Off-line operations

Figure 8.18 Schematic diagrams showing (a) serially connected multilayer OPLC employing different types of stationaiy phase of decreasing polarity (A > B > C) as a type of MD-FFPC (b) fully off-line operational mode (c) fully on-line operational mode.

Figure S.ll shows the flow diagram of the microprocessor-controlled preconcentration equipment, which is configured here for off-line operation, and consists of a sample changer, three separate peristaltic pumps (PI, P2 and P3) for the sample solution, buffer and add, three magnetic valves (VI, V2 and V3), the preconcentration column filled with chelating ion-exchange material (7 mm i.d., 10—30 mm height) and a fractionating unit for the addic column eluate. The flow-rates for the sample solution, buffer and add are adjusted to S ml/min.

Operations that are conducted while the RO system is off line are just as important as on-line operations in keeping an RO system functioning well. Off-line operations covered in this chapter include system flush, membrane cleaning, and membrane lay-up. 

An important advantage of the off-line operation of TLC is the flexibility afforded by the use of multiple methods for zone detection and identification. For example, the layer can be viewed under long- and short-wave UV light, followed by one or more chromogenic, fluorogenic, or biological detection methods. 

Figure 7.16A depicts a flexible SFE-HPLC coupled assembly developed by Ischi and Haerdi [106] that consists of three main parts [viz. the SFE system (Al), the interface (A2) and the HPLC system (A3)] each furnished with appropriate valves operating as shown in Fig. 7.16B. Thus, valve 5 in Fig. 7.16A is used to provide extraction with or without a modifier, via a tee connector on the other hand, valve 10 allows switching between static and dynamic extraction. The former is done by having the valve close the outlet of the extraction cell after the desired temperature is reached. By switching the valve back, the dynamic state is restored. Valve 13 enables trapping of the extracted analytes, either on a C, silica column placed in an oven for on-line preconcentration and insertion of non-polar or low-polar analytes into the chromatograph after elution or into a liquid phase to implement an off-line operation. When polar ionic analytes are to be preconcentrated, the eluent from the extractor is diverted to valve 18 and retained on the ion-exchange material packed in the column. Preconcentration of both non-polar, low-polar and polar ionic analytes can be accomplished by using both valves (13 and 18) [106],...

Sample quantitation is a difficult operation to automate. Weighing Is In fact an off-line operation that cannot be readily Implemented by analyser modules with the exception of robot stations. On the other hand, electronic balances do allow the measured weight to be passed on to the analyser or Instrument microprocessor. In this case, the operation Is essentially manual as only data transfer Is automated. The quantitation of solid samples can be based on ... 

Generally, analog computations are on-line operations. This may sometimes be preferred to off-line operations. The results are simultaneously recorded and can be immediately controlled. The influence of any modification of the parameters can be registered at... 

Figure 7-50. The many off-line operations that are performed on extruded plastics.

The combination of fast atom bombardment and liquid secondary-ion mass spectrometry with TLC is the subject of Chapter 3 of this volume. It should be noted that less difficulties were encountered in developing combinations TLC/MS than with the corresponding coupling with HPLC, due to the off-line operation of TLC separation and MS detection. 

In TLC, data acquisition is one, independent, off-line operation in a sophisticated analytical procedure that starts with application, continues with development (and occasionally postrun plate treatment), and finishes with scanning of developed plates. As a developed and dried plate can no longer be changed, the operator must find and select the best scanning conditions. It is necessary, in the name of correctness, to prepare SOPs (system operating procedures) so that operator is allowed, according to his skill and feel, to select freely the correct scanning conditions, since the best choice of conditions very often depends on the results of previous operations. The purist in CLP may take issue with this tactic, but this is the only way to eliminate systematic errors. 

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