Off-line mode

When multiple development is performed on the same monolayer stationary phase, the development distance and the total solvent strength and selectivity values (16) of the mobile phase (17) can easily be changed at any stage of the development sequence to optimize the separation. These techniques are typically fully off-line modes, because the plates must be dried between consecutive development steps only after this can the next development, with the same or different development distances and/or mobile phases, be started. This method involves the following stages ... 

In the off-line mode of use of the drug file the data-base, stored In a time-shared computer. Is Interrogated through a remote device such as a cathode ray terminal The questions that may be asked Include all the affects of a drug, all the drugs that affect a test, or more specifically does a particular drug affect a particular test procedure by a particular mechanism ... 

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... 

Klink [135] recently discussed sample preparation procedures for LC-MS. SPE can be so well integrated into the concept of LC-MS, that in many automated applications no clear distinction exists between SPE and LC [135]. In on-line LC-MS mode, the possibilities for changing the eluent are rather limited, because of the tolerance of the eluent for the interface. Moreover, the conventional gradient mode may lead to strong fluctuations in the response of the MS detector. Here the off-line mode, using SPE for concentration followed by selective elution, enables very far-reaching preseparation, due to the differences in the polarity of the eluents applied and their mixtures. Although the overall benefits of SPE for LC-MS applications are positive, extracts... 

There are two general types of multidimensional chromatography separation schemes those in which the effluent from one column flows directly on to a second column at some time during the experiment, and those in which some type of trap exists between the two columns to decouple them (off-line mode). The purpose of a trap is often to allow collection of a fixed eluate volume to reconcentrate the analyte zone prior to the second separation step, or to allow a changeover from one solvent system to another. The use of offline multidimensional techniques (conventional sample cleanup) with incompatible mobile phases, is common in the literature, and replacing these procedures with automated on-line multidimensional separations will require continuous development efforts. 

Principles and Characteristics Multidimensional gas chromatography (MDGC) is widely used, due to the mobile-phase compatibility between the primary and secondary separating systems, which allows relatively simple coupling with less-complicated interfaces. In its simplest form, 2DGC can be carried out in the off-line mode. The most elementary procedure involves manual collection of effluent from a column, followed by reinjection into another column of a different selectivity (e.g. from an apolar to a polar column). Selecting proper GC-column combinations is critical. In on-line mode, the interface in MDGC must provide for the quantitative transfer of the effluent from one column... 

In this case the model is run in off-line mode and used to search for the raw and recycle brine flows which minimise the cost of chlorine production for a whole range of electricity prices and loads. Optimisation is simplified by the fact that the brine price is effectively fixed. 

Liquid chromatographic clean up [441,443,450] has been used either in normal phase flow using alumina, silica, or florisil [22,189,403,481,484] or with reverse-phase (RP) columns [409,452,480]. In most cases these techniques are well established and are used in an off-line mode, primarily to remove the bulk of co-extracted materials prior to a more refined clean-up prior to the final determination. These columns may be prepared in the laboratory [22,403 -405] or commercial solid phase extraction (SPE) cartridges can be used [409,452, 463,470,485,486]. In both cases, the normal phase cartridges and column materials are disposable since many of the polar co-extractants bind firmly to the substrate surface and are difficult to remove. This has been overcome to some... 

In most applications chemometric methods are applied to analytical data in an off-line mode that is, data has already been obtained by conventional techniques and is then applied to a particular chemometric method. Examples of this use are in cluster analysis and in pattern recognition. They are applied to spectroscopic, chromatographic, and other analytical data. 

Both types of detector systems can be used in two modes of applications, i.e., on-line and off-line. In the on-line mode the fractionation and detection systems are directly coupM. It is possible to use more than one detector either in series or in parallel. Examples of this mode of detection will be given in the section dealing with combined techniques. The off-line mode involves the collection of the fractions with subsequent determination of the constituents. The advantage of this approach is that further sample pretreatment procedures could be applied where necessary before the constituents are detected. In addition, quantitative estimates of the recoveries can be made. 

A recent review [30] has demonstrated that in the last 5 years there has been a trend in the MISPE literature to go away from mere proof-of-principle demonstrations towards applications in real samples. The authors found 110 original papers on MISPE applications in real samples in this period of time. These have dealt with four main types of samples environmental, biological, food, and drug. The MIPs used were mainly prepared by noncovalent methods and were used in off-line mode. The review gives a nice tabulated summary of all these papers with information about the templates, sample matrices, and detection methods. 

Modern ion-trap spectrometers are capable of performing MS" experiments which yield unique information on the molecular weight of individual sugar units in the oligosaccharide chain of saponins. While fragmentations up to MS5 could be performed in off-line mode, MS/MS spectra were recorded in on-flow LC-NMR-MS runs by default (auto-MS/MS mode). 

Transferring the procedure to an off-line mode, quantification by reversed phase HPLC showed that all analytes except -nicotyrine were quantitatively recovered. Due to the enhanced stability of the nicotine MIP, achieved by thermal annealing, and the reusability of the MIP, the procedure may be suited for automation. 

The off-line mode is to be preferred when a deep knowledge of the features of the extraction process concerned is required as this will allow such experimental variables as pressure, temperature, SF polarity and flow-rate, extractor volume and dimensions, extraction time and sample size to be optimized. 

MI-SPE has been applied in either on-line or off-line mode for several analytes. In the first case, MIP is packed as an HPLC precolumn. Column switching and pulsed elution modes are employed in on-line MI-SPE. One advantage of this design is the automation capability and the direct coupling to HPLC or other separation modes. Despite these advantages, the off-line mode, which has been successfully applied to bio and environmental and food analysis, is the most commonly practiced method. 

In the absence of online MS, fractionation is usually accompanied by an off-line mode of quality analysis. The typical workflow comprises of (1) a prepreparative analysis of the unpurified material, (2) the purification/ fractionation of the compound, and (3) a postpreparative analysis of the individual fractions. The pre- and postpreparative analysis can be performed with analytical HPLC, MS, and activity testing of the fractions, if an assay is available. 

Can only be interfaced with LC in an off-line mode.. Cannot analyze noncovalently bound protein complexes. 

By using ion-exchange resins, a selected metal ion can be isolated (separated) and pre-concentrated from its matrix. This process can be carried out in two ways, i.e. by (i) batch, or (ii) column processes. In the batch process, the ion-exchange resin is added to the aqueous sample, whereas in the second process the resin is packed into a chromatographic column. The former would be always carried out off-line, while the latter could be carried out in either on-line or off-line mode. 

Off-line mode consists of depressurising the supercritical fluid following extraction. By returning to the gaseous state, it leaves the analytes in a concentrated form with the undesirable impurities on the internal wall of the extraction vessel. The concentrate is dissolved using some classical solvent and submitted to a selective extraction on a solid-phase cartridge. 

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