Liquid sampling in continuous analysers

In completely continuous analysers, samples are taking from an evolving system such as a pipe pouring out waste water or an industrial effluent. A peristaltic pump continuously aspirates the sample, In which the evolution of one or several analytes is monitored as a function of time. This is the simplest possible alternative and Is characterized by the absence of discontinuities however, the system can be programmed to operate In a discrete fashion if required. Reverse FIA and completely continuous assemblies (Fig. 3.2a) are representative examples. 

Normal flow-injection analysis (FIA) involves using a peristaltic pump to aspirate the sample Into the loop of an Injection valve, which also quantizes the aspirated volume, and subsequently Inserting the loop contents into a carrier or reagent stream. 

The discrete aspiration of a fixed sample volume, determined by the time during which the sample is withdrawn from the sampler and by the flow-rate of the peristaltic pump, can be accomplished in two manners. In the assembly developed by Rocks and co-workers [3,4], the moving articulated needle features two aspiration positions (see Fig. 3.2c). In the first, the sample Is aspirated from the sampler vial. Once the programmed volume has been taken, the 

To the right of Fig. 3.2 Is shown the flow profile obtained after the system has sequentially taken two samples, Si and S2 —except if sampling is carried out continuously. Cross-contamination between samples In the above-described alternatives is discussed in the corresponding chapters. 

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