On-line technique

Since the reliability of gas turbines in the power industry has been lower than desired in recent years because of hot-corrosion problems, techniques have been developed to detect and control the parameters that cause these problems. By monitoring the water content and corrosive contaminant in the fuel line, any changes in fuel quality can be noted and corrective measures initiated. The concept here is that Na contaminants in the fuel are caused from external sources such as seawater thus, by monitoring water content, Na content is automatically being monitored. This on-line technique is adequate for lighter distillate fuels. For heavier fuels, a more complete analysis of the fuel should be carried out at least once a month using the batch-type system. The data should be input directly to the computer. The water and corrosion detecting systems also operate in conjunction with the batch analysis for the heavier fuels. 

The quantitative analysis of additives in a polycarbonate homopolymer has been carried out by micro-SEC-capillary GC and by a conventional precipitation technique (14). The validity of the on-line technique was demonstrated and equivalent... 

The main aims in environmental analysis are sensitivity (due to the low concentration of microcontaminants to be determined), selectivity (due to the complexity of the sample) and automation of analysis (to increase the throughput in control analysis). These three aims are achieved by multidimensional chromatography sensitivity is enhanced by large-volume injection techniques combined with peak compression, selectivity is obviously enhanced if one uses two separations with different selectivi-ties instead of one, while on-line techniques reduce the number of manual operations in the analytical procedure. 

The category of algebraic equation models is quite general and it encompasses many types of engineering models. For example, any discrete dsmamic model described by a set of difference equations falls in this category for parameter estimation purposes. These models could be either deterministic or stochastic in nature or even combinations of the two. Although on-line techniques are available for the estimation of parameters in sampled data systems, off-line techniques... 

Coupled on-line techniques (GC-MS, LC-MS, MS/ MS, etc.) provide for indirect mixture analysis, while many of the newer desorption/ionisation methods are well suited for direct analysis of mixtures. DI techniques, applied either directly or with prior liquid chromatographic separations, provide molecular weight information up to 5000 Da, but little or no additional structural information. Higher molecular weight (or more labile) additives can be detected more readily in the isolated extract, since desorption/ionisation techniques (e.g. FD and FAB) can be used with the extract but not with the compounded polymer. Major increases in sensitivity will be needed to support imaging experiments with DI in which the spatial distribution of ions in the x — y plane are followed with resolutions of a few tens of microns, and the total ion current obtained is a few hundreds of ions. 

Trace analysis is particularly attractive for SFE-HPLC since quantitative transfer of all analytes extracted to the chromatographic system becomes possible. At present, on-line SFE-HPLC appears to be feasible for qualitative analysis only quantitation is difficult due to possible pump and detector precision problems. Sample size restrictions also appear to be another significant barrier to using on-line SFE-HPLC for quantitative analysis of real samples. On-line SFE-HPLC has therefore not proven to be a very popular hyphenated sample preparatory/separation technique. Although online SFE-HPLC has not been quantitatively feasible, SFE is quite useful for quantitative determination of those analytes that must be analysed by off-line HPLC, and should not be ruled out when considering sample preparatory techniques. In most cases, all of the disadvantages mentioned with the on-line technique (Table 7.15) are eliminated. On- and off-line SFE-HPLC were reviewed [24,128]. 

With on-line techniques, the column switching operations are done using valves. Fig. 5.3d shows a simple arrangement for zone cutting that could be used for sample clean-up. The zone marked Y is to be determined and all other zones are to go to waste (this type of cut is called a heart cut). Initial separation takes place on column Cl so that early zones (X) are routed to waste. When zone Y is eluted from Cl, valve V2 is switched to elute this zone onto column C2. After complete transfer of Y onto C2, valve VI is switched to prevent further elution of unwanted zones (Z, for instance). Zone Y is eluted to the detector and Cl can be cleaned and re-equilibrated with mobile phase. 

Comparing on-line and off-line methods of column switching, which of the following advantages and limitations do you think would apply to the on-line technique ... 

On-line techniques are easily automated, but are more expensive as they require additional valves with associated switching equipment. Off-line methods are rather easier to carry out, but, because of the sample collection and re-injection steps, they are slower and tend... 

Off-line measurements differ significantly from on-line techniques. Offline measurements include safety testing in a laboratory, which has the major advantage of providing available time to redesign the process and/or equipment if a risk is identified. Of course, immediate process response is not possible. 

Analytical-scale SFE can be divided into off-line and on-line techniques. Off-line SFE refers to any method where the analytes are extracted using SFE and collected in a device independent of the chromatograph or other measurement instrument. On-line SF techniques use direct transfer of the extracted analytes to the analytical instrument, most frequently a chromatograph. While the development of such on-line SFE methods of analysis has great potential for eventual automation and for enhancing method sensitivities [159-161], the great majority of analytical SFE systems described use some form of off-line SFE followed by conventional chromatographic or spectroscopic analysis. 

The classical off-line procedures for sample preparations are time consuming and analytical precision depends on the skill of the investigator. With on-line techniques, using a combination of an elemental analyzer directly coupled to the mass spectrometer many problems of the off-hne preparation can be overcome and minimized. Differences in both techniques are summarized in Table 1.5. 

NIR Vibrational overtones and combination bands Very high S/N Relatively low sensitivity facilitates minimum sample preparation Most popular on-line technique, but not as sensitive or diagnostic as IR and Raman. Sensitivity to physical as well as chemical status can be a problem or an opportunity, depending on application... 

Methods for the detection of placement and quantitation of Si-H, Si-OH and Si-0 groups in molecular weight components of silicones of the polydimethylsiloxane type have been developed using SEC-IR and SEC-UV on-line techniques. In the case of silanols, problems with partial hydrogen bonding were overcome by use of 1,4-dioxane mobile phase. For phenyl groups, methylene chloride was effective for qualitative measurements and THF yielded quantitative results at high sensitivity. 

This contribution concentrates on a discussion of some on-line techniques for ground state studies which are now under development and will be applied at ISOLDE in the near future. The plans for a nuclear-orientation set up at ISOLDE are outlined in the contribution to this symposium by N. Stone. 

TABLE 1 On-line techniques for Optical Spectroscopy at ISOLDE... 

T. Sakai, The Development of On-line Techniques and Novel Processing Systems for the Monitoring and Handling of the Evolution of Microstructure in Nonreactive and Reactive Polymer Systems, Adv. Polym. Techn., 14, 277 (1995). 

Most of the currently applied LC-MALDI interfaces use off-line spot deposition and a number of commercial instruments are available (Mukhopadhyay 2005). Compared to on-line techniques, this interface type is technically less demanding but only careful adjustment guarantees optimal performance. 

This paper demonstrates the fantastic power of py-GC-MS-C and TGA-MS-C for analysis of complex, nonvolatile systems such as coal. These techniques make it possible to obtain both qualitative and quantitative information on saturates, aromatics, and some hetero compounds without any prior treatment. These are on-line techniques, i.e., pyrolysis is followed by chromatographic or weight analysis which is followed by mass analysis without interruption. 

However, laser diffiaction can only be used as an off-line measurement technique, as it requires relatively low particle concentrations. In contrast to it, ultrasonic spectroscopy can be applied as an on-line technique, but the sensitivity towards coarse particles is not as high as for laser diffraction (Fig. 6). 

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