falls into fonr categories scanning instruments, diode-array instrnments, photometers, and fiber-optic spectrophotometers with photodiode-array (PDA) and charge-conpled device (CCD) detectors. The former two are more typically enconntered in at-line or near-line applications, whereas the latter two are better snited to actnal on-line analyses. 

Fiber-optic-coupled spectrophotometers (single beam and double beam) are the best choice for on-line analyses. The advent of nonsolarizing optical fiber has made possible on-line analyses in which the spectrophotometer may be located remotely from the process and light is carried to/from the process by the optical fiber. A rugged probe or flow cell provides the sample interface. 

This metho dology was previously successfully used for express on-line analyses of phosphate ores. The commercial TRACER 2100 Laser Element Analyzer, a laser-induced breakdown spectroscopy instrument, was utilized for rapid analyses of phosphate ores at the mine site (Rosenwasser et al. 2001). Excellent calibrations were achieved for P and Mg with correlation coefficients significantly above 0.98. The instrument demonstrated strong potential of the LIBS for use in on-site, real-time or grading. LIBS apparatus was developed for ap-... 

This technique encompasses a large scope of instruments ranging from mobile spectrometers to high resolution spectrometers. On-line analysers and probes that can be fitted to scanning electron microscopes (SEMs) allow instant analyses to be performed (i.e. microanalysis by X-ray emission). 

One may need to account for labile intermediates as found, for instance, in penicillin production [196,304]. Then, on-line analyses will best avoid artifacts due to storage of materials even though the samples are cooled to 4 or 5 °C. 

The numerical values of the time constants summarized in Table 1 confirm that there is no particular time for significant mutarotation to take place in rapid on-line analyses. If glucose uptake is anomerically specific, as has been shown by Benthin et al. [23], analytical results obtained with a glucose oxidase method must be corrected accordingly. The same holds true for analyses of rapid biological transients and for any spiking method (if the spiking solution happens to be freshly prepared). 

TR 61 831 Ed. 1 On-line analyser systems - Guide to design and installation... 

Vibrational spectroscopy is successfully employed to quantitative analysis of gases, especially if real time and on-line analyses are needed. In order to compensate the effects of pressure broadening, it is worthwhile to carry out all measurements at the same total pressure. To this end, the sample is placed in an inert gas, such as nitrogen or a noble gas, and the pressure raised to a defined value. The partial pressure instead of the concentration is used in the Lambert-Beer law. The calibration curve is valid only at the calibration temperature. If the temperature of the sample deviates from this temperature, the partial pressure has to be corrected by the Gay-Lussac law. 

Hiac PC4000 portable liquid particle counter is a contamination measurement tool, designed to run on-line analyses of hydraulic systems and fluids. The fully self-contained counter operates in the light-blocking mode using a laser diode and reports contamination levels at 4, 6, 10, 14, 21,38 and 70 pm at a flow rate of 60 ml min. ... 

Figure 2.4.1 Pastel UV (portable instrument) and STAC (on-line analyser) (reproduced by permission of Secomam)...

In this type of analysis, the result must be obtained rapidly as a strategy of feedback is used to optimize or to ensure the stability of the operation. The on-line analyser can control the procedure either up or down stream of the reactor (Figure 21.9). 

The problems associated with the sampling and automation of this stage were dealt with in their corresponding sections, so this section Is exclusively devoted to the automated instrumentation available for this type of sensor. As with water analysers, air analysers are classified into off- and on-line analysers, and the latter in turn Into single-parameter and multiparameter. 

On-line analysers are usually placed In purpose-built houses [7]. These provide a suitable environment for the analyser, operators and maintenance personnel alike, a conditioned atmosphere ensuring the stability of the analyser, and the Ideal way to link and allocate additional supplies such as electricity, air, water, vacuum, etc. 

Trace element analyses are often required for the determination of toxic metals such as chromium, mercury and lead in environmental samples and for monitoring the workplace environment. Conventional methods requiring extraction and separation procedures are time consuming. However, recent developments in GC and HPLC interfaced to atomic absorption and plasma emission spectrometers have enabled on-line analyses to be carried out. Ideally, the GC or HPLC column should be connected directly to the spectrometer sample cell or sample area to avoid dilution and loss of resolution. In practice a short heated transfer line of stainless steel or silica is used which has an internal diameter smaller than the column i.d. 

The purity of the reactor water is checked by measurement of pH, conductivity, turbidity, oxygen concentration, radioactivity, etc., either by sampling or - more usual — by on-line analyses. 

Because operation is not steady state, much more attention must be devoted to running the plant or on-line analysing equipment must be fitted and depended upon. 

Moreover ion-selective electrodes can be manufactured in very robust probes and, provided they are protected from fouling and physical attrition, they can often be used for continuous and on-line analyses as well as laboratory determinations. Their application is expected to increase substantially in the coming years. 

Murai, T. Iwabuchi, H. Ikeda T. Identification of gemfibrozil metabolites, produced as positional isomers in human liver microsomes, by on-line analyses using liquid chroma-tography/mass spectrometry and liquid chromatography/ nuclear magnetic resonance spectroscopy. J. Mass. Spectrom. Soc. Jpn. 2004, 52 (5), 277-283. 

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