Discontinuous sampling techniques

FIGURE 1.2 Different types of discontinuous sampling techniques. 

A comparison of active (using pumps) and passive (relying on diffusion) sampling techniques for the determination of nitrobenzene, benzene and aniline in air was mentioned in Section IV.A77. Several LLE methods for nitroaromatic compounds dissolved in water were evaluated. High recoveries were achieved with discontinuous or continuous extraction with dichloromethane, adsorption on a 1 1 1 mixture of Amberlite XAD-2, -4 and -8 resins and elution with dichloromethane445. 

The way the analysis is performed, by using a discontinuous sampling process while the crystallization proceeds, provides the possibility to easily automate the technique, as shown in Fig. 23 where five different samples introduced in separate crystallization vessels can be analyzed simultaneously. 

Disc Electrophoresis. Resolution in zone electrophoresis depends critically on getting sample components to migrate in a focused band, thus some techniques ate employed to concentrate the sample as it migrates through the gel. The most common technique is referred to as discontinuous pH or disc electrophoresis. Disc electrophoresis employs a two-gel system, where the properties of the two gels are different. 

Automated titrations can be divided into discontinuous and continuous, the former representing a discrete sample analysis, as a batch titration is the usual laboratory technique and the latter a flow technique, which is used less frequently in the laboratory, e.g., in kinetic studies, but is of greater importance in plant and environment control. 

Liquid-liquid extraction is by far the most popular separation method for the cleanup and preconcentration of samples because it is simple, reproducible, and versatile. There are several ways to achieve these objectives, from the original discontinuous ( batch ) and nonautomatic techniques to continuous separation techniques incorporated with automated methods of analysis. The methodologies can be classified into two general types ... 

Continuous analytical methods (amperometric and UV-absorption methods) are advantageous. However, sometimes only discontinuous methods (titrimetric and some photometric methods) are available due to expense. In such cases it is important to measure immediately after sampling to avoid the decay of ozone and in the case of liquid ozone to avoid degassing. Discontinuous photometric methods requiring the addition of chemicals to the sample can be converted to a continuous method by combination with flow injection analysis (FIA). This analytical technique requires instrumentation and is not easy to handle. 

Whereas SDS-PAGE and other discontinuous techniques are generally quite tolerant of sample impurities and buffer and ionic variations, the quality of the sample and the nature of the solution it is loaded in have a strong influence on the quality of an IEF separation. The sample must be as free as possible of salts, buffers, and other small charged molecules,... 

A last remark must be made about the use of the word continuous electrophoresis, covering collecting two-dimensional techniques in which both sample and background buffer feeding are continuous. As there is a two-dimensional technique with discontinuous application of the sample but a continuous feeding of buffer, the use of the term continuous is difficult, and again the terms collecting and star electrophoresis are preferred. 

The choice of the technique used in monitoring or detection of corrosion depends upon many factors such as the size of the sample, the particular features of the defect, accessibility, sensitivity of the technique, the limitations of the techniques, knowledge and skills of the operator and the availability of the instrument. The following is a general guide for the application and suitability of the technique in the detection of discontinuity.52... 

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