On-line method

On-Hne Procedures The growing trend toward automation in industry has resiilted in many studies of rapid procedures for generating size information so that feedback loops can be instituted as an integral part of a process. Many of these techniques are modifications of more traditional methods. The problems associated with on-line methods include allocation and preparation of a representative sample analysis of the sample evaluation of the results. The interface between the measuring apparatus and the process has the potential of high complexity, and consequently, high costs [Leschonsld, Paiticle Cha racterization, 1, 1 (July 1984)]. 

Coupled liquid chromatography-gas chromatography is an excellent on-line method for sample enrichment and sample clean-up. Recently, many authors have reviewed in some detail the various LC-GC transfer methods that are now available (1, 43-52). For the analysis of normal phase eluents, the main transfer technique used is, without doubt, concurrent eluent evaporation employing a loop-type interface. The main disadvantage of this technique is co-evaporation of the solute with the solvent. 

Statistical process control (SPC) is an important on-line method in real time by which a production process can be monitored and control plans can be initiated to keep quality standards within acceptable limits. Statistical quality control (SQC) provides off-line analysis of the big picture such as what was the impact of previous improvements. It is important to understand how SPC and SQC operate. 

Successful combination of a chromatographic procedure for separating and isolating additive components with an on-line method for obtaining the IR spectrum enables detailed compositional and structural information to be obtained in a relatively short time frame, as shown in the case of additives in PP [501], and of a plasticiser (DEHP) and an aromatic phenyl phosphate flame retardant in a PVC fabric [502], RPLC-TSP-FTIR with diffuse reflectance detection has been used for dye analysis [512], The HPLC-separated components were deposited as a series of concentrated spots on a moving tape. HPLC-TSP-FTIR has analysed polystyrene samples [513,514], The LC Transform has also been employed for the identification of a stain in carpet yarn [515] and a contaminant in a multiwire cable [516], HPLC-FTIR can be used to maintain consistency of raw materials or to characterise a performance difference. 

Progress in the Study of Methanol Oxidation by In Situ, Ex Situ and On-Line Methods... 

The mechanism of the cocatalytic effect is still a matter of investigation. For most of the systems of interest in electrocatalysis, data for characterization of the surface by means of spectroscopic UHV methods are still missing. Also measurements of changes in the electronic properties of the metal in the presence of adatoms in addition to more intensive application of in situ and on-line methods are desirable for a systematic search of new catalytic materials. 

Although the library of catalysts was actually very small, this combinatorial approach was shown to work surprisingly well. It remains to be seen if truly high throughput can be put into practice, which would require on-line methods for the detection and characterization of the particles bearing the polymers. 

The most important on-line methods are discussed below along with the hazard criteria used by each technique. 

Both urban and industrial wastewater often contains high concentrations of surfactants. Cationic (like alkylbenzene sulphonates) and non-ionic surfactants (like alcohol ethoxylates) are among the most-used surfactants and are discharged into sewers in widely varying concentrations. Two on-line methods have been designed for the monitoring of cationic surfactants with UV spectrophotometry [46] and non-ionic surfactants by on-line titration [47]. The detection limits are around 10 mg L. ... 

Westerink BH, De Vries JB. 1989. On the origin of extracellular GABA collected by brain microdialysis and assayed by a simplified on-line method. Naunyn Schmiedeberg s Arch Pharmacol 339(6) 603-607. 

The main advantage of this method compared with many other on-line methods for process monitoring is the use of noninvasive, so-called clamp-on sensors which can be easily mounted onto the process eqnip-ment (pipelines, reactors). The sensor, which often is a standard accelerometer, has no moving parts, and can, for example, withstand high temperatnres, a dirty enviromnent and is easy to maintain. The aconstic spectra measured with one sensor will often contain information abont several process-relevant properties and/or analytes which makes it possible to predict several parameters of interest from the same aconstic spectrum. 

In some manufacturing process analysis applications the analyte requires sample preparation (dilution, derivatization, etc.) to afford a suitable analytical method. Derivatization, emission enhancement, and other extrinsic fluorescent approaches described previously are examples of such methods. On-line methods, in particular those requiring chemical reaction, are often reserved for unique cases where other PAT techniques (e.g., UV-vis, NIR, etc.) are insufficient (e.g., very low concentrations) and real-time process control is imperative. That is, there are several complexities to address with these types of on-line solutions to realize a robust process analysis method such as post reaction cleanup, filtering of reaction byproducts, etc. Nevertheless, real-time sample preparation is achieved via an on-line sample conditioning system. These systems can also address harsh process stream conditions (flow, pressure, temperature, etc.) that are either not appropriate for the desired measurement accuracy or precision or the mechanical limitations of the inline insertion probe or flow cell. This section summarizes some of the common LIF monitoring applications across various sectors. 

Real process standards, but reference concentrations obtained from process sampling/lab analysis or temporary on-line method... 

Despite the remarks above, in a well-defined reaction system on-line analysis methods are very valuable. The most important on-line methods are, in this order, UV-VIS spectrophotometry, fluorimetry, and polarimetry. All three methods are based on the validity of equivalent laws linking the measured optical parameter with concentration ... 

There are two basic approaches off-line and on-line. The off-line method, as discussed in the chapters on sample pretreatment, are most often used because they involve either manually or automatically collecting a fraction from a sample cleanup sorbent. The appropriate fraction is transferred and then assayed by a second chromatographic method. The manual steps are time-consuming and potentially introduce significant error to the j precision and accuracy of the method. The on-line method, when fully automated, would have the chromatography system perform sample pre- j treatment by column switching between two or more columns. j... 

Quantitative PCR without using an internal standard has become possible with the recent devdopment of real-time thermocyclers. The advantage of this approach is that the amplified DNA is detected in the exponential phase of the amplification rather than the plateau phase, by sensitive on-line methods using fluorescent oligo-nudeotide probes or fluorescent dyes (Meuer et al. 2001). 

Thorough colloidal/surface characterisation is fundamental to the success of research on polymer colloids. A wide range of complementary techniques are available for colloidal/surface characterisation of polymer colloids and access to several is necessary since no single technique can provide full characterisation. There is an ongoing need for experimental and theoretical work on improvements to existing methods and on development of new techniques to support the needs of research. Additionally, the necessary improvements in process modelling will naturally lead to a demand for advances in on-line analysis to support feedback loops for process control and manufacturing. Thus, further developments in on-line methods for measurement of particle... 

The most accurate measurements of creatinine involve some form of separation, so an on-line method is very useful. Figure 10.2 shows a simple cation exchange isocratic HPLC assay that can be easily set up in a metabolic laboratory. The urine is simply diluted prior to injection. Creatinine is detected by its UV absorption but the figure clearly shows that trace amounts of other compounds must be resolved if a precise assay is to be achieved. 

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